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Cell_Biology_Alberts. The guidance of commissural axons illustrates how axons rarely navigate directly to their targets. Instead, they use intermediate targets, or guideposts, and switch their sensitivities as they move from one local guidepost to the next, steering their way through a complex environment to a far-away destination. the Formation of Orderly Neural Maps Depends on Neuronal Specificity In many cases, neurons of a similar type are laid out in a broad array of different positions, but send out axons that come together for their journey and arrive at the target region in a tight bundle. There the axons disperse again, to terminate at different sites in the target territory. This they do in an orderly way, creating a regular mapping from one territory to another—a neural map.


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Cell_Biology_Alberts. The axon projection from the eye to the brain provides an important example. The neurons in the retina that convey visual information back to the brain are called retinal ganglion cells (RGCs). There are more than a million of them in humans, each one reporting on a different part of the visual field. Their axons converge on the optic nerve head at the back of the eye and travel together along the developing optic nerve toward the brain. Their main site of termination, in most vertebrates other than mammals, is the optic tectum—a broad expanse of cells in the midbrain. In connecting with tectal neurons, the RGC axons distribute themselves in a predictable pattern according to the arrangement of their cell bodies in the retina: RGCs that are neighbors in the retina connect with target cells that are neighbors in the tectum. The orderly projection creates a retinotopic map of visual space on the tectum (Figure 21–75).


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Cell_Biology_Alberts. Orderly maps of this sort are found in many brain regions. In the auditory system, for example, the neurons that project from the ear to the brain form a tonotopic map in which brain cells receiving information about sounds of different pitch are ordered along a line, like the keys of a piano. And in the somatosensory Figure 21–75 The neural map from eye to brain in a young zebrafish.


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Cell_Biology_Alberts. Figure 21–75 The neural map from eye to brain in a young zebrafish. (a) Diagrammatic view, looking down on the top of the head. (B) Fluorescence micrograph. Fluorescent tracer dyes have been injected into each eye—red into the anterior part, green into the posterior part. the tracer molecules have been taken up by the neurons in the retina and carried along their axons, revealing the paths they take to the optic tectum in the brain and the map that they form there. (courtesy of chi-Bin chien, from D.h. Sanes, t.a. reh and W.a. harris, Development of the Nervous System. San Diego, ca: academic press, 2000.) genitalia leg toesfoot intra-abdominalpharynx tongue teeth, gums, and jawlower lip lips upper lip facenoseeyethumbindexmiddleringlittlehandwristforearmelbowarmshoulderheadnecktrunkhip


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Cell_Biology_Alberts. Figure 21–76 A map of the body surface in the human brain. the surface of the body is mapped onto the somatosensory region of the cerebral cortex by using an orderly system of nerve cell connections to pair body sites with the brain sites that receive their sensory information. this means that the map in the brain is largely faithful to the topology of the body surface, even though different body regions are represented at different magnifications according to their density of innervation. the homunculus (the “little man” in the brain) has big lips, for example, because the lips are a particularly large and important source of sensory information. the map was determined by stimulating different points in the cortex of conscious patients during brain surgery and recording what they said they felt. (after W. penfield and t. rasmussen, the cerebral cortex of Man. New York: Macmillan, 1950.) system, neurons conveying information about touch map onto the cerebral cortex so as to mark out a


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Cell_Biology_Alberts. felt. (after W. penfield and t. rasmussen, the cerebral cortex of Man. New York: Macmillan, 1950.) system, neurons conveying information about touch map onto the cerebral cortex so as to mark out a “homunculus”—a small, distorted, two-dimensional image of the body surface (Figure 21–76).


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Cell_Biology_Alberts. The retinotopic map of visual space in the optic tectum is the best characterized of all these maps. How does it arise? A famous experiment in the 1940s on frogs provided an important clue. If the optic nerve of a frog is cut, it will regenerate. The retinal axons grow back to the optic tectum, restoring normal vision. If, however, the eye is in addition rotated in its socket at the time of cutting of the nerve, so as to put originally ventral retinal cells in the position of dorsal retinal cells, vision is still restored, but with an awkward flaw: the animal behaves as though it sees the world upside down and left–right inverted (Figure 21–77). If food is dangled in front of it, for example, it will lunge perversely backward. This is because the misplaced retinal cells make the connections appropriate to their original, not their actual, positions. It seems that the retinal ganglion cells (RGCs) have positional values—position-specific biochemical properties representing records of


Cell_Biology_Alberts

Cell_Biology_Alberts. appropriate to their original, not their actual, positions. It seems that the retinal ganglion cells (RGCs) have positional values—position-specific biochemical properties representing records of their original location in the retina, assigned perhaps by earlier morphogen gradients, and making RGCs on opposite sides of the retina intrinsically different.


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Cell_Biology_Alberts. Such nonequivalence among neurons is referred to as neuronal specificity. It is this intrinsic characteristic that guides the retinal axons to their appropriate target sites in the tectum. Those target sites themselves are distinguishable by the Figure 21–77 Neurons in different regions of the retina project axons to different regions in the tectum. Neurons (rGcs) in the anterior retina project axons to the posterior tectum (as shown in Figure 21–75 for zebrafish). regeneration experiments show that retinal neurons have an intrinsic preference for the part of the tectum they normally connect to. If the eye is surgically rotated when the optic nerve is cut, the regenerating retinal axons connect to their original targets, creating an inverted map. (after e. Kandel et al., principles of Neural Science, 5th ed., New York: McGraw hill Medical, 2012.)


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Cell_Biology_Alberts. Figure 21–78 Selectivity of retinal axons growing over tectal membranes. (a) Diagram of an experiment performed with cells from a chick embryo. the culture substratum is coated with alternating stripes of membrane prepared either from posterior tectum or from anterior tectum. axons from posterior retina grow on anterior tectal membrane but are repelled by posterior tectal membrane. axons from anterior retina show different (less selective) behavior. (B) photograph of results. the retinal axons, growing out from the left, are made visible by staining them with a fluorescent marker. the selective pattern of outgrowth shows that anterior tectum differs from posterior tectum, and anterior retina correspondingly differs from posterior retina. In the intact organism, this serves to orient a retinotopic map; the map is refined by subsequent competitive interactions among the anterior and posterior retinal axons, which push the anterior retinal cells off anterior tectal territory. (From J.


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Cell_Biology_Alberts. map; the map is refined by subsequent competitive interactions among the anterior and posterior retinal axons, which push the anterior retinal cells off anterior tectal territory. (From J. Walter et al., Development 101:685–696, 1987.With permission from the company of Biologists.) retinal axons because the tectal cells also carry positional labels. Thus, the neural map depends on a correspondence between two systems of positional markers, one in the retina and the other in the tectum.


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Cell_Biology_Alberts. How are these markers used to make the map? When posterior axons are allowed to grow out over a carpet of anterior or posterior tectal membranes in a culture dish, they show selectivity. Posterior axons strongly prefer the anterior tectal membranes, as in vivo, whereas anterior axons show no preference or prefer posterior tectal membranes (Figure 21–78). The key difference between anterior and posterior tectum is not an attractive factor on the anterior tectum but a repulsive factor on the posterior tectum, to which posterior retinal axons are sensitive but anterior retinal axons are not. If a posterior retinal growth cone touches posterior tectal membrane, it collapses its filopodia and withdraws.


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Cell_Biology_Alberts. In this system, as in others that we have mentioned, the repulsive interactions are mediated by ephrin–Eph signaling—specifically, EphrinA–EphA signaling for the anteroposterior axis (Figure 21–79). An analogous mechanism based on EphB–EphrinB signaling orients the dorsoventral axis of the retinotopic map. These mechanisms serve to orient the map along both axes, but they are not enough by themselves to ensure accurate point-to-point detail. This is brought about through a long process of adjustment that fills in and refines the map through interactions among the RGC axon terminals as they compete for territory on the tectum. This refinement of the pattern of connections involves electrical signaling in the system of developing synapses—a topic that we return to shortly. Both Dendrites and axonal Branches From the Same Neuron avoid One another


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Cell_Biology_Alberts. Both Dendrites and axonal Branches From the Same Neuron avoid One another Axons and dendrites from different neurons can repel one another, or they can cohere; they can collaborate to form synapses, or they can compete. Remarkably, Figure 21–79 Ephrin signaling orients the retinotopic map. (a) Neurons in the posterior retina express epha. as their axons reach the tectum, they are repelled by high levels of ephrina protein in the posterior tectum and project preferentially to the anterior tectum. (B) In epha-mutant mice, posterior retinal axons feel no such repulsion and project more widely within the tectum. (after e. Kandel et al., principles of Neural Science, 5th ed., New York: McGraw hill Medical, 2012.) axons or dendrites can also repel each other when they arise from a single neuron. Such self-avoidance prevents the neuron from making purposeless synapses with itself; it also helps the cell spread out its processes widely so as to innervate a broad territory.


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Cell_Biology_Alberts. Self-avoidance poses a problem. If the same self-recognition molecule were used in every neuron, all neurons in the brain would repel each other. Some classes of neurons do show this sort of mutual repulsion, creating solitary territories—a phenomenon called tiling; but in most cases, axons and dendrites from different neurons can overlap with one another. How then can the processes put out by a single neuron distinguish between self and non-self? This conundrum has been partially resolved by the discovery of a remarkable set of proteins that endow each neuron with a label unlike that of its neighbors. These are the DSCAM proteins in Drosophila and the protocadherins in vertebrates. As described in Chapter 7, DSCAM proteins are extraordinary for the number of isoforms that can be generated by alternative RNA splicing—more than 30,000 variants for DSCAM1 (see Figure 7–57). Diversity arises from alternative exons that code for three highly variable extracellular immunoglobulin domains.


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Cell_Biology_Alberts. by alternative RNA splicing—more than 30,000 variants for DSCAM1 (see Figure 7–57). Diversity arises from alternative exons that code for three highly variable extracellular immunoglobulin domains. Each DSCAM1 isoform engages in homophilic binding (see Figure 19–5), but remarkably, all the variable domains need to be identical for this to occur. Thus, one cell surface will bind to another via DSCAM only when the two cell surfaces express identical isoforms. The result of binding is repulsion, although the detailed mechanisms are poorly understood.


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Cell_Biology_Alberts. If alternative splicing occurs in a random fashion in each cell, neighboring processes from different neurons are unlikely to express the same DSCAM1 variant, so only the processes of the same cell will repel one another. Neurons that lack all DSCAM1 variants have severe defects in neuronal self-avoidance. Engineering Drosophila so that all of its neurons produce a single isoform restores self-avoidance; but now the processes of neighboring neurons express the same isoform and repel each other, resulting in the phenomenon of tiling (Figure 21–80).


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Cell_Biology_Alberts. Vertebrate neurons use a similar self-avoidance strategy to pattern their axons and dendrites, but instead of DSCAMs, they use protocadherins for self/non-self discrimination. The Protocadherin locus encodes 58 related cadherin-like trans-membrane proteins that are expressed in different combinations in single neurons. Homophilic recognition results in self-avoidance of dendrites emanating from the same neuron; neighboring dendrites of different neurons express different protocadherins and thus evade repulsion. Thus, although insect DSCAM and vertebrate protocadherin proteins share no sequence homology, they mediate similar self-avoidance strategies.


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Cell_Biology_Alberts. Eventually, axonal growth cones reach the target region where they must halt and make synapses. These synapses, as a rule, are destined to transmit neural signals in one direction, from axon to target cell. The development of synapses, however, depends on signaling in both directions: signals from the target tissue not only help control which growth cones synapse where (as we discuss shortly), but can also regulate how many of the innervating neurons survive. Many types of vertebrate neurons are produced in excess; up to 50% or more of some of them die soon after they reach their target, even though they appear perfectly normal and healthy up to the time of their death. About half of all the motor neurons that send axons to skeletal muscle, for example, die within a few days after making contact with their target muscle cells. A similar proportion of the sensory neurons that innervate the skin die after their growth cones have arrived there.


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Cell_Biology_Alberts. This large-scale normal neuronal death often seems to reflect the outcome of a competition, in which the target tissue releases a limited amount of a specific neurotrophic factor that the neurons innervating the tissue require to survive; those that do not get enough die by programmed cell death. If the amount of target tissue is increased—for example, by grafting an extra limb bud onto the side of the embryo—more limb-innervating neurons survive; conversely, if the limb bud is cut off, the same neurons all die (Figure 21–81). In this way, although individuals may vary in their bodily proportions, they always retain the right number of motor neurons to innervate all their muscles and the right number of sensory neurons to innervate their body surface. The strategy of overproduction followed by death of surplus cells may seem wasteful, but it provides a simple and effective means to adjust the number of innervating neurons according to the amount of tissue requiring innervation.


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Cell_Biology_Alberts. The first neurotrophic factor to be identified, and still the best characterized, is called nerve growth factor (NGF )—the founding member of the neurotrophin family of signal proteins. It promotes the survival and growth of specific classes of sensory neurons and of sympathetic neurons (a subclass of peripheral neurons that control contractions of smooth muscle and secretion from exocrine glands). Figure 21–80 DSCAM mediates self-avoidance of dendrites. (a) Sensory neurons in the Drosophila peripheral nervous system extend dendrites along the larval body wall. the image shows the dendrites of a regular array of photosensing neurons (red), which allow the larva to detect and avoid harmful light. the posterior epidermal cells of each segment are labeled in blue. there are many neurons, and those shown here spread out their dendrites into overlapping fields.


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Cell_Biology_Alberts. (B) Mutations at the Dscam locus upset the way the various dendrites interact, changing the rules of self-avoidance and the distribution of innervation. (a, courtesy of chun han; B, after D. hattori et al., Annu. Rev. Cell Dev. Biol. 24:597–620, 2008. With permission from annual reviews.) 2.5 day chick section of spinal cord 9 day chick (A) section of spinal cord NGF is produced by the tissues that these neurons innervate. When extra NGF is provided, extra sensory and sympathetic neurons survive, just as if extra target tissue were present. Conversely, in a mouse with a mutation that inactivates the gene for NGF or for its receptor (a receptor tyrosine kinase called TrkA), almost all sympathetic neurons and the NGF-dependent sensory neurons are lost. There are many neurotrophic factors, only a few of which belong to the neurotrophin family, and they act in different combinations to promote the survival and growth of different classes of neurons.


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Cell_Biology_Alberts. Formation of Synapses Depends on two-Way communication Between Neurons and their target cells


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Cell_Biology_Alberts. At journey’s end, the task of a growth cone is to halt its travels and make synapses with specific target cells. Synapses were introduced in Chapter 11, where we discussed channels and the electrical properties of membranes. Two main classes of synapses are found in vertebrates; those made with muscle cells and those made with other neurons. Synapse formation is best understood in the case of the highly specialized connections between motor neurons and skeletal muscle cells—so-called neuromuscular junctions (see Figure 11–38). During synapse formation, the axonal growth cone differentiates into a nerve terminal that contains synaptic vesicles filled with the neurotransmitter acetylcholine, while acetylcholine receptors become clustered in the muscle cell plasma membrane at the site of synapse formation. A synaptic cleft separates the preand postsynaptic plasma membranes, and a thin sheet of basal lamina lies in this space between them (Figure 21–82).


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Cell_Biology_Alberts. Figure 21–81 The survival of motor neurons depends on signals provided by the target muscles. (a) removal of the limb bud shortly after arrival of motor axons results in the death of motor neurons in the spinal cord on the amputated side. (B) transplantation of an extra limb bud increases the survival of motor neurons. (after e. Kandel et al., principles of Neural Science, 5th ed., New York: McGraw hill Medical, 2012.)


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Cell_Biology_Alberts. Formation of the synapse involves two-way communication between the muscle cell and axonal growth cone: each of them, under the influence of the other, must reorganize the molecules on its side of the junction. The growth cone releases the signal protein Agrin, while the muscle expresses the Agrin receptor LRP4. Agrin binding to LRP4 stimulates association of LRP4 with MuSK, a receptor tyrosine kinase. LRP4 also serves as a signal in the reverse direction, from the muscle to the axon (Figure 21–83). During synapse formation, MuSK and LRP4 cluster in the muscle cell plasma membrane in the general neighborhood of the future synapse. As the growth cone approaches, it recognizes LRP4, which stimulates the differentiation of presynaptic structures in the nerve cell. At the same time, Agrin released from the growth cone binds to LRP4 in the muscle cell; this activates MuSK, and promotes a more focused clustering of acetylcholine receptors in the muscle cell membrane. Through these


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Cell_Biology_Alberts. Agrin released from the growth cone binds to LRP4 in the muscle cell; this activates MuSK, and promotes a more focused clustering of acetylcholine receptors in the muscle cell membrane. Through these mechanisms, the reciprocal signaling of LRP4 from muscle to growth cone—and of Agrin from growth cone to muscle—induces the coordinated, localized differentiation of preand postsynaptic structures.


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Cell_Biology_Alberts. Synapse formation between neurons in the CNS is far more challenging, both for the neurons and for the scientists trying to understand the molecular basis of its specificity, and it remains poorly understood.


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Cell_Biology_Alberts. LRP4 SIGNALS BACK TO AXON CLUSTERING OF LRP4 AND MuSK BY AGRIN CLUSTERING OF ACETYLCHOLINE RECEPTORS Agrin Agrin axon terminal axon terminal basal lamina MuSK (Agrin co-receptor) MuSK LRP4 (Agrin receptor) LRP4acetylcholine receptor axon of motor neuron muscle fber (A) (C) (B) new synapse muscle fber Figure 21–83 Reciprocal signaling during neuromuscular synapse differentiation. (a) the agrin receptor Lrp4 and its co-receptor MuSK cluster in the muscle cell membrane in the general neighborhood of the future synapse. (B) as the growth cone approaches, it recognizes Lrp4, which stimulates differentiation of presynaptic structures. reciprocally, agrin is released from the nerve terminal, binds to a complex of Lrp4 and MuSK in the muscle, and (c) promotes the further and more focused clustering of the Lrp4 and acetylcholine receptors in the muscle cell. although the agrin/MuSK/Lrp4 machinery organizes the synapse, the process also depends on electrical signaling via the acetylcholine


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Cell_Biology_Alberts. of the Lrp4 and acetylcholine receptors in the muscle cell. although the agrin/MuSK/Lrp4 machinery organizes the synapse, the process also depends on electrical signaling via the acetylcholine receptors. It is not yet known how Lrp4 signals to the motor axon.


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Cell_Biology_Alberts. Figure 21–82 Formation of the neuromuscular junction. (a) the growth cone of a motor axon approaches the muscle fiber. (B) Initial synapse formation is characterized by the accumulation of synaptic vesicles at the axon terminal and the formation of a specialized basal lamina in the synaptic cleft. (c) as the neuromuscular junction matures, the synaptic cleft accumulates basal lamina and extracellular matrix proteins, synaptic vesicles cluster at presynaptic release sites, and neurotransmitter receptors cluster at postsynaptic sites. Schwann (glial) cells accompany the motor axon and wrap around its terminus outside the region of synaptic contact. (D) transmission electron micrograph of the region of synaptic contact. [D, courtesy of John heuser, from J. Electron Microsc. 60 (Suppl 1), 2011. With permission from Oxford University press.]


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Cell_Biology_Alberts. The two-way exchange of signals between axon growth cones and muscle cells controls the initial formation of neuromuscular junctions, but it is only the first step in the establishment of the final pattern of the synaptic connections. Each muscle cell at first receives synapses from several motor neurons, but in the end it is left innervated by only one. This process of synapse elimination depends on active synaptic communication and electrical activity. If synaptic transmission is blocked by a toxin that binds to the acetylcholine receptors in the muscle cell membrane, or if axonal electrical activity is blocked by a toxin that binds to sodium channels in the axon plasma membrane, the muscle cell retains its multiple innervation beyond the normal time of elimination.


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Cell_Biology_Alberts. The phenomenon of activity-dependent synapse elimination is encountered in almost every part of the developing vertebrate nervous system (Figure 21–84). It has a key role, for example, in the refinement of the retinotopic map discussed earlier. Synapses are first formed in abundance and distributed over a broad target field; then the system of connections is pruned back and remodeled by competitive processes that depend on electrical activity and synaptic signaling. The elimination of synapses in this way is distinct from the elimination of surplus neurons by cell death, and it occurs after the period of normal neuronal death is over. Synapse remodeling during neural development, however, involves more than just synapse elimination; it also involves synapse reinforcement, as we discuss next.


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Cell_Biology_Alberts. Throughout the nervous system, and throughout life, activity-dependent elimination and reinforcement of synapses plays a fundamental part in adjusting the detailed anatomy of the neural network according to functional requirements. The importance of these processes, and their underlying rules, emerged half a century ago from a groundbreaking series of experiments on the developing visual system of young mammals.


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Cell_Biology_Alberts. In the brain of most mammals, axons relaying visual inputs from the two eyes are brought together in a specific neuronal layer in the visual region of the cerebral cortex. Here, they form two overlapping maps of the external visual field, one as perceived through the right eye, the other as perceived through the left. Although there may be a tendency for rightand left-eye inputs to be segregated even before synaptic communication begins, a large proportion of the axons carrying information from the two eyes at early stages form synapses together on shared target neurons in the visual cortex. A period of early electrical signaling activity, is quiet: cell C gets excited simultaneously: cell C gets excited synapse made by B on C synapses made by both A and B is weakened or eliminated on C are strengthened BB


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Cell_Biology_Alberts. Figure 21–84 Synapse modification and its dependence on electrical activity. experiments in several systems indicate that synapses are strengthened or weakened by electrical activity according to the rule shown in the diagram. the underlying principle appears to be that each excitation of a target cell tends to weaken any synapse where the presynaptic axon terminal has been quiet, but to strengthen any synapse where the presynaptic axon terminal has just been active. as a result, any synapse that is repeatedly weakened and rarely strengthened is eventually eliminated altogether.


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Cell_Biology_Alberts. Figure 21–85 Ocular dominance columns in the visual cortex of a monkey’s brain, and their sensitivity to visual experience. (a) Normally, stripes of cortical cells driven by the right eye alternate with stripes, of equal width, driven by the left eye. the stripes, set up before birth, are revealed here by injecting a radioactive tracer molecule into one eye, allowing time for this tracer to be transported to the visual cortex, and detecting radioactivity there by autoradiography, in sections cut parallel to the cortical surface. (B) If one eye is kept covered after birth, during the sensitive period of development, and thus deprived of visual experience, its stripes shrink and those of the active eye expand. In this way, the deprived eye may lose the power of vision almost entirely. (From


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Cell_Biology_Alberts. D.h. hubel, t.N. Wiesel and S. LeVay, Philos. Trans. R. Soc. Lond. B Biol. Sci. 278:377–409, 1977. With permission from the royal Society.) however, occurring spontaneously and independently in each retina before birth, leads to a remarkable pattern of ocular dominance columns in the visual cortex: stripes of cells driven by inputs from the right eye alternating with stripes driven by inputs from the left eye (Figure 21–85).


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Cell_Biology_Alberts. The basis for these phenomena became clear from ingenious experiments interfering artificially with visual experience and altering the coordination of electrical signaling in the two eyes. These studies, and many others subsequently, have highlighted a simple but profoundly important principle that seems to govern synapse reinforcement and elimination throughout the nervous system. When two (or more) neurons synapsing on the same target cell fire at the same time, they reinforce their connections to that cell; when they fire at different times, they compete, so that all but one of them tend to be eliminated. This firing rule is expressed in the catchphrase “neurons that fire together wire together.”


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Cell_Biology_Alberts. The firing rule provides a simple interpretation of the developmental phenomenon we have just described in the mammalian visual system. A pair of axons bringing information from neighboring sites in the left eye will frequently fire together, and therefore wire together, as will a pair of axons from neighboring sites in the right eye; but a right-eye axon and a left-eye axon will rarely fire together, and will instead compete. Indeed, if activity from both eyes is silenced using toxins that block axonal electrical activity or synaptic signaling, as described above, the inputs fail to segregate correctly.


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Cell_Biology_Alberts. The segregation of inputs from the two eyes is only the first of a series of activity-dependent adjustments of visual connections, whose maintenance is extraordinarily sensitive to experience early in life. If, during a certain sensitive period (ending at about 5 years of age in humans), one eye is kept covered for a time so as to deprive it of visual stimulation, while the other eye is allowed normal stimulation, the deprived eye loses its synaptic connections to the cortex and becomes almost entirely, and irreversibly, blind. In accordance with what the firing rule would predict, a competition has occurred in which synapses in the visual cortex made by inactive axons are eliminated while synapses made by active axons are consolidated. In this way, cortical territory is allocated to axons that carry information and is not wasted on those that are silent.


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Cell_Biology_Alberts. Activity-dependent synaptic changes are not confined to early life. They also occur in the adult brain, where many synapses show both functional and morphological alterations with use. This synaptic plasticity is thought to have a fundamental role in learning and memory. Clearly, for the nervous system as for other parts of the body, developmental processes do not end at birth, as we discuss in the next chapter. The development of the nervous system proceeds in four phases. First, neurons and glial cells are generated from dividing neural progenitor cells. Then, the newborn neurons send out axons and dendrites toward their targets. Next, they make synaptic connections with appropriate target cells so that communication can begin. Finally, excessive neurons are eliminated by normal neuronal cell death, after which the system of synaptic connections is refined and remodeled according to the pattern of electrical and synaptic activity in the neural network.


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Cell_Biology_Alberts. Neurons born at different times and places are specialized to express different sets of genes, and they have a cell memory that plays a major role in determining the connections they will form. Their specialization depends not only on spatial patterning by morphogens but also on intrinsic developmental programs that unfold as the neural progenitors proliferate. Axons and dendrites grow out from the neurons by means of growth cones, which follow specific pathways delineated by attractive and repellant signals along the way, including cell-surface and extracellular matrix molecules and soluble signal proteins to which growth cones from different classes of neurons respond differently. In many parts of the nervous system, neural maps are set up—orderly projections of one array of neurons onto another. In the retinotopic system, the map is based on the matching of complementary systems of position-specific cell-surface markers—ephrins and Eph receptors—possessed by the two sets of cells.


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Cell_Biology_Alberts. another. In the retinotopic system, the map is based on the matching of complementary systems of position-specific cell-surface markers—ephrins and Eph receptors—possessed by the two sets of cells. Other cell-surface molecules such as DSCAM proteins in Drosophila and protocadherins in vertebrates mediate self-avoidance between the branches arising from a single neuron, helping the cell spread out its processes.


Cell_Biology_Alberts

Cell_Biology_Alberts. The formation of synapses involves back-and-forth signaling between target cells and the growth cone. After the growth cones have reached their targets and initial connections have formed, individual synapses are eliminated in some places and reinforced in others by mechanisms that depend on synaptic and electrical activity. These mechanisms adjust the architecture of the neural network according to the way in which it is used. Which statements are true? Explain why or why not. What regulates the pace of development? Why does a mouse embryo develop faster than a human embryo, for example? What are the mechanisms that allow cell memory to be stored during development, explaining how each cell’s history determines its future behavior? how do signals move through tissues? What are the roles of the extracellular matrix and of elongated cell projections?


Cell_Biology_Alberts

Cell_Biology_Alberts. how do signals move through tissues? What are the roles of the extracellular matrix and of elongated cell projections? how does a cell know exactly where it is in a multicellular organism? how does it know that its neighbors are the correct ones and that, if not, it should move or kill itself? how do cells respond to tiny gradients of molecules in their environment, as required for knowing their positions? how are morphogen gradients reliably interpreted? What are the genetic changes that allow the repurposing of existing body parts during evolution? For example, how did bat wings evolve from arms? how do cells use genetic instructions to form the shape of something as complex as the human nose?


Cell_Biology_Alberts

Cell_Biology_Alberts. how do cells use genetic instructions to form the shape of something as complex as the human nose? 21–1 In the early cleavage stages, when the embryo 21–5 Changes in the coding regions of genes involved in cannot yet feed, the developmental program is driven and development are primarily responsible for the differences controlled entirely by the material deposited in the egg by between species. the mother. 21–6 The cell cycle is the ticking clock that sets the tempo of developmental processes, with maturational 21–2 Because of the many later developmental trans-changes in gene expression being dependent on cell-cycle formations that produce the elaborately structured organs, progression. the body plan set up during gastrulation bears little resemblance to the body plan in the adult.


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Cell_Biology_Alberts. the body plan set up during gastrulation bears little resemblance to the body plan in the adult. Discuss the following problems. 21–3 As development progresses, individual cells 21–7 Name the four processes that are fundamental to become more and more restricted in the range of cell types animal development, and describe each of them in a single they can give rise to. sentence. 21–4 At different stages of embryonic development, 21–8 What are the three germ layers formed during gas-the same signals are used over and over again by different trulation, and what are the principal structures each gives cells, but with different biological outcomes. rise to in the adult? 21–9 In the early Drosophila embryo, there seems to be no requirement for the usual forms of cell–cell signaling; instead, transcriptional regulators and mRNA molecules move freely between nuclei. How can that be?


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Cell_Biology_Alberts. 21–10 Morphogens play a key role in development, creating concentration gradients that inform cells of where they are and how to behave. Examine the simple patterns represented by the flags in Figure Q21–1. Which do you suppose could be created by a gradient of a single morphogen? Which would require gradients of two morphogens? Assuming that such patterns were present in a sheet of cells, explain how they could be created by morphogens. Figure Q21–1 National flags from three countries (problem 21–10).


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Cell_Biology_Alberts. 21–11 Two adjacent cells in the nematode worm normally differentiate into an anchor cell (AC) and a ventral uterine precursor (VU) cell, but which of the two becomes the AC and which becomes the VU cell is completely random: the cells have an equal chance of adopting either fate, but they always adopt different fates. Mutations of Lin12 alter these fates. In hyperactive Lin12 mutants, both cells become VU cells, while in inactive Lin12 mutants, both cells become ACs. Thus, Lin12 is central to the decision-making process. In genetic mosaics in which one precursor cell has the hyperactive Lin12 and the other precursor has the inactive Lin12, the cell with the hyperactive Lin12 always becomes the VU cell and the cell with inactive Lin12 always becomes the AC. Assuming that one cell sends a signal and the other cell receives it, explain how these results suggest that Lin12 encodes a protein required to receive the signal. Offer a suggestion for how the fates of these two precursor cells


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Cell_Biology_Alberts. signal and the other cell receives it, explain how these results suggest that Lin12 encodes a protein required to receive the signal. Offer a suggestion for how the fates of these two precursor cells are normally decided in wild-type worms.


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Cell_Biology_Alberts. 21–12 It was clear from the early days of studying development that certain “morphogenetic” substances were present in the egg and segregated asymmetrically into cells of the developing embryo. One such investigation in ascidian (sea squirt) embryos examined endodermal alkaline phosphatase, which could be visualized by a histochemical stain. Treatment of embryos with cytochalasin B stopped cell division, but did not block expression of alkaline phosphatase at the appropriate time. Treatment with actinomycin D, which blocks transcription, did not interfere with expression of alkaline phosphatase. Treatment with puromycin, which blocks translation, eliminated expression of alkaline phosphatase. What is the likely nature of the morphogenetic substance that gives rise to alkaline phosphatase?


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Cell_Biology_Alberts. 21–13 The mouse HoxA3 and HoxD3 genes are paralogs that occupy equivalent positions in their respective Hox gene clusters and share roughly 50% identity in their pro-tein-coding sequences. Mice with defects in HoxA3 have deficiencies in pharyngeal tissues, whereas mice with defects in HoxD3 have deficiencies in the axial skeleton, suggesting quite different functions for the paralogs. Thus, it came as a surprise when it was found that replacing a defective HoxD3 gene with the normal HoxA3 gene corrected the deficiency, as did the reciprocal experiment of replacing a mutant HoxA3 gene with a normal HoxD3 gene. Neither transplaced gene, however, could supply its normal function; that is, a normal HoxA3 gene at the HoxD3 locus could not correct the deficiency caused by a mutant HoxA3 gene at the HoxA3 locus. The same was true for the HoxD3 gene. If the HoxA3 and HoxD3 genes are equivalent, how do you suppose they can play such distinct roles in development? Why do you suppose they cannot


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Cell_Biology_Alberts. the HoxA3 locus. The same was true for the HoxD3 gene. If the HoxA3 and HoxD3 genes are equivalent, how do you suppose they can play such distinct roles in development? Why do you suppose they cannot perform their normal function in a new location?


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Cell_Biology_Alberts. 21–14 The segmentation of somites in vertebrate embryos is thought to depend on oscillations in the expression of the Hes7 gene. Mathematical modeling explains these oscillations in terms of the delays in production of the unstable Hes7 protein, which acts as a transcription regulator to shut off its own expression. Once Hes7 decays, with a half-life of about 20 minutes, its transcription resumes. To test this model, you decide to reduce the total delay by removing one, two, or all three of the introns from the Hes7 gene in mice. Why do you expect that intron removal would reduce the delay? What would you predict would happen to the oscillation time, and somite formation, if the model were correct?


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Cell_Biology_Alberts. 21–15 The oscillatory clock that drives somite formation in vertebrates involves three essential components Her7 (an unstable repressor of its own synthesis), Delta (a transmembrane signaling molecule), and Notch (a trans-membrane receptor for Delta). Notch is bound by Delta on neighboring cells, activating the Notch signaling pathway, which then activates Her7 transcription. Normally, this system works flawlessly to create sharply defined somites (Figure Q21–2A). In the absence of Delta, however, only the first five somites form normally, and the rest are poorly defined (Figure Q21–2B). If a pulse of Delta is supplied later, somite formation returns to normal in the regions where Delta was present (Figure Q21–2C). A diagram of the connections between the components of the clock and how they interact in adjacent cells is shown in Figure Q21–2D. In the absence of Delta, why do the cells become unsynchronized? What is it about the presence of Delta that keeps adjacent cells oscillating


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Cell_Biology_Alberts. interact in adjacent cells is shown in Figure Q21–2D. In the absence of Delta, why do the cells become unsynchronized? What is it about the presence of Delta that keeps adjacent cells oscillating in synchrony?


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Cell_Biology_Alberts. 21–16 The extracellular protein factor Decapentaplegic (Dpp) is critical for proper wing development in Drosophila (Figure Q21–3A). It is normally expressed in a narrow stripe in the middle of the wing, along the anterior–posterior boundary. Flies that are defective for Dpp form stunted “wings” (Figure Q21–3B). If an additional copy of the gene is placed under control of a promoter that is active in the anterior part of the wing, or in the posterior Figure Q21–2 Somite formation in zebrafish embryos (problem 21–15).


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Cell_Biology_Alberts. Figure Q21–2 Somite formation in zebrafish embryos (problem 21–15). (a) Wild-type embryos with normal somites. (B) Somite formation in embryos lacking Delta. the bracket indicates normal-looking somites where they initially form. (c) Somite formation in embryos lacking Delta, but receiving a pulse of Delta expression at the time indicated by the right-hand bracket. (D) Interactions among components of the oscillatory clock in adjacent cells. (adapted from c. Soza-ried et al., Development 141:1780–1788, 2014. With permission from the company of Biologists.) carroll SB (2006) endless Forms Most Beautiful: the New Science of evo Devo. New York: W.W. Norton & co., Inc. Gilbert SF (2013) Developmental Biology, 10th ed. Sunderland, Ma: Sinauer associates, Inc. Wolpert L & tickle c (2010) principles of Development, 3rd ed. Oxford, UK: Oxford University press. Overview of Development


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Cell_Biology_Alberts. Wolpert L & tickle c (2010) principles of Development, 3rd ed. Oxford, UK: Oxford University press. Overview of Development Gurdon JB (2013) the egg and the nucleus: a battle for supremacy (Nobel Lecture). Angew. Chem. Int. Ed. Engl. 52, 13890–13899. Istrail S & Davidson eh (2005) Logic functions of the genomic cisregulatory code. Proc. Natl Acad. Sci. USA 102, 4954–4959. Levine M (2010) transcriptional enhancers in animal development and evolution. Curr. Biol. 20, r754–r763. Lewis J (2008) From signals to patterns: space, time, and mathematics in developmental biology.Science 322, 399–403. Meinhardt h & Gierer a (2000) pattern formation by local self-activation and lateral inhibition. Bioessays 22, 753–760. rogers KW & Schier aF (2011) Morphogen gradients: from generation to interpretation. Annu. Rev. Cell Dev. Biol. 27, 377–407. Shubin N, tabin c & carroll S (2009) Deep homology and the origins of evolutionary novelty. Nature 457, 818–823.


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Cell_Biology_Alberts. Shubin N, tabin c & carroll S (2009) Deep homology and the origins of evolutionary novelty. Nature 457, 818–823. Figure Q21–3 effects of Dpp expression on wing development in Drosophila (problem 21–16). (a) Normal Dpp expression. (B) absence of Dpp expression. (c) additional anterior Dpp expression. (D) additional posterior Dpp expression. (From M. Zecca, K. Basler and G. Struhl, Development 121:2265–2278, 1995. With permission from the company of Biologists.) part of the wing, a large mass of wing tissue composed of normal-looking cells is produced at the site of Dpp expression (Figure Q21–3C and D). Does Dpp stimulate cell division, cell growth, or both? How can you tell? 21–17 The highly branched structures of neurons would seem to make it almost inevitable that they should make unproductive synapses with themselves, yet they manage to avoid this outcome very effectively. How is this accomplished in vertebrates?


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Cell_Biology_Alberts. Mechanisms of Pattern Formation andrey G & Duboule D (2014) SnapShot: hox gene regulation. Cell 156, 856–856.e1. Baker Ne (2011) proximodistal patterning in the Drosophila leg: models and mutations. Genetics 187, 1003–1010. chan YF, Marks Me, Jones Fc et al. (2010) adaptive evolution of pelvic reduction in sticklebacks by recurrent deletion of a pitx1 enhancer. Science 327, 302–305. Davis rL, Weintraub h & Lassar aB (1987) expression of a single transfected cDNa converts fibroblasts to myoblasts. Cell 51, 987–1000. De robertis eM (2006) Spemann’s organizer and self-regulation in amphibian embryos. Nat. Rev. Mol. Cell Biol. 4, 296–302. DiNardo S, heemskerk J, Dougan S & O’Farrrell ph (1994) the making of a maggot: patterning the Drosophila embryonic epidermis. Curr. Opin. Genet. Dev. 4, 529–534. Driever W & Nüsslein-Volhard c (1988) a gradient of bicoid protein in Drosophila embryos. Cell 54, 83–93.


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Cell_Biology_Alberts. Driever W & Nüsslein-Volhard c (1988) a gradient of bicoid protein in Drosophila embryos. Cell 54, 83–93. Fowlkes cc, Luengo cL, Keränen Ve et al. (2008) a quantitative spatiotemporal atlas of gene expression in the Drosophila blastoderm. Cell 133, 364–74. Furman Dp & Bukharina ta (2008) how Drosophila Melanogaster forms its mechanoreceptors. Curr. Genomics 9, 312–323. Gaudet J & Mango Se (2002) regulation of organogenesis by the Caenorhabditis elegans Foxa protein pha-4. Science 295, 821–825. halder G, callaerts p & Gehring WJ (1995) Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science 267, 1788–1792. Kornberg tB & roy S (2014) cytonemes as specialized signaling filopodia. Development 141, 729–36. Knoblich Ja (2010) asymmetric cell division: recent developments and their implications for tumour biology. Nat. Rev. Mol. Cell Biol. 11, 849–860. Lander aD (2013) how cells know where they are. Science 339, 923–27.


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Cell_Biology_Alberts. Lander aD (2013) how cells know where they are. Science 339, 923–27. Lewis eB (1978) a gene complex controlling segmentation in Drosophila. Nature 276, 565–570. Müller p, rogers KW, Jordan BM et al. (2012) Differential diffusivity of Nodal and Lefty underlies a reaction-diffusion patterning system. Science 336, 721–724. Nüsslein-Volhard c & Wieschaus e (1980) Mutations affecting segment number and polarity in Drosophila. Nature 287, 795–801. ringrose L & paro r (2007) polycomb/trithorax response elements and epigenetic memory of cell identity. Development 134, 223–232. Shulman JM & St Johnston D (1999) pattern formation in single cells. Trends Cell Biol. 9, M60–64. von Dassow G, Meir e, Munro eM & Odell GM (2000) the segment polarity network is a robust developmental module. Nature 406, 188–192. Brown DD & cai L (2007) amphibian metamorphosis. Dev. Biol. 306, 20–33.


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Cell_Biology_Alberts. Brown DD & cai L (2007) amphibian metamorphosis. Dev. Biol. 306, 20–33. Giraldez aJ, Mishima Y, rihel J et al. (2006) Zebrafish Mir-430 promotes deadenylation and clearance of maternal mrNas. Science 312, 75–79. Isshiki t, pearson B, holbrook S & Doe cQ (2001) Drosophila neuroblasts sequentially express transcription factors which specify the temporal identity of their neuronal progeny. Cell 106, 511–521. Lee rc, Feinbaum rL & ambros V (1993) the C. elegans heterochronic gene lin-4 encodes small rNas with antisense complementarity to lin-14. Cell 75, 843–854. Lewis J (2003) autoinhibition with transcriptional delay: a simple mechanism for the zebrafish somitogenesis oscillator. Curr. Biol. 13, 1398–1408. pourquié O (2011) Vertebrate segmentation: from cyclic gene networks to scoliosis. Cell 145, 650–663. Song J, Irwin J & Dean c (2013) remembering the prolonged cold of winter.Curr. Biol. 23, r807–r811.


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Cell_Biology_Alberts. Song J, Irwin J & Dean c (2013) remembering the prolonged cold of winter.Curr. Biol. 23, r807–r811. Wightman B, ha I & ruvkun G (1993) posttranscriptional regulation of the heterochronic gene lin-14 by lin-4 mediates temporal pattern formation in C. elegans. Cell 75, 855–862. Green aa, Kennaway Jr, hanna aI et al. (2010) Genetic control of organ shape and tissue polarity. PLoS Biol. 8, e1000537. Le Douarin NM & Kalcheim c (1999) the Neural crest, 2nd ed. cambridge, UK: cambridge University press. Matis M & axelrod JD (2013) regulation of pcp by the fat signaling pathway. Genes Dev. 27, 2207–20. Ochoa-espinosa a & affolter M (2012) Branching morphogenesis: from cells to organs and back. Cold Spring Harb. Perspect. Biol. 4, pii: a008243. raz e & reichman-Fried M (2006) attraction rules: germ cell migration in zebrafish. Curr. Opin. Genet. Dev. 16, 355–359. revenu c & Gilmour De (2009) Mt2.0: shaping epithelia through collective migration. Curr. Opin. Genet. Dev. 19, 338–342.


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Cell_Biology_Alberts. revenu c & Gilmour De (2009) Mt2.0: shaping epithelia through collective migration. Curr. Opin. Genet. Dev. 19, 338–342. Simons M & Mlodzik M (2008) planar cell polarity signaling: from fly development to human disease. Annu. Rev. Genet. 42, 517–540. Solnica-Krezel L & Sepich DS (2012) Gastrulation: making and shaping germ layers. Annu. Rev. Cell Dev. Biol. 28, 687–717. takeichi M (2011) Self-organization of animal tissues: cadherinmediated processes. Dev. Cell 21, 24–26. Walck-Shannon e & hardin J (2014) cell intercalation from top to bottom. Nature 15, 34–48. andersen DS, colombani J & Léopold p (2013) coordination of organ growth: principles and outstanding questions from the world of insects. Trends Cell Biol. 23, 336–344. enderle L & McNeill h (2013) hippo gains weight: added insights and complexity to pathway control. Sci. Signal. 6, re7.


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Cell_Biology_Alberts. enderle L & McNeill h (2013) hippo gains weight: added insights and complexity to pathway control. Sci. Signal. 6, re7. hariharan IK & Bilder D (2006) regulation of imaginal disc growth by tumor-suppressor genes in Drosophila. Annu. Rev. Genetics 40, 335–61. Johnston La (2009) competitive interactions between cells: death, growth, and geography. Science 324, 1679–1682. Lawrence pa & casal J (2013) the mechanisms of planar cell polarity, growth and the hippo pathway: some known unknowns. Dev. Biol. 377, 1–8. pan D (2010) the hippo signaling pathway in development and cancer. Dev. Cell 19, 491–505. restrepo S, Zartman JJ & Basler K (2014) coordination of patterning and growth by the morphogen Dpp. Curr. Biol. 24, r245–r255. Burden SJ, Yumoto N & Zhang W (2013) the role of MuSK in synapse formation and neuromuscular disease. Cold Spring Harb. Perspect. Biol. 5, a009167.


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Cell_Biology_Alberts. Burden SJ, Yumoto N & Zhang W (2013) the role of MuSK in synapse formation and neuromuscular disease. Cold Spring Harb. Perspect. Biol. 5, a009167. Dessaud e, McMahon ap & Briscoe J (2008) pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network. Development 135, 2489–2503. hubel Dh & Wiesel tN (1965) Binocular interaction in striate cortex of kittens reared with artificial squint. J. Neurophysiol. 28, 1041–1059. Kolodkin aL & tessier-Lavigne M (2011) Mechanisms and molecules of neuronal wiring: a primer. Cold Spring Harb. Perspect. Biol. 3, pii: a001727. Luo L & Flanagan JG (2007) Development of continuous and discrete neural maps. Neuron 56, 284–300. rakic p (1988) Specification of cerebral cortical areas. Science 241, 170–176. reichardt LF (2006) Neurotrophin-regulated signalling pathways. Philos. Trans. R. Soc. Lond. B Biol. Sci. 361, 1545–1564.


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Cell_Biology_Alberts. reichardt LF (2006) Neurotrophin-regulated signalling pathways. Philos. Trans. R. Soc. Lond. B Biol. Sci. 361, 1545–1564. Sanes Dh, reh ta & harris Wa (2011) Development of the Nervous System, 3rd ed. San Diego, ca: academic press. Sperry rW (1963) chemoaffinity in the orderly growth of nerve fiber patterns and connections. Proc. Natl Acad. Sci. USA 50, 703–710. Zipursky SL & Sanes Jr (2010) chemoaffinity revisited: dscams, protocadherins, and neural circuit assembly. Cell 143, 343–353.


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Cell_Biology_Alberts. Zipursky SL & Sanes Jr (2010) chemoaffinity revisited: dscams, protocadherins, and neural circuit assembly. Cell 143, 343–353. Cells evolved originally as free-living individuals, and such cells still dominate the Earth and its oceans. But the cells that matter most to us, as human beings, are specialized members of a multicellular community. These cells have lost features needed for independent survival and acquired peculiarities that serve the needs of the body as a whole. Although they share the same genome, they are spectacularly diverse in structure, chemistry, and behavior. There are more than 200 different named cell types in the human body that collaborate with one another to form many different tissues, arranged into organs performing widely varied functions. To understand them, it is not enough to analyze cells in a culture dish: we need also to know how they live, work, and die in their natural habitat, the intact body.


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Cell_Biology_Alberts. In Chapters 7 and 21, we saw how the various cell types become different in the embryo and how cell memory and signals from their neighbors enable them to remain different thereafter. In Chapter 19, we discussed the technology used to build multicellular tissues—the devices that bind cells together and the extracellular materials that give them support. But the adult body is not static: it is a structure in dynamic equilibrium, where new cells are continually being born, differentiating, and dying. Homeostatic mechanisms maintain a proper balance, so that the tissue architecture is preserved despite the constant replacement of old cells by new. In this chapter, we focus on these developmental processes that continue throughout life. In doing so, we shall illustrate some of the diversity of specialized cell types and see how they work together to perform their tasks.


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Cell_Biology_Alberts. We shall examine in particular the role played in many tissues by stem cells— cells that are specialized to provide a fresh supply of differentiated cells where these need to be continually replaced, or when they are required in great number for purposes of repair and regeneration. We shall see that while many tissues renew and repair themselves, some others do not; there, lost cells are lost forever, causing deafness, blindness, dementia, and other ills. In the final section of the chapter, we discuss how stem cells can be generated and manipulated artificially, and we confront the practical question that underlies the current storm of interest in stem-cell technology: How can we use our understanding of the processes of cell differentiation and tissue renewal to improve upon nature, and make good those injuries and failings of the human body that have hitherto seemed to be beyond repair?


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Cell_Biology_Alberts. Among all the self-renewing tissues in a mammal, the champion—for speed at least—is the lining of the small intestine: the long, convoluted portion of the gut tube that is chiefly responsible for absorption of nutrients from the gut lumen. To introduce stem cells, we take the small intestine as our starting point—not only because it renews itself at a greater rate than any other tissue in the body, but also because the molecular mechanisms that control its organization are particularly well understood. It thereby provides a beautiful illustration of the principles of stem-cell systems that have broad applicability. FibRoblaStS and theiR tRanSFoRmationS: the ConneCtive-tiSSue Cell Family blood veSSelS, lymphatiCS, and endothelial CellS a hieRaRChiCal Stem-Cell SyStem: blood Cell FoRmation epithelial cell migration from “birth” at the bottom the lining of the Small intestine is Continually Renewed through Cell proliferation in the Crypts


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Cell_Biology_Alberts. The lining of the small intestine (and of most other regions of the gut) is a sin-gle-layered epithelium, only one cell thick. This epithelium covers the surfaces of the villi that project into the lumen, and it lines the crypts that descend into the underlying connective tissue (Figure 22–1). Dividing cells are restricted to the crypts, and differentiated cells, no longer dividing, pour out of the crypts in a steady stream onto the villi. There are four main types of nondividing differentiated cells—one absorptive and three secretory (Figure 22–2): 1. Absorptive cells (also called brush-border cells or enterocytes) have densely packed microvilli on their exposed surfaces. Their job is to take up nutrients from the gut lumen. To this end, they also produce hydrolytic enzymes that perform some of the final steps of extracellular digestion. They are the majority cell type in the epithelium. 2.


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Cell_Biology_Alberts. 2. Goblet cells secrete mucus into the gut lumen that covers the epithelium with a protective coat. 3. Paneth cells form part of the innate immune defense system (discussed in Chapter 24) and secrete proteins that kill bacteria. 4. Enteroendocrine cells, of more than 15 different subtypes, secrete serotonin and peptide hormones that act on neurons and other cell types in the gut wall and regulate the growth, proliferation, and digestive activities of cells of the gut and other tissues. As if on a conveyor belt, the absorptive, goblet, and enteroendocrine cells travel mainly upward from their site of birth in the crypt, by a sliding movement in the plane of the epithelial sheet, to cover the surfaces of the villi. Within 3–5 days (in the mouse) after emerging from the crypts, the cells reach the tips of the villi, where they undergo apoptosis and are finally discarded into the gut lumen (see Figure 22–1 Renewal of the gut lining.


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Cell_Biology_Alberts. (a) the pattern of cell turnover and proliferation in the epithelium that forms the lining of the small intestine. Stem cells (red) lie at the crypt base, interspersed among nondividing differentiated cells (paneth cells). progeny of the stem cells move mainly upward from the crypts onto the villi; after a few quick divisions, they cease dividing and differentiate—some of them while still in the crypt, most of them as they emerge from the crypt. the paneth cells, like the other nondividing differentiated cells, are continually replaced by progeny of the stem cells, but they migrate downward to the crypt base and survive there for many weeks. (b) photograph of a section of part of the lining of the small intestine, showing the crypts and villi. note the mixture of differentiated cell types, all generated from the stem cells; these are primarily absorptive cells, with mucus-secreting goblet cells (stained red) interspersed among them. enteroendocrine cells (not labeled) are less


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Cell_Biology_Alberts. types, all generated from the stem cells; these are primarily absorptive cells, with mucus-secreting goblet cells (stained red) interspersed among them. enteroendocrine cells (not labeled) are less numerous and less easy to identify without special stains.


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Cell_Biology_Alberts. Movie 20.6). The Paneth cells in the crypts are produced in much smaller numbers and have a different migration pattern. They live at the bottom of the crypts, where they too are continually replaced, although not so rapidly, persisting for several weeks (in the mouse) before undergoing apoptosis and being phagocytosed by their neighbors. The central problem is to understand the processes in the crypt that generate a continual supply of all these nondividing, terminally differentiated cell types. Stem Cells of the Small intestine lie at or near the base of each Crypt


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Cell_Biology_Alberts. The general pattern of cell proliferation and migration in the gut lining is revealed by a simple labeling method that uses injected pulses of tritiated (radioactive) thymidine or of a thymidine analog that can be detected in tissue sections. Cells that are in S phase of the division cycle incorporate the marker molecule into their DNA, and their fate can then be followed over subsequent hours and days. If a cell divides after incorporation of the label, the label becomes diluted, halving with each cell cycle. This can be quantified. Experiments based on this labeling method confirm, first of all, that dividing cells are confined to the crypts and that the differentiated cell types listed above do not divide. Second, the most rapidly dividing cells, with a cycle time of about 12 hours in the mouse, are shown to lie in the middle and upper parts of the crypt, and these cells are all fated to differentiate and stop dividing (see Figure 22–1A). Just above the base of the crypt,


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Cell_Biology_Alberts. hours in the mouse, are shown to lie in the middle and upper parts of the crypt, and these cells are all fated to differentiate and stop dividing (see Figure 22–1A). Just above the base of the crypt, interspersed among the Paneth cells, lie cells that divide more slowly. These are the stem cells, which feed some of their progeny into the higher levels of the crypt destined for differentiation, while other progeny remain at the crypt base to continue the whole process. The rapidly dividing cells above these stem cells are derived from them, but already committed to differentiation. These cells are called committed precursors or transit amplifying cells, since their divisions serve to amplify the number of differentiated cells that ultimately result from each stem-cell division.


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Cell_Biology_Alberts. the two daughters of a Stem Cell Face a Choice Stem cells have a critical role in a variety of tissues, and it is useful to list their defining properties: 1. A stem cell is not itself terminally differentiated: that is, it is not at the end of a pathway of differentiation. 2. It can divide without limit (or at least for the lifetime of the animal). 3. When it divides, each daughter has a choice: it can either remain a stem cell, or it can embark on a course that commits it to terminal differentiation (Figure 22–3).


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Cell_Biology_Alberts. Figure 22–2 The four main differentiated cell types found in the epithelial lining of the small intestine. all cells are oriented with the gut lumen at top. broad orange arrows indicate direction of secretion or uptake of materials for each type of cell. all of these cells are generated from undifferentiated multipotent stem cells living near the bottoms of the crypts (see Figure 22–1). Absorptive (brush-border) cells outnumber the other cell types in the epithelium by about 10:1 or more. the microvilli on their apical surface provide a 30-fold increase of surface area, not only for the import of nutrients but also for the anchorage of enzymes that perform the final stages of extracellular digestion, breaking down small peptides and disaccharides into monomers that can be transported across the cell membrane. Goblet cells secrete mucus; these are the commonest of the secretory cell types. Paneth cells secrete (along with some growth factors) cryptdins—proteins of the defensin family


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Cell_Biology_Alberts. the cell membrane. Goblet cells secrete mucus; these are the commonest of the secretory cell types. Paneth cells secrete (along with some growth factors) cryptdins—proteins of the defensin family that kill bacteria. different subtypes of enteroendocrine cells secrete serotonin and peptide hormones into the gut wall (and thence the blood). Cholecystokinin is a hormone released from enteroendocrine cells in response to the presence of nutrients in the gut. it binds to receptors on nearby sensory nerve endings, which relay a signal to the brain to stop the feeling of hunger once one has eaten enough. (after t.l. lentz, Cell Fine Structure. philadelphia: Saunders, 1971;


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Cell_Biology_Alberts. R. krsti´c, illustrated encyclopedia of human histology. berlin: Springer-verlag, 1984.) Stem cells are required wherever there is a recurring need to replace differentiated cells that cannot themselves divide. Although a stem cell must be able to divide, it does not necessarily have to divide rapidly; in fact, many stem cells divide at a relatively slow rate.


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Cell_Biology_Alberts. Stem cells are of many types, specialized for the genesis of different classes of terminally differentiated cells—intestinal stem cells for intestinal epithelium, epidermal stem cells for epidermis, hematopoietic stem cells for blood, and so on. Each stem-cell system nevertheless raises similar fundamental questions. What are the distinguishing features of the stem cell in molecular terms? What conditions serve to keep the stem cell in its proper place and to maintain its stem-cell character? What decides whether a given daughter cell commits to differentiation or remains a stem cell? In a tissue where several distinct types of differentiated cells must be produced, are they all derived from a single type of stem cell, or is there a distinct type of stem cell for each one? wnt Signaling maintains the Gut Stem-Cell Compartment


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Cell_Biology_Alberts. For the gut, the beginnings of an answer to these questions came from studies of cancer of the colon and rectum (the lower end of the gut, also known as the large intestine). Some people have a hereditary predisposition to colorectal cancer and, in advance of the invasive disease, develop large numbers of small precancerous tumors (adenomas) in the lining of this part of the gut (Figure 22–4). The appearance of these tumors suggests that they have arisen from intestinal crypt cells that have failed to halt their proliferation in the normal way. As discussed in Chapter 20, the cause has been traced to mutations in the Apc (adenomatous polyposis coli) gene: the tumors arise from cells that have lost both gene copies. Because Apc codes for a protein that prevents inappropriate activation of the Wnt signaling pathway (see Figure 15–60), this loss of Apc is presumed to mimic the effect of continual exposure to a Wnt signal. The suggestion, therefore, is that Wnt signaling normally keeps


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Cell_Biology_Alberts. the Wnt signaling pathway (see Figure 15–60), this loss of Apc is presumed to mimic the effect of continual exposure to a Wnt signal. The suggestion, therefore, is that Wnt signaling normally keeps crypt cells in a proliferative state, and that a cessation of exposure to Wnt signaling normally makes them stop dividing as they leave the crypt.


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Cell_Biology_Alberts. Stem Cells at the Crypt base are multipotent, Giving Rise to the Full Range of differentiated intestinal Cell types It has long been suspected that all the differentiated cell types in the lining of the intestine derive from a single type of stem cell. But firm proof was lacking, and the precise nature and location of the stem cells were disputed. To solve the problem, and indeed to understand the organization of any stem-cell system, we need to discover how its cells are related to one another—who Figure 22–3 The definition of a stem cell. each daughter produced when a stem cell divides can either remain a stem cell or go on to become terminally differentiated. in many cases, the daughter that opts for terminal differentiation undergoes additional cell divisions before terminal differentiation is completed; such cells are called transit amplifying cells.


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Cell_Biology_Alberts. Figure 22–4 An adenoma in the human colon, compared with normal tissue from an adjacent region of the same person’s colon. the specimen is from a patient with an inherited mutation in one of his two copies of the Apc gene. a mutation in the other Apc gene copy, occurring in a colon epithelial cell during adult life, has given rise to a clone of cells that behave as though the wnt signaling pathway is permanently activated. as a result, the cells of this clone form an adenoma—an enormous, steadily expanding mass of giant cryptlike structures.


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Cell_Biology_Alberts. Figure 22–5 Clonal analysis using a genetic marker. a modern method for tracking cell lineage uses transgenic animals containing two transgenes, which together drive expression of a readily detected and heritable marker protein in a small subset of stem cells. the first transgene (top) carries two adjacent protein-coding sequences, GFP and CreERT2, both expressed under the control of the Lgr promoter that is active only in stem cells and not in their differentiated progeny. GFP encodes green fluorescent protein (see Chapter 9), which is used here simply to confirm expression in the entire stem-cell population. the CreERT2 gene encodes a chimeric form of the Cre recombinase called CreeRt, which consists of Cre recombinase linked to the estrogen receptor protein; this enzyme becomes active as a recombinase only when it binds the artificial estrogen analog tamoxifen.


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Cell_Biology_Alberts. the second transgene (bottom) carries a marker gene, LacZ, under the control of a promoter that is active in all cells. the LacZ gene encodes β-galactosidase, an enzyme that can be detected histochemically in tissues (see Figure 7–28). however, LacZ expression in the transgene shown here is prevented by a blocking sequence (red) that is flanked by LoxP sites (pink; see Figure 5–66). when tamoxifen is provided, CreeRt becomes active––leading to a recombination event that removes the blocking dna sequence (and leaves one LoxP site behind). as a result, the lacZ marker is expressed. because this change is heritable, the marker continues to be expressed in all cells descended from those in which a recombination event has occurred. with a low dose of the inducer molecule tamoxifen, it is possible to activate the marker at random in just a few widely spaced cells, which, in the course of time, give rise to widely separated and easily distinguished clones of progeny (see Figure 22–6).


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Cell_Biology_Alberts. is descended from whom, or, equivalently, what progeny will be produced from any given cell. This can best be done using a heritable marker that can be activated in an individual cell, thus allowing the identification of the clone of progeny descended from that cell. A modern method uses transgenic animals to create a visible genetic mark in just a few widely spaced cells, which, in the course of time, give rise to widely separated and easily distinguished clones of progeny, as explained in Figure 22–5.


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Cell_Biology_Alberts. A search among genes that are strongly upregulated in response to Wnt signaling revealed one, called Lgr5, that is expressed in gut stem cells specifically. The technique described in Figure 22–5 can be used to create a genetic mark in a random subset of Lgr5-expressing cells—a mark that is inherited by the progeny of each cell. These Lgr5 cells divide with a cycle time of about 24 hours, and within a few days marked clones are seen extending from the crypt bases up along the sides of the villi. After as long as 60 days or more, many of these clones still persist, retaining one or more members at the crypt base and extending all the way up to the tips of the villi (Figure 22–6). Moreover, each single clone typically contains all the major differentiated gut cell types—absorptive, goblet, Paneth, and enteroendocrine—in their normal proportions. The Lgr5-expressing cells, therefore, are true stem cells that are multipotent—that is, able to generate a diverse set of differentiated cell


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Cell_Biology_Alberts. Paneth, and enteroendocrine—in their normal proportions. The Lgr5-expressing cells, therefore, are true stem cells that are multipotent—that is, able to generate a diverse set of differentiated cell types.


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Cell_Biology_Alberts. 1 day 5 days 60 days Figure 22–6 Lgr5-expressing stem cells and their progeny in the small intestine.


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Cell_Biology_Alberts. the method shown in Figure 22–5 was used here to mark single intestinal stem cells and trace the fates of their progeny. the Lgr5 gene encodes a member of the family of G-protein-linked transmembrane receptors, and it is expressed specifically in stem cells near the crypt base. because the Lgr5 promoter was used to drive expression of CreERT2, treatment with a low dose of tamoxifen resulted in occasional stem cells expressing LacZ. these cells and all of their progeny could subsequently be detected with a blue histochemical stain. all of the blue cells in these images derive from a single Lgr5expressing stem cell. after 60 days, the blue progeny of this cell are seen to extend all the way up a villus. these progeny 100 µm can be shown to include all types of differentiated cells, as well as persistent the two daughters of a Stem Cell do not always have to Lgr5-expressing cells at the crypt base. become different this proves that Lgr5-expressing cells are multipotent stem cells. (From


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Cell_Biology_Alberts. as persistent the two daughters of a Stem Cell do not always have to Lgr5-expressing cells at the crypt base. become different this proves that Lgr5-expressing cells are multipotent stem cells. (From n. barker et If the number of stem cells in a crypt is to remain stable, each stem-cell division al., Nature 449:1003–1007, 2007. with must on average generate one daughter that remains a stem cell and one that permission from macmillan publishers ltd.) becomes committed to differentiation. In principle, this could be achieved in at least two ways (Figure 22–7).


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Cell_Biology_Alberts. One mechanism—the simplest at first sight—would be through asymmetric division: processes internal to the dividing stem cell could distribute regulatory factors asymmetrically to its two daughters, as occurs in Drosophila neuroblast divisions (see Figure 21–36). The factors inherited by one daughter would cause it to remain a stem cell, while those inherited by the other would drive it toward differentiation. This strategy would guarantee that the original stem cell would give rise to precisely one stem cell in every subsequent cell generation.


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Cell_Biology_Alberts. An alternative strategy would be based on a choice that each daughter makes independently of its sister: in normal circumstances, each would have a 50% probability of remaining as a stem cell and a 50% probability of commitment to differentiation. Sometimes the two daughters of a stem cell would thus have opposite fates, sometimes the same. The choice that each cell makes might either be stochastic, like the flip of a coin, or governed by the environment in which the cell finds itself. A strategy of independent choices is more flexible than that of strict asymmetric division. In particular, environmental factors can control the balance of probabilities, adjusting them in favor of the stem-cell option where more stem cells are needed, as they often are, either for growth or for damage repair.


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Cell_Biology_Alberts. Clonal analysis gives a way to distinguish between the two strategies, since they give quite different predictions as to the expected number of clones of different sizes produced from individual stem cells (see Figure 22–7). For the gut, the findings seem clear: the independent-choice theory fits the observations, and the asymmetric-division theory does not. paneth Cells Create the Stem-Cell niche There are about 15 Lgr5-expressing stem cells in each crypt. They are slim and columnar, and they sit at the crypt base interspersed among the Paneth cells (see Figure 22–6). This is the intestinal stem-cell niche: the Paneth cells generate signals, including a strong Wnt signal, that act over a short range to maintain the stem-cell state. Signal proteins from the connective tissue surrounding the crypt base help to reinforce the localizing signal from the Paneth cells; Lgr5 itself is a receptor for one of these proteins, called R-spondin.


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Cell_Biology_Alberts. In the intestine, it seems that the niche created by the Paneth cells has space for only a limited number of stem cells, and when these divide, it is a random POSSIBLE OUTCOMES after frst division cycle of stem cell after frst division cycle of stem cell terminally differentiated cell after second division cycle of stem cell after second division cycle of stem cell POSSIBLE OUTCOMES etc. etc. terminally differentiated cell environmental factors help determine cell fate or or or or or or or or choice determined by asymmetry in dividing stem cell choice determined stochastically and/or by environment localizeddeterminant


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Cell_Biology_Alberts. Figure 22–7 Two ways for a stem cell to produce daughters with different fates: asymmetric division and independent choice. (a) the asymmetric-division strategy gives a clone consisting of precisely one stem cell plus a steadily increasing number of differentiating cells, in proportion to the number of cell divisions. (b) the independent-choice strategy is more variable in its outcome. with a choice made at random by each daughter and with a 50% probability for each one to remain a stem cell or differentiate, there is, for example, a 25% chance at the first division that both daughters will differentiate, so that the clone eventually goes extinct. or, at this division or later, a preponderance of daughters may chance to retain stem-cell character, creating a clone that persists and increases in size. with the help of some mathematics, the probability distribution of clone sizes generated from a single stem cell at any given time can be predicted on this stochastic assumption. the


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Cell_Biology_Alberts. in size. with the help of some mathematics, the probability distribution of clone sizes generated from a single stem cell at any given time can be predicted on this stochastic assumption. the observations in the gut and elsewhere fit the stochastic independent-choice strategy, but not the asymmetric-division strategy.


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Cell_Biology_Alberts. matter which of them are pushed out of the nest and condemned to differentiation and which stay in place as stem cells for the future. In most other stem-cell systems where the question has been examined, it appears that the fates of the daughters of a stem cell are assigned in a similar way, independently and subject to influence from the cells’ environment.


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Cell_Biology_Alberts. The Paneth cells themselves are progeny of the stem cells, suggesting that the intestinal stem-cell system is in some way self-maintaining and self-organizing. This is demonstrated in a striking way by taking single dissociated Lgr5-expressing cells and allowing them to proliferate in culture, embedded in a cell-free matrix rich in the basal-lamina component laminin (mimicking basal lamina). The cells proliferate, forming at first small, round epithelial vesicles. Within a few days, however, one or another of the cells in the vesicle, at random, begins to differentiate as a Paneth cell. This induces its neighbors to behave as stem cells and initiates transformation of the simple vesicle into an organized structure, or organoid (Figure 22–8A,B). Protrusions resembling crypts grow out into the surrounding matrix and contain Paneth cells, Lgr5-expressing stem cells, and the transit amplifying cells derived from them; these cell types are confined to the cryptlike structures. Terminally


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Cell_Biology_Alberts. the surrounding matrix and contain Paneth cells, Lgr5-expressing stem cells, and the transit amplifying cells derived from them; these cell types are confined to the cryptlike structures. Terminally differentiated, nondividing absorptive cells line the other parts of the organoid epithelium, with their microvilli facing the lumen. Goblet and enteroendocrine cells are also present, scattered through the epithelium, and the whole “minigut” structure, with all its cell types, grows and renews itself in much the same way as the lining of the normal intestine.


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Cell_Biology_Alberts. Figure 22–8 Genesis of a minigut from a single Lgr5-expressing cell cultured in a cell-free matrix. (a,b) the founder cell first divides to form a small vesicle. at random, one or more of the cells in this vesicle differentiates as a paneth cell (blue). this cell maintains Lgr5 expression (yellow) in its immediate neighbors, which persist as stem cells that generate the full range of intestinal cell types. (C ) Schematic diagram of the key organizing signals. the paneth cells organize crypts by producing a wnt signal that acts on neighboring cells and keeps them proliferating in the stem-cell state. a repulsive interaction based on ephrin–eph binding causes the crypt cell types (which express ephb, induced by wnt) to segregate from the nondividing differentiated villus cell types (which express ephrinb). both ephrin and eph are cell-surface proteins attached to the plasma membrane; in many tissues, two cells that contain a different member of this pair repel each other when they touch


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Cell_Biology_Alberts. ephrinb). both ephrin and eph are cell-surface proteins attached to the plasma membrane; in many tissues, two cells that contain a different member of this pair repel each other when they touch (see Figure 21–49). (adapted from t. Sato and h. Clevers, Science 340:1190– 1194, 2013. with permission from aaaS.) ephrin–eph Signaling drives Segregation of the different Gut Cell types


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Cell_Biology_Alberts. The remarkable self-organizing behavior of the cultured organoids suggests that some interaction among the different epithelial cells drives them to segregate from one another. The ephrin–Eph signaling pathway (discussed in Chapter 15) appears to be responsible. The cells that live in the crypts express EphB receptor proteins, while absorptive, goblet, and enteroendocrine cells, as they begin to differentiate, switch off expression of this receptor and instead switch on expression of its ligands, cell-surface proteins of the EphrinB family (Figure 22–8C). In various other tissues, cells expressing Eph proteins are repelled by contacts with cells expressing ephrins on their surface (see Figures 21–49 and 21–79). It seems that the same is true in the gut lining, and that this mechanism serves to keep the cells segregated and in their proper places. In EphB knockout mutants, the populations become mixed, so that, for example, Paneth cells wander out onto the villi.


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Cell_Biology_Alberts. notch Signaling Controls Gut Cell diversification and helps maintain the Stem-Cell State If a single type of stem cell generates all the differentiated cell types in the gut lining, what causes the progeny of this stem cell to diversify? Notch signaling has this role in many other systems, where it mediates lateral inhibition—a competitive interaction that drives neighboring cells toward different fates (see Figure 15–58 and Figure 21–35). All the essential components of the Notch pathway are expressed in the crypts; it seems that Wnt signaling maintains them there. If Notch signaling is abruptly blocked, within a few days all the cells in the crypts differentiate as goblet cells, and absorptive cells cease to be produced; conversely, if Notch signaling is artificially activated in all the cells, absorptive cells continue


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Cell_Biology_Alberts. IN CRYPT, to be generated but no goblet cells are produced. This reflects the lateral inhibition mechanism operating in normal animals: the nascent goblet (and other secretory) cells express the Notch ligand Delta and thereby activate Notch in their neighbors, inhibiting them from differentiating as secretory (Figure 22–9). Delta-Notch signaling is crucial not only in the transit amplifying population, but also at the crypt base: the Paneth cells express Delta and this activates Notch in the stem cells, inhibiting differentiation. Without this influence, the stem cells lose their special character and differentiate as secretory cells. Thus maintenance of the intestinal stem-cell state requires a combination of signals, with both Wnt and Notch acting as central players. the epidermal Stem-Cell System maintains a Self-Renewing waterproof barrier


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Cell_Biology_Alberts. Stem-cell systems are organized in many different ways, but they share some underlying principles. Consider the epidermis, for example—the outer, epithelial covering of the body. The epidermis undergoes continual renewal, but, unlike the lining of the gut, it is multilayered or stratified. Stem cells are located in the basal layer, and their progeny move outward toward the exposed surface, differentiating as they go. They end up as lifeless scales or squames, which are eventually shed from the surface of the skin (Figure 22–10). Even though the architecture of this tissue is very different from that of the intestine, many of the same basic principles apply. The stem cells depend for their existence on signals from a specific niche, in this case the basal lamina and underlying connective tissue. The daughters of stem cells that are committed to differentiation undergo several divisions as transit amplifying cells (while still in the basal layer) before differentiating. Finally, a


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Cell_Biology_Alberts. tissue. The daughters of stem cells that are committed to differentiation undergo several divisions as transit amplifying cells (while still in the basal layer) before differentiating. Finally, a stochastic independent-choice mechanism dictates the fates of the daughters of a stem-cell division, allowing for increase in the number of stem cells when needed for growth or wound healing. Most of the same signaling pathways that organize the intestinal stem-cell system are also involved in regulating the epidermal stem-cell system, although with different individual roles.


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Cell_Biology_Alberts. Figure 22–9 How Notch signaling, in combination with Wnt, maintains stem cells and drives cell diversification in the intestine. wnt signaling leads to expression of notch and delta in the cells of the crypt, and delta-notch signaling in the crypt mediates lateral inhibition between adjacent cells. Cells expressing higher levels of delta eventually activate notch in their neighbors, adopt a secretory fate, and stop dividing; their neighbors, with activated notch, are prevented from differentiating and keep on dividing. essentially the same process operates at the crypt base, where the paneth cells express higher levels of delta to prevent stem cells from differentiating, and in the transit amplifying population, where nascent secretory cells express higher levels of delta. division continues in the notch-activated cells as they move up the crypt, until they escape from the influence of wnt and emerge onto the villi to become absorptive cells.


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Cell_Biology_Alberts. squame about to fake off from surface keratin flaments connective tissue of basal cell basal cell dividing


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Cell_Biology_Alberts. Figure 22–10 The multilayered structure of the epidermis, as seen in thin skin of a mouse. (a) the epidermis forms the outer covering of the skin, creating a waterproof barrier that is self-repairing and continually renewed. beneath this lies a relatively thick layer of connective tissue, which includes the tough, collagen-rich dermis (from which leather is made) and the underlying fatty subcutaneous layer or hypodermis. the cells of the epidermis are called keratinocytes, because their characteristic differentiated activity is the synthesis of keratin intermediate filament proteins, which give the epidermis its toughness. these cells change their appearance and properties from one layer to the next, progressing through a regular program of differentiation. those in the innermost layer, attached to an underlying basal lamina, are termed basal cells, and it is usually only these that divide: the basal cell population includes relatively small numbers of stem cells along with larger


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Cell_Biology_Alberts. attached to an underlying basal lamina, are termed basal cells, and it is usually only these that divide: the basal cell population includes relatively small numbers of stem cells along with larger numbers of transit amplifying cells derived from them. above the basal cells are several layers of larger prickle cells, shown in top view in (b), whose numerous desmosomes—each a site of anchorage for thick tufts of keratin filaments—are just visible in the light microscope as tiny prickles around the cell surface. beyond the prickle cells lies the thin, darkly staining granular cell layer, where the cells are sealed together to form a waterproof barrier; this marks the boundary between the inner, metabolically active strata and the outermost layer of the epidermis, consisting of dead cells whose intracellular organelles have disappeared. these outermost cells are reduced to flattened scales, or squames, filled with densely packed keratin, which are eventually shed from the surface of the


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Cell_Biology_Alberts. intracellular organelles have disappeared. these outermost cells are reduced to flattened scales, or squames, filled with densely packed keratin, which are eventually shed from the surface of the skin. the time from exit of a cell from the basal layer to its loss by shedding at the surface is a week or two, depending on body region and species.


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Cell_Biology_Alberts. in addition to the cells destined for keratinization, the deep layers of the epidermis include small numbers of cells (not shown) that invade this tissue and have quite different origins and functions. these immigrants include dendritic cells, called langerhans cells, derived from bone marrow and belonging to the immune system; melanocytes (pigment cells) derived from the neural crest; and merkel cells, which are associated with nerve endings in the epidermis. (b, from R.v. krsti´c, ultrastructure of the mammalian Cell: an atlas. berlin: Springer-verlag, 1979.) tissue Renewal that does not depend on Stem Cells: insulin-Secreting Cells in the pancreas and hepatocytes in the liver


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Cell_Biology_Alberts. Some types of cells can divide even though fully differentiated, allowing for renewal and regeneration without the use of stem cells. The insulin-secreting cells (β cells) of the pancreas are one example. Their mode of renewal has a special importance, because it is the loss of these cells (through autoimmune attack) that is responsible for type 1 (juvenile-onset) diabetes; they are also a significant factor in the type 2 (adult-onset) form of the disease. The β cells are normally sequestered in cell clusters called islets of Langerhans. These islets contain no obvious subset of cells specialized to act as stem cells, yet fresh β cells are continually generated within them. Lineage tracing studies, similar to those described above for the gut, show that the renewal of this population normally occurs by simple duplication of the existing insulin-expressing cells, and not by means of stem cells.


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Cell_Biology_Alberts. Another tissue that can renew by simple duplication of fully differentiated cells is the liver. The main cell type in the liver is the hepatocyte, a large cell that performs the liver’s metabolic functions. Hepatocytes normally live for a year or more and renew themselves through cell division at a very slow rate. Powerful homeostatic mechanisms operate to adjust the rate of cell proliferation or the rate of cell death, or both, so as to keep the organ at its normal size or restore it to that size in case of damage. A dramatic effect is seen if large numbers of hepatocytes are removed surgically or are killed by poisoning with carbon tetrachloride. Within a day or so after either sort of damage, a surge of cell division occurs among the surviving hepatocytes, quickly replacing the lost tissue. If two-thirds of a rat’s liver is removed, for example, a liver of nearly normal size can regenerate from the remainder by hepatocyte proliferation within about two weeks.


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Cell_Biology_Alberts. Both the pancreas and the liver contain small populations of stem cells that can be called into play as a backup mechanism for production of the differentiated cell types in more extreme circumstances. This imparts resilience to the mechanisms of renewal and repair. Some tissues lack Stem Cells and are not Renewable


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Cell_Biology_Alberts. Some tissues lack Stem Cells and are not Renewable The variety among tissues in the capacity for self-renewal is illustrated in a striking way by comparing the olfactory epithelium in the nose, the auditory epithelium of the inner ear, and the photoreceptive epithelium of the retina. These three sensory structures, which like the epidermis develop from the ectodermal layer of the early embryo, differ radically in their self-renewal capabilities. The olfactory epithelium contains a population of stem cells that give rise to differentiated cells that have a limited life-span and are continually replaced. But unlike the epidermis, these differentiated cells (the olfactory receptor cells) are neurons, with cell bodies lying in the olfactory epithelium and axons that extend back to the olfactory lobes in the brain. The continual renewal of this epithelium therefore involves continual production of fresh axons, which have to navigate back to the appropriate sites in the brain.


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Cell_Biology_Alberts. In contrast, in mammals at least, the auditory epithelium and the retinal epithelium lack stem cells, and their sensory receptor cells—the sensory hair cells in the ear, the photoreceptors in the retina—are irreplaceable. If they are destroyed— whether by too much exposure to loud noise, by looking into the beam of a laser, or through degenerative processes in old age—the loss is permanent.


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Cell_Biology_Alberts. Many tissues in the adult mammalian body are continually renewed by stem cells. Stem cells, by definition, are not terminally differentiated and have the ability to divide throughout the organism’s lifetime, yielding some progeny that differentiate and others that remain stem cells. The lining of the gut renews itself more rapidly than any other tissue in the mammalian body and provides a paradigm for the workings of stem-cell systems. In the small intestine, there is a continual upward flow from crypts, where new cells are generated by cell division, onto villi that are composed of nondividing differentiated cells. Wnt signaling maintains cell proliferation in the crypts, and overactivation of the Wnt pathway gives rise to tumors. Stem cells lie at each crypt base and are distinguished by expression of Lgr5 and certain other genes. The Lgr5+ stem cells are multipotent, each capable of generating several different types of differentiated cells as well as new stem cells. The balance of


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Cell_Biology_Alberts. expression of Lgr5 and certain other genes. The Lgr5+ stem cells are multipotent, each capable of generating several different types of differentiated cells as well as new stem cells. The balance of fate choices is adjusted according to need, allowing increase in the number of stem cells where more are needed for growth or repair. In a suitable cell-free culture medium, a single Lgr5+ stem cell can generate a self-organizing “minigut,” containing all the standard intestinal epithelial cell types.


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Cell_Biology_Alberts. Other self-renewing epithelia, such as the epidermis with its multilayered (stratified) architecture, have stem cells and their differentiating progeny arranged in different ways but are governed by similar basic principles. However, tissue renewal and repair does not always have to depend on stem cells. Thus, the population of insulin-producing cells in the pancreas is enlarged and renewed by simple duplication of existing insulin-producing cells. Similarly, in the liver, differentiated hepatocytes remain able to divide throughout life and can dramatically increase their division rate when the need arises. At an opposite extreme, some tissues, such as the sensory epithelia of the ear and the eye, do not undergo any turnover and are not renewable: their cells, once lost, are lost forever. FibRoblaStS and theiR tRanSFoRmationS: the ConneCtive-tiSSue Cell Family