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Cell_Biology_Alberts. The cell proliferation that follows the initial specification accounts for the size increase, while the refinement of the initial pattern is explained by a series of local inductions plus other interactions that add successive levels of detail on an initially simple sketch. For example, as soon as two types of cells are present in a developing tissue, one of them can produce a signal that induces a subset of the neighboring cells to specialize in a third way. The third cell type can in turn signal 1. asymmetric division: sister cells born different uniform feld of cells long-range inhibitor (red) blocks production of activator by other cells in the neighborhood specialized clusters of cells 2. symmetric division: sister cells become different as a result of Figure 21–12 Two ways of making sister cells different.


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Cell_Biology_Alberts. Figure 21–12 Two ways of making sister cells different. D and E are induced by signal C is induced on A and B, B acting on A back to the other two cell types nearby, generating a fourth and a fifth cell type, and so on (Figure 21–13). This strategy for generating a progressively more complicated pattern is called sequential induction. It is chiefly through sequential inductions that the body plan of a developing animal, after being first roughed out in miniature, becomes elaborated with finer and finer details as development proceeds.


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Cell_Biology_Alberts. The rapid progress in understanding animal development has been one of the great success stories in biology over the last few decades, and it has important practical implications. Some 2 to 5% of all human babies are born with anatomical abnormalities, such as heart malformations, truncated limbs, cleft palate, or spina bifida. Advances in developmental biology help us understand how these defects arise, even if we cannot yet prevent or cure most of them. Less obvious, but even more important from a practical point of view, is that developmental biology provides insights into the workings of cells and tissues in the adult body. Developmental processes do not halt at birth; they continue throughout life, as tissues are maintained and repaired. The fundamental mechanisms of cell growth and division, cell–cell signaling, cell memory, cell adhesion, and cell movement are involved in adult tissue maintenance and repair—just as they are in embryo development.


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Cell_Biology_Alberts. Embryos are simpler than adults, and they allow us to analyze such basic processes more easily. Studies of the early Drosophila embryo, for example, were crucial to the discovery of several conserved signaling pathways, including the Wnt, Hedgehog, and Notch pathways. They also provided the key to understanding the central role of these pathways in the maintenance of normal adult human tissues and in diseases such as cancer. In Chapter 22, we shall consider how other developmental mechanisms operate in the normal adult body, especially in tissues that are continually renewed by means of stem cells—including the gut, skin, and the hematopoietic system. But now, we must look more closely at the way in which an early embryo generates its spatial pattern of specialized cells, beginning with the transformations that create the adult body plan.


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Cell_Biology_Alberts. Animal development is a self-assembly process, in which the cells of the embryo become different from one another and organize themselves into increasingly complex structures. The process begins with a single large cell—the fertilized egg. This cell cleaves to form many smaller cells, producing a blastula. The blastula undergoes gastrulation to generate the three germ layers of the embryo—ectoderm, mesoderm, and endoderm—consisting of cells determined for different fates. As development continues, the cells become more and more narrowly specialized according to their locations and their interactions with one another. Through cell memory, these cell–cell interactions, even though transient, can have lasting effects on each Figure 21–13 Patterning by sequential induction. a series of inductive interactions can generate many types of cells, starting from only a few.


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Cell_Biology_Alberts. Figure 21–13 Patterning by sequential induction. a series of inductive interactions can generate many types of cells, starting from only a few. cell’s internal state. Thus, a succession of simple cues that a cell receives at different times can direct it along a complex developmental pathway. At each step, the cell becomes further restricted in the range of final states open to it. The process reaches its limit when the cell differentiates to form one of the specialized cell types of the adult body.


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Cell_Biology_Alberts. Differences between developing cells arise in various ways and have to be properly coordinated in space. In one common strategy, initially similar cells within a group become different by exposure to different levels of an inductive signal or morphogen emanating from a source outside the group. Neighboring cells can also become different by lateral inhibition, in which a cell signals to its neighbors not to follow the same fate. These cell–cell interactions are mediated by a small number of highly conserved signaling pathways, which are used repeatedly in different organisms and at different times during development. Not all cell diversification arises by cell–cell interactions, however: daughter cells can be born different as a result of asymmetric cell division.


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Cell_Biology_Alberts. Regulators of transcription and chromatin structure bind to regulatory DNA and determine the fate of each cell. Differences of body plan seem to arise to a large extent from differences in the regulatory DNA associated with each gene. This DNA has a central role in defining the sequential program of development, calling genes into action at specific times and places according to the pattern of gene expression that was present in each cell at the previous developmental stage. Development has been most thoroughly studied in a handful of model organisms. But most of the genes and mechanisms thereby identified are used in all animals and repeatedly at different stages of development. Thus, insights from worms, flies, fish, frogs, and mice deeply inform our understanding of embryology, birth defects, and adult tissue maintenance in humans.


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Cell_Biology_Alberts. A developing multicellular organism has to create a pattern in fields of cells where there was little or none before. Some of the early microscopists imagined the entire shape and structure of the human body to be already present in the sperm as a “homunculus,” a miniature human; after fertilization, the homunculus would simply grow and generate a full-sized human. We now know that this view is incorrect and that development is a progression from simple to complex, through a gradual refinement of an animal’s anatomy. To see how the whole sequence of events of spatial patterning and cell determination is set in train, we must return to the egg and the early embryo. Different animals Use Different Mechanisms to establish their primary axes of polarization


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Cell_Biology_Alberts. Different animals Use Different Mechanisms to establish their primary axes of polarization Surprisingly, the earliest steps of animal development are among the most variable, even within a phylum. A frog, a chicken, and a mammal, for example, even though they develop in similar ways later, make eggs that differ radically in size and structure, and they begin their development with different sequences of cell divisions and cell specializations. Gastrulation occurs in all animal embryos, but the details of its timing, of the associated pattern of cell movements, and of the shape and size of the embryo as gastrulation proceeds are highly variable. Likewise, there is great variation in the time and manner in which the primary axes of the body become marked out. However, this polarization of the embryo usually becomes discernible very early, before gastrulation begins: it is the first step of spatial patterning.


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Cell_Biology_Alberts. Three axes generally have to be established. The animal-vegetal (A-V) axis, in most species, defines which parts are to become internal (through the movements of gastrulation) and which are to remain external. (The bizarre name dates from a century ago and has nothing to do with vegetables.) The anteroposterior (A-P) axis specifies the locations of future head and tail. The dorsoventral (D-V) axis specifies the future back and belly. Figure 21–14 The frog egg and its asymmetries. (a) Side view of a Xenopus egg photographed just before fertilization.


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Cell_Biology_Alberts. Figure 21–14 The frog egg and its asymmetries. (a) Side view of a Xenopus egg photographed just before fertilization. (B) the asymmetric distribution of molecules inside the egg, and how this changes following fertilization so as to define a dorsoventral as well as an animal-vegetal asymmetry. Fertilization, through a reorganization of the microtubule cytoskeleton, triggers a rotation of the egg cortex (a layer a few μm deep) through about 30° relative to the core of the egg; the direction of rotation determined by the site of sperm entry. Some components are carried still further to the future dorsal side by active transport along microtubules. the resulting dorsal concentration of Wnt11 mrNa leads to dorsal production of the Wnt11 signal protein and defines the dorsoventral polarity of the future embryo. Vegetally localized Vegt defines the vegetal source of signals that will induce endoderm and mesoderm. (a, courtesy of tony Mills.)


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Cell_Biology_Alberts. At one extreme, the egg is spherically symmetrical, and the axes only become defined during embryogenesis. The mouse comes close to being an example, with little obvious sign of polarity in the egg. Correspondingly, the blastomeres produced by the first few cell divisions seem to be all alike and are remarkably adaptable. If the early mouse embryo is split in two, a pair of identical twins can be produced—two complete, normal individuals from a single cell. Similarly, if one of the cells in a two-cell mouse embryo is destroyed by pricking it with a needle and the resulting “half-embryo” is placed in the uterus of a foster mother to develop, in many cases a perfectly normal mouse will emerge.


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Cell_Biology_Alberts. At the opposite extreme, the structure of the egg defines the future axes of the body. This is the case for most species, including insects such as Drosophila, as we shall see shortly. Many other organisms lie between the two extremes. The egg of the frog Xenopus, for example, has a clearly defined A-V axis even before fertilization: the nucleus near the top defines the animal pole, while the mass of yolk (the embryo’s food supply, destined to be incorporated in the gut) toward the bottom defines the vegetal pole. Several types of mRNA molecules are already localized in the vegetal cytoplasm of the egg, where they produce their protein products. After fertilization, these mRNAs and proteins act in and on the cells in the lower and middle part of the embryo, giving the cells there specialized characters, both by direct effects and by stimulating the production of secreted signal proteins. For example, mRNA encoding the transcription regulator VegT is deposited at the vegetal pole


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Cell_Biology_Alberts. characters, both by direct effects and by stimulating the production of secreted signal proteins. For example, mRNA encoding the transcription regulator VegT is deposited at the vegetal pole during oogenesis. After fertilization, this mRNA is translated, and the resulting VegT protein activates a set of genes that code for signal proteins that induce mesoderm and endoderm, as discussed later.


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Cell_Biology_Alberts. The D-V axis of the Xenopus embryo, by contrast, is defined through the act of fertilization. Following entry of the sperm, the outer cortex of the egg cytoplasm rotates relative to the central core of the egg, so that the animal pole of the cortex becomes slightly shifted to one side (Figure 21–14). Treatments that block the rotation allow cleavage to occur normally but produce an embryo with a central gut and no dorsal structures or D-V asymmetry. Thus, this cortical rotation is required to define the D-V axis of the future body by creating the D-V axis of the egg.


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Cell_Biology_Alberts. The site of sperm entry that biases the direction of the cortical rotation in Xenopus, perhaps through the centrosome that the sperm brings into the egg— inasmuch as the rotation is associated with a reorganization of the microtubules nucleated from the centrosome in the egg cytoplasm. The reorganization leads to a microtubule-based transport of several cytoplasmic components, including the mRNA coding for Wnt11, a member of the Wnt family of signal proteins, moving cytoplasm of animal pole VENTRAL DORSAL it toward the future dorsal side (see Figure 21–14). This mRNA is soon translated and the Wnt11 protein secreted from cells that form in that region of the embryo activates the Wnt signaling pathway (see Figure 15–60). This activation is crucial for triggering the cascade of subsequent events that will organize the dorsoventral axis of the body. (The A-P axis of the embryo will only become clear later, in the process of gastrulation.)


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Cell_Biology_Alberts. Although different animal species use a variety of different mechanisms to specify their axes, the outcome has been relatively well conserved in evolution: head is distinguished from tail, back from belly, and gut from skin. It seems that it does not much matter what tricks the embryo uses to break the initial symmetry and set up this basic body plan. Studies in Drosophila have revealed the Genetic control Mechanisms Underlying Development


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Cell_Biology_Alberts. It is the fly Drosophila, more than any other organism, that has provided the key to our present understanding of how genes govern development. Decades of genetic study culminated in a large-scale genetic screen, focusing especially on the early embryo and searching for mutations that disrupt its pattern. This revealed that the key developmental genes fall into a relatively small set of functional classes defined by their mutant phenotypes. The discovery of these genes and the subsequent analysis of their functions was a famous tour de force and had a revolutionary impact on all of developmental biology, earning its discoverers a Nobel Prize. Some parts of the developmental machinery revealed in this way are conserved between flies and vertebrates, some parts not. But the logic of the experimental approach and the general strategies of genetic control that it revealed have transformed our understanding of multicellular development in general.


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Cell_Biology_Alberts. To understand how the early developmental machinery operates in Drosophila, it is important to note a peculiarity of fly development. Like the eggs of other insects, but unlike most vertebrates, the Drosophila egg—shaped like a cucumber—begins its development with an extraordinarily rapid series of nuclear divisions without cell division, producing multiple nuclei in a common cytoplasm—a syncytium. The nuclei then migrate to the cell cortex, forming a structure called the syncytial blastoderm. After about 6000 nuclei have been produced, the plasma membrane folds inward between them and partitions them into separate cells, converting the syncytial blastoderm into the cellular blastoderm (Figure 21–15).


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Cell_Biology_Alberts. We shall see that the initial patterning of the Drosophila embryo depends on signals that diffuse through the cytoplasm at the syncytial stage and exert their actions on genes in the rapidly dividing nuclei, before the partitioning of the egg into separate cells. Here, there is no need for the usual forms of cell–cell signaling; neighboring regions of the syncytial blastoderm can communicate by means of transcription regulatory proteins that move through the cytoplasm of the giant multinuclear cell. egg-polarity Genes encode Macromolecules Deposited in the egg to Organize the axes of the early Drosophila embryo As in most insects, the main axes of the future body of Drosophila are defined before fertilization by a complex exchange of signals between the developing egg, Figure 21–15 Development of the Drosophila egg from fertilization to the cellular blastoderm stage.


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Cell_Biology_Alberts. Figure 21–15 Development of the Drosophila egg from fertilization to the cellular blastoderm stage. or oocyte, and the follicle cells that surround it in the ovary. In the stages before fertilization, the anteroposterior and dorsoventral axes of the future embryo become defined by four systems of egg-polarity genes that create landmarks— either mRNA or protein—in the developing oocyte. Following fertilization, each landmark serves as a beacon, providing a signal that organizes the developmental process in its neighborhood.


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Cell_Biology_Alberts. The nature of the genes emerged from studies of mutants in which the patterning of the embryo was altered. One class of mutations gave embryos with disrupted polarity—for example, tail-end structures at both ends of the body, with no head-end structures. This class of mutations identified the set of egg-polarity genes. The egg-polarity gene responsible for the signal that organizes the anterior end of the embryo is called Bicoid. A deposit of Bicoid mRNA molecules is localized, before fertilization, at the anterior end of the egg. Upon fertilization, the mRNA is translated to produce Bicoid protein. This protein is an intracellular morphogen and transcription regulator that diffuses away from its source to form a concentration gradient within the syncytial cytoplasm, with its maximum at the head end of the embryo (Figure 21–16). The different concentrations of Bicoid along the A-P axis help determine different cell fates by regulating the transcrip tion of genes in the nuclei of the


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Cell_Biology_Alberts. the head end of the embryo (Figure 21–16). The different concentrations of Bicoid along the A-P axis help determine different cell fates by regulating the transcrip tion of genes in the nuclei of the syncytial blastoderm (discussed in Chapter 7).


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Cell_Biology_Alberts. Of the three other egg-polarity gene systems, two contribute to patterning the syncytial nuclei along the A-P axis and one to patterning them along the D-V axis. Together with the Bicoid group of genes, and acting in a broadly similar way, their gene products mark out three fundamental partitions of body regions—head versus rear, dorsal versus ventral, and endoderm versus mesoderm and ectoderm— as well as a fourth partition, no less fundamental to the body plan of animals: the distinction between germ cells and somatic cells (Figure 21–17).


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Cell_Biology_Alberts. The egg-polarity genes have a further special feature: they are all maternal-effect genes, in that it is the mother’s genome rather than the zygote’s genome that is critical. For example, a fly whose chromosomes are mutant in both copies of the Bicoid gene but who is born from a mother carrying one normal copy of Bicoid develops perfectly normally, without any defects in the head pattern. However, if that offspring is a female, she cannot deposit any functional Bicoid mRNA into her own eggs, which will therefore develop into headless embryos, regardless of the father’s genotype. The egg-polarity genes act first in a hierarchy of gene systems that define a progressively more detailed pattern of body parts. In the next few pages, we begin with the molecular mechanisms that pattern the developing Drosophila embryo and larva along the A-P axis, before considering the patterning along the D-V axis.


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Cell_Biology_Alberts. Figure 21–16 The Bicoid protein gradient. (a) Bicoid mrNa is deposited at the anterior pole during oogenesis. Local translation followed by diffusion generates the Bicoid protein gradient. absence of the Bicoid protein gradient in embryos from Bicoid homozygous mutant mothers. (a and B, courtesy of Stephen Small.)


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Cell_Biology_Alberts. absence of the Bicoid protein gradient in embryos from Bicoid homozygous mutant mothers. (a and B, courtesy of Stephen Small.) Figure 21–17 The organization of the four egg-polarity gradient systems in Drosophila. Nanos is a translational repressor that governs the formation of the abdomen. Localized Nanos mrNa is also incorporated into the germ cells as they form at the posterior of the embryo, and Nanos protein is necessary for germ-line development. Bicoid protein is a transcriptional activator that determines the head and thoracic regions. toll and torso are receptor proteins that are distributed all over the membrane but are activated only at the sites indicated by the coloring, through localized exposure to the extracellular ligands Spaetzle (the ligand for toll) and trunk (the ligand for torso). toll activity determines the mesoderm and torso activity determines the formation of terminal structures.


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Cell_Biology_Alberts. Figure 21–18 The origins of the Drosophila body segments. (a) at 3 hours, the embryo (shown in side view) is at the blastoderm stage and no segmentation is visible, although a fate map can be drawn showing the future segmented regions (color). (B) at 10 hours, all the segments are clearly defined (t1: first thoracic segment; a1: first abdominal segment). See Movie 21.3. (c) the segments of the Drosophila larva and their correspondence with regions in the embryo. (D) the segments of the Drosophila adult and their correspondence with regions in the embryo. three Groups of Genes control Drosophila Segmentation along the a-p axis


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Cell_Biology_Alberts. The body of an insect is divided along its A-P axis into a series of segments. The segments are repetitions of a theme with variations: each segment forms highly specialized structures, but all built according to a similar fundamental plan (Figure 21–18). The gradients of transcription regulators set up along the A-P axis in the early embryo by the egg-polarity genes are the prelude to creation of the segments. These regulators initiate the orderly transcription of segmentation genes, which refine the pattern of gene expression to define the boundaries and ground plan of the individual segments. Segmentation genes are expressed by subsets of cells in the embryo, and their products are the first components that the embryo’s own genome contributes to embryonic development; they are therefore called zygotic-effect genes, to distinguish them from the earlier-acting maternal-effect genes. Mutations in segmentation genes can alter either the number of segments or their basic internal


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Cell_Biology_Alberts. called zygotic-effect genes, to distinguish them from the earlier-acting maternal-effect genes. Mutations in segmentation genes can alter either the number of segments or their basic internal organization.


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Cell_Biology_Alberts. The segmentation genes fall into three groups according to their mutant phenotypes (Figure 21–19). It is convenient to think of the three groups as acting in sequence, although in reality their functions overlap in time. First to be expressed is a set of at least six gap genes, whose products mark out coarse A-P subdivisions of the embryo. Mutations in a gap gene eliminate one or more groups of adjacent segments: in the mutant Krüppel, for example, the larva lacks eight segments. Next comes a set of eight pair-rule genes. Mutations in these genes cause a series of deletions affecting alternate segments, leaving the embryo with only half as many segments as usual; although all the mutants display this two-segment periodicity, they differ in the precise pattern. Finally, there are at least 10 segment-polarity genes, in which mutations produce a normal number of segments but with a part of each segment deleted and replaced by a mirror-image duplicate of all or part of the rest of the


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Cell_Biology_Alberts. 10 segment-polarity genes, in which mutations produce a normal number of segments but with a part of each segment deleted and replaced by a mirror-image duplicate of all or part of the rest of the segment.


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Cell_Biology_Alberts. In parallel with the segmentation process, a further set of genes—the homeotic selector, or Hox, genes—serves to define and preserve the differences between one segment and the next, as we describe shortly. The phenotypes of the various segmentation mutants suggest that the segmentation genes form a coordinated system that subdivides the embryo progressively into smaller and smaller domains along the A-P axis, each distinguished by a different pattern of gene expression. Molecular genetics has helped to reveal how this system works. a hierarchy of Gene regulatory Interactions Subdivides the Drosophila embryo


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Cell_Biology_Alberts. a hierarchy of Gene regulatory Interactions Subdivides the Drosophila embryo Like Bicoid, most of the segmentation genes encode transcription regulator proteins. Their control by the egg-polarity genes and their actions on one another and on still other genes can be deciphered by comparing gene expression in normal and mutant embryos. By using appropriate probes to detect RNA transcripts or their protein products, one can observe genes switch on and off in changing patterns. By comparing these patterns in different mutants, one can begin to discern the logic of the entire gene control system. The products of the egg-polarity genes provide the global positional signals in the early embryo (see Figure 21–17). The Bicoid protein, as we have seen, acts as 0.2 mm


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Cell_Biology_Alberts. Figure 21–19 Examples of the phenotypes of mutations affecting egg-polarity genes and the three types of segmentation genes. In each case, the areas shaded in green on the normal larva (left) are deleted in the mutant or are replaced by mirror-image duplicates of the unaffected regions. (Modified from c. Nüsslein-Volhard and e. Wieschaus, Nature 287:795–801, 1980. With permission from Macmillan publishers Ltd.) a morphogen and activates different sets of genes at different positions along the A-P axis: some gap genes are only activated in regions with high levels of Bicoid, others only where levels of Bicoid are lower. After the gap gene products refine their positions by mutual repression, they provide a second tier of positional signals that act more locally to regulate finer details of patterning. Gap genes act by controlling the expression of yet other genes, including the pair-rule genes. The pair-rule genes, in turn, collaborate with one another and with the gap genes to set up


Cell_Biology_Alberts

Cell_Biology_Alberts. patterning. Gap genes act by controlling the expression of yet other genes, including the pair-rule genes. The pair-rule genes, in turn, collaborate with one another and with the gap genes to set up a regular, periodic pattern of expression of the segment-polarity genes, which collaborate with one another to define the internal pattern of each individual segment (Figure 21–20).


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Cell_Biology_Alberts. The initial steps in creation of the segmental pattern occur before cellularization of the syncytial blastoderm and are governed by the combinatorial effects of transcription regulators, as discussed in detail in Chapter 7 for the regulation of the expression of the pair-rule gene Even-skipped (see pp. 394–396). After cellularization, the segment-polarity genes further subdivide each segment into smaller domains. A large subset of the segment-polarity genes codes for components of two signaling pathways—the Wnt pathway and the Hedgehog pathway, including the secreted signal proteins Wingless (the first-named member of the Wnt family) and Hedgehog. (The Hedgehog pathway was first discovered through study of Drosophila segmentation, and it takes its name from the prickly appearance of the surface of the Hedgehog mutant embryo.) Wingless and Hedgehog are synthesized in different bands of cells that serve as signaling centers within each segment. The two proteins mutually maintain each


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Cell_Biology_Alberts. surface of the Hedgehog mutant embryo.) Wingless and Hedgehog are synthesized in different bands of cells that serve as signaling centers within each segment. The two proteins mutually maintain each other’s expression, while regulating the expression of genes such as Engrailed in neighboring cells (Figure 21–21). In such a manner, a series of sequential inductions creates a fine-grained pattern of gene expression within each segment.


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Cell_Biology_Alberts. egg-polarity, Gap, and pair-rule Genes create a transient pattern that Is remembered by Segment-polarity and Hox Genes


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Cell_Biology_Alberts. The gap genes and pair-rule genes are activated within the first few hours after fertilization. Their mRNA products initially appear in patterns that only approximate the final picture; then, within a short time, this fuzzy initial pattern resolves itself into a regular, crisply defined system of stripes. But this pattern itself is unstable and transient: as the embryo proceeds through gastrulation and beyond, the pattern disintegrates. The genes’ actions, however, have passed on an enduring memory of their patterns of expression by inducing the expression of certain segment polarity genes along with Hox genes (discussed shortly). After a period of pattern refinement mediated by cell–cell interactions, the expression patterns of these new groups of patterning genes is stabilized to provide positional labels that serve to maintain the segmental organization of the larva and adult fly.


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Cell_Biology_Alberts. The segment-polarity gene Engrailed provides a good example. Its RNA transcripts form a series of 14 bands in the cellular blastoderm, each approximately one-cell wide. These stripes lie immediately anterior to similar stripes of expression of another segment polarity gene, Wingless. As the cells in the developing embryo continue to grow, divide, and move, a mutually reinforcing signal between the Wingless expressing cells and the Engrailed expressing cells maintains narrow stripes of their expression (see Figure 21–21). After three cell cycles, newly expressed regulators stabilize an Engrailed expression pattern that will last Figure 21–20 An example of the regulatory hierarchy of egg-polarity, segmentation, and Hox genes. as discussed in the text, there are three groups of segmentation genes. the photographs show mrNa expression patterns of representative examples of genes of each type. (courtesy of Stephen Small.)


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Cell_Biology_Alberts. Figure 21–21 Mutual maintenance of Hedgehog and Wingless expression. engrailed is a transcription regulator (blue) that drives the expression of Hedgehog. Hedgehog encodes a secreted protein (red) that activates its signaling pathway in neighboring cells and thereby drives them to express the Wingless gene. In turn, Wingless encodes a secreted protein (green) that acts back on neighbors of the Wingless-expressing cell to maintain their expression of Engrailed and Hedgehog. as indicated, the same control loop repeats along the a-p axis of the fly. (Based on S. Dinardo et al., Curr. Opin. Genet. Dev. 4:529–534, 1994.) throughout the life of the fly, long after the signals that induced and refined it have disappeared. The segment borders will form at the posterior edge of each such Engrailed stripe (Figure 21–22).


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Cell_Biology_Alberts. In addition to regulating the segment-polarity genes, the products of pair-rule genes collaborate with those of gap genes to induce the precisely localized activation of a further set of genes—originally called homeotic selector genes and now often called Hox genes, for reasons that will become clear shortly. It is the Hox genes that permanently distinguish one segment from another. In the next section, we examine these important genes in detail and consider their role in cell memory; we shall see that this role is critical in a wide range of animals, including ourselves. Hox Genes permanently pattern the a-p axis


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Cell_Biology_Alberts. Hox Genes permanently pattern the a-p axis As animal development proceeds, the body becomes more and more complex. But again and again, in every species and at every level of organization, we find that complex structures are made by repeating a few basic themes, with variations. Thus, a limited number of basic differentiated cell types, such as muscle cells or fibroblasts, recur with subtle individual variations in different sites. These cell types are organized into a limited variety of tissue types, such as muscle or tendon, which again are repeated with subtle variations in different regions of the body. From the various tissues, organs such as teeth or digits are built— molars and incisors, fingers and thumbs and toes—a few basic kinds of structure, repeated with variations.


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Cell_Biology_Alberts. Wherever we find this phenomenon of modulated repetition, we can break down the developmental biologist’s problem into two kinds of questions: what is the basic construction mechanism common to all the objects of the given class, and how is this mechanism modified to give the observed variations in different animals? The segments of the insect body provide a good example. We have thus far sketched the way in which the rudiment of a single body segment is constructed and how cells within each segment become different from one another. We now consider how one segment becomes determined, or specified, to be different from another.


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Cell_Biology_Alberts. The first glimpse of the answer to this problem came over 80 years ago, with the discovery of a set of mutations in Drosophila that cause bizarre disturbances in the organization of the adult fly. In the Antennapedia mutant, for example, legs sprout from the head in place of antennae, whereas in the Bithorax mutant, portions of an extra pair of wings appear where normally there should be the much smaller appendages called halteres (Figure 21–23). These mutations transform parts of the body into structures appropriate to other positions, and they are called homeotic mutations (from the Greek “homoios,” meaning similar) because the transformation is between structures of a recognizably similar general type, changing one kind of limb, or one kind of segment, into another. It was eventually discovered that a whole set of genes, the homeotic selector genes, or Hox genes, serve to permanently specify the A-P characters of the whole set of animal segments. These genes are all related to one


Cell_Biology_Alberts

Cell_Biology_Alberts. that a whole set of genes, the homeotic selector genes, or Hox genes, serve to permanently specify the A-P characters of the whole set of animal segments. These genes are all related to one another as members of a multigene family.


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Cell_Biology_Alberts. There are eight Hox genes in the fly, and they all lie in one or the other of two gene clusters known as the Bithorax complex and the Antennapedia complex. wild type loss of Ubx gain of Ubx Figure 21–22 The pattern of expression of Engrailed, a segment-polarity gene. the Engrailed pattern is shown in a 10hour embryo and an adult (whose wings have been removed in this preparation). the pattern is revealed by constructing a strain of Drosophila containing the control sequences of the Engrailed gene coupled to the coding sequence of the reporter LacZ, whose product is detected histochemically through the brown product generated by immunohistochemistry against LacZ (10-hour embryo) or through the blue product generated by a reaction that LacZ catalyzes (adult). Note that the Engrailed pattern, once established, is preserved throughout the animal’s life. (courtesy of tom Kornberg.) Figure 21–23 Homeotic mutations.


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Cell_Biology_Alberts. Figure 21–23 Homeotic mutations. Ultrabithorax, or Ubx, is one of three genes in the Bithorax gene complex (a Hox gene cluster). Ubx is responsible for all of the differences between the second and third thoracic segments. (a, B) Ubx loss-of-function mutations transform the haltere-bearing segment (a) into a wing-bearing segment, resulting in four-winged flies (B). (c) Ubx gain-of-function in the second thoracic segment transforms this wing-bearing segment into a halterebearing segment, resulting in wingless flies. (courtesy of richard Mann.)


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Cell_Biology_Alberts. The genes in the Bithorax complex control the differences among the abdominal and thoracic segments of the body, while those in the Antennapedia complex control the differences among thoracic and head segments. Comparisons with other species show that the same genes are present in essentially all animals, including humans. These comparisons also reveal that the Antennapedia and Bithorax complexes are the two halves of a single entity, called the Hox complex, that has become split in the course of the fly’s evolution, and whose members operate in a coordinated way to exert their control over the head-to-tail pattern of the body.


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Cell_Biology_Alberts. The products of the Hox genes, the Hox proteins, are transcription regulators, all of which possess a highly conserved, 60-amino-acid-long DNA-binding homeodomain (see p. 376). The corresponding motif in the DNA sequence is called a “homeobox,” from which, by abbreviation, the Hox complex takes its name. There are many homeobox-containing genes, but only those located in a Hox complex are Hox genes. The Hox proteins can be viewed as molecular address labels possessed by the cells of each segment: these labels give the cells in each region a positional value—that is, an intrinsic character that differs according to a cell’s location. If the address labels in a developing Drosophila segment are changed, the segment behaves as though it were located somewhere else; if all the Hox genes in an embryo are deleted, the body segments in the larva will all be alike.


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Cell_Biology_Alberts. To a first approximation, each Hox gene is normally expressed in those regions that develop abnormally when that gene is mutated or absent. How does each Hox protein give a segment its permanent identity? All the Hox proteins are similar in their DNA-binding regions, but they are very different in the regions that interact with the other proteins with which the Hox proteins form transcriptional regulatory complexes. The different protein partners act together with the Hox proteins to dictate which DNA binding sites will be recognized, as well as whether the effect on transcription at those sites will be activation or repression. Acting in this way, the Hox proteins modulate the actions of many other transcription regulators. Hundreds of genes are under this type of Hox-modulated control, including genes for cell–cell signaling, transcriptional regulation, cell polarity, cell adhesion, cytoskeletal function, cell growth, and cell death, all conspiring (in ways that are not yet


Cell_Biology_Alberts

Cell_Biology_Alberts. including genes for cell–cell signaling, transcriptional regulation, cell polarity, cell adhesion, cytoskeletal function, cell growth, and cell death, all conspiring (in ways that are not yet understood) to give each segment its distinctive Hox-dependent character.


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Cell_Biology_Alberts. Hox Genes are expressed according to their Order in the Hox complex How, then, is the expression of the Hox genes themselves regulated? The coding sequences of the eight Hox genes in the Antennapedia and Bithorax complexes in Drosophila are interspersed amid a much larger quantity of regulatory DNA. This DNA includes binding sites for the products of the egg-polarity and segmentation genes, thereby serving as an interpreter of the multiple items of spatial information supplied to it by all these transcription regulators. The net result is that the particular set of Hox genes transcribed is appropriate for each location along the A-P body axis.


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Cell_Biology_Alberts. The pattern of Hox gene expression exhibits a remarkable regularity that suggests an additional form of control. The sequence in which the genes are ordered along the chromosome, in both the Antennapedia and the Bithorax complexes, corresponds almost exactly to the order in which they are expressed along the A-P axis of the body (Figure 21–24). This hints at some process of gene activation, perhaps dependent on chromatin structures that propagate along the Hox complexes, switching on one Hox gene after another according to their order along the chromosome. The most “posterior” of the Hox genes that are expressed in a cell generally dominates, driving down expression and activity of the “anterior” genes and dictating the character of the segment. The gene regulatory mechanisms underlying these phenomena are still not well understood, but their consequences are profound. We shall see that the serial organization of gene expression in the Hox complex is a fundamental feature that has


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Cell_Biology_Alberts. these phenomena are still not well understood, but their consequences are profound. We shall see that the serial organization of gene expression in the Hox complex is a fundamental feature that has been highly conserved in the course of animal evolution.


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Cell_Biology_Alberts. trithorax and polycomb Group proteins enable the Hox complexes to Maintain a permanent record of positional Information The spatial pattern of expression of the genes in the Hox complex is set up by signals acting early in development, but the consequences are long lasting. Although the pattern of expression undergoes complex adjustments as development proceeds, the Hox complexes serve to stamp each cell and its progeny with a permanent record of the A-P position that the cell occupied in the early embryo. In this way, the cells of each segment are equipped with a long-term memory of their location along the A-P axis of the body. This memory trace is somehow imprinted on the Hox complexes, and it governs the segment-specific identity not only of the larval segments, but also of the structures of the adult fly.


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Cell_Biology_Alberts. The molecular mechanism of this memory of positional information relies on two types of regulation. One is from the Hox genes themselves: many of the Hox proteins autoactivate the transcription of their own genes, thereby helping to keep the genes on indefinitely. Another crucial input is from two large, complementary sets of proteins, called the Trithorax group and the Polycomb group, which stamp the chromatin of the Hox complex with a heritable record of its embryonic state of activation or repression. These are key general regulators of chromatin structure that can be shown to be critical for cell memory: if genes of the Trithorax or Polycomb group are defective, the pattern of expression of the Hox genes is set up correctly at first, but it is not correctly maintained as the embryo grows older.


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Cell_Biology_Alberts. The two sets of regulators act in opposite ways. Trithorax group proteins are needed to maintain the transcription of Hox genes in cells where transcription has already been switched on. In contrast, Polycomb group proteins form stable complexes that bind to the chromatin of the Hox complex and maintain the repressed state in cells where Hox genes have not been activated at the critical time (Figure 21–25). How such changes in chromatin can store developmental cell memory is discussed in Chapters 4 and 7. the D-V Signaling Genes create a Gradient of the transcription regulator Dorsal As with the patterning along the Drosophila A-P axis just discussed, the patterning along the dorsoventral (D-V) axis begins with maternal gene products that define


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Cell_Biology_Alberts. As with the patterning along the Drosophila A-P axis just discussed, the patterning along the dorsoventral (D-V) axis begins with maternal gene products that define Figure 21–24 The patterns of expression compared to the chromosomal locations of the genes of the Hox complex. the diagram shows the sequence of genes in each of the two subdivisions of the chromosomal complex. this corresponds, with minor deviations, to the spatial sequence in which the genes are expressed, shown in the photograph of a Drosophila embryo at the so-called germ band retraction stage, about 10 hours after fertilization. the embryo has been stained by in situ hybridization with differently labeled probes to detect the mrNa products of different Hox genes in different colors. (photograph courtesy of William McGinnis, adapted from D. Kosman et al., Science 305:846, 2004. With permission from aaaS.)


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Cell_Biology_Alberts. Figure 21–25 The role of genes of the Polycomb group. (a) photograph of a wild-type Drosophila embryo. (B) photograph of a mutant embryo defective for the gene Extra sex combs (Esc) and derived from a mother also lacking this gene. the gene belongs to the Polycomb group. essentially all segments have been transformed to resemble the most posterior abdominal segment. In the mutant, the pattern of expression of the homeotic selector genes, which is roughly normal initially, is unstable in such a way that all these genes soon become switched on all along the body axis. (From G. Struhl, Nature 293:36–41, 1981. With permission from Macmillan publishers Ltd.) this axis in the egg (see Figure 21–17), and it then progresses through zygotic gene products that further subdivide the D-V axis in the embryo.


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Cell_Biology_Alberts. Initially, a protein that is produced by follicle cells underneath the future ventral region of the embryo leads to the localized activation of a transmembrane receptor, called Toll, on the ventral side of the egg membrane. The various maternal genes required for this process are called D-V egg-polarity genes. (Curiously, Drosophila Toll and vertebrate Toll-like proteins also operate in innate immune responses, as discussed in Chapter 24). The localized activation of Toll controls the distribution of Dorsal, a transcription regulator of the NFκB family discussed in Chapter 15. The Toll-regulated activity of Dorsal, like that of NFκB, depends on the translocation of Dorsal from the cytosol, where it is held in an inactive form, to the nucleus, where it regulates gene expression (see Figure 15–62). In the newly laid egg, both Dorsal mRNA and protein are distributed uniformly in the cytosol. After the nuclei in the syncytial blastoderm have migrated to the surface of the embryo, but


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Cell_Biology_Alberts. 15–62). In the newly laid egg, both Dorsal mRNA and protein are distributed uniformly in the cytosol. After the nuclei in the syncytial blastoderm have migrated to the surface of the embryo, but before cellularization (see Figure 21–15), Toll receptor activation on the ventral side induces a remarkable redistribution of the Dorsal protein. On the dorsal side, the protein remains in the cytosol, but ventrally it becomes concentrated in the nuclei, with a smooth gradient of nuclear localization between these two extremes (Figure 21–26).


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Cell_Biology_Alberts. Once inside the nucleus, the Dorsal protein acts as a morphogen and turns on or off the expression of different sets of genes depending on Dorsal’s concentration. The expression of each responding gene depends on its regulatory DNA— specifically, on the number and affinity of the binding sites that this DNA contains for Dorsal and other transcription regulators. In this way, the regulatory DNA interprets the positional signal provided by the nuclear Dorsal protein gradient, so as to define a D-V series of territories—distinctive bands of cells that run the length of the embryo. Most ventrally—where the nuclear concentration of Dorsal protein is highest—it switches on, for example, the expression of a gene called Twist, which is specific for mesoderm. Most dorsally, where the nuclear concentration of Dorsal protein is lowest, the cells switch on a gene called Decapentaplegic (Dpp). And in an intermediate region, where the nuclear concentration of Dorsal protein is high enough to


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Cell_Biology_Alberts. concentration of Dorsal protein is lowest, the cells switch on a gene called Decapentaplegic (Dpp). And in an intermediate region, where the nuclear concentration of Dorsal protein is high enough to repress Dpp but too low to activate Twist, the cells switch on another set of genes, including one called Short gastrulation (Sog) (Figure 21–27A).


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Cell_Biology_Alberts. Products of the genes directly regulated by the Dorsal protein generate in turn more local signals, which define finer subdivisions along the D-V axis. These signals act during cellularization and take the form of conventional extracellular Figure 21–26 The concentration gradient of Dorsal protein in the nuclei of the blastoderm. In wild-type Drosophila embryos, the protein is present in the dorsal cytoplasm and absent from the dorsal nuclei; ventrally, it is depleted in the cytoplasm and concentrated in the nuclei. In a mutant in which the toll pathway is activated everywhere and not just ventrally, Dorsal protein is everywhere concentrated in the nuclei; the result is a ventralized embryo. conversely, in a mutant in which the toll signaling pathway is inactivated, Dorsal protein everywhere remains in the cytoplasm and is absent from the nuclei; the result is a dorsalized embryo. (From


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Cell_Biology_Alberts. S. roth, D. Stein and c. Nüsslein-Volhard, Cell 59:1189–1202, 1989. With permission from elsevier.) 1166 Chapter 21: Development of Multicellular Organisms (B) gradients of Dpp and Sog proteins dorsoventral territories are specifed signal proteins. In particular, Dpp codes for a secreted TGFβ-family protein, which forms a local morphogen gradient in the dorsal part of the embryo. Sog encodes another secreted protein that is produced by the neurogenic ectoderm (which gives rise to the nervous system) and acts as an antagonist of Dpp protein. The opposing diffusion gradients of these two signal proteins create a steep gradient of Dpp activity: the highest Dpp activity levels, in combination with certain other factors, cause development of the most dorsal tissue of all—an extraembryonic membrane. Intermediate levels cause development of dorsal epidermis; and the absence of Dpp activity allows the development of neurogenic ectoderm (Figure 21–27B).


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Cell_Biology_Alberts. a hierarchy of Inductive Interactions Subdivides the Vertebrate embryo The molecular genetic analysis of Drosophila development has uncovered how a cascade of transcription regulators and signaling pathways subdivides the embryo. The same principle of progressive pattern refinement is used during the development of all animal embryos, including vertebrates. Remarkably, conservation is not restricted to the general strategy of pattern formation, but also extends to many of the molecules involved. As mentioned previously, the earliest phases of vertebrate development are surprisingly variable, even between closely related species, and it is even hard to say precisely how the axes of an early fly embryo correspond to those of an early frog or mouse embryo. Nevertheless, we shall see that amid this display of evolutionary plasticity, some features of early development turn out to be highly


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Cell_Biology_Alberts. Figure 21–27 How morphogen gradients guide a patterning process along the dorsoventral axis of the Drosophila embryo. (a) Initially, a gradient of Dorsal protein defines three broad territories of gene expression, marked here by the expression of three representative genes—Dpp, Sog, and Twist. (B) Slightly later, the cells expressing Dpp and Sog secrete, respectively, the signal proteins Dpp (a tGFβ family member) and Sog (an antagonist of Dpp). these two proteins then diffuse and interact with one another (and with certain other factors) to create the dorsoventral (D–V) territories shown. conserved. The same is true of later developmental stages also, often to an astonishing degree. From our own anatomy, it is obvious that we are cousins to birds and fish. But looking at molecular mechanisms, we see that we are cousins to flies and worms too.


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Cell_Biology_Alberts. In the following pages, we discuss how vertebrate embryos are patterned by the interplay of signaling molecules and transcription regulators. We begin by discussing the formation and patterning of the embryonic axes in amphibians, taking the frog Xenopus as our example. We have already broached this topic earlier in the chapter. Here, we pick up the thread and draw comparisons with the fly.


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Cell_Biology_Alberts. As noted earlier, the origins of the embryonic axes and the three germ layers in the frog can be traced back to the blastula (see Figure 21–3A). By labeling individual blastomeres, we can track cells through all their divisions, transformations, and migrations and see what they become and where they come from. The precursors of ectoderm, mesoderm, and endoderm are arranged in order along the animal-vegetal axis of the blastula: the endoderm derives from the most vegetal blastomeres, the ectoderm from the most animal, and the mesoderm from a middle set. Within each of these territories, the cells have diverse fates according to their positions along the D-V axis of the later embryo. For ectoderm, epidermal precursors are located ventrally, and future neurons are found dorsally; for mesoderm, precursors for notochord, muscle, kidney, and blood are arranged from dorsal to ventral. All this can be represented by a fate map that shows which cell types derive from which regions of the


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Cell_Biology_Alberts. precursors for notochord, muscle, kidney, and blood are arranged from dorsal to ventral. All this can be represented by a fate map that shows which cell types derive from which regions of the blastula (Figure 21–28). The fate map confronts us with the central question: how are the cells in different positions driven toward their different fates? We have already explained how maternal factors deposited in the developing frog egg define its animal-vegetal axis, and how cortical rotation triggered by fertilization defines the orientation of the dorsoventral axis (see Figure 21–14). But how does the establishment of axes lead on to the subdivision of the embryo into the future body parts?


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Cell_Biology_Alberts. The maternal gene products lead to the formation of signaling centers on the vegetal and dorsal sides of the embryo. The dorsal signaling center in particular has a special place in the history of developmental biology. Experiments in the early twentieth century identified it as a small cluster of cells, located on the dorsal side of the amphibian embryo, with an extraordinary property: when the cells were transplanted to an opposite site, they could trigger a radical reorganization of the neighboring tissue, causing it to form a second whole-body axis (Figure 21–29). The discovery of this signaling center, called the Organizer, led the way to a pioneering analysis of the chain of inductive interactions that establish the framework of the vertebrate body.


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Cell_Biology_Alberts. In contrast to the Drosophila syncytial embryo, the fertilized frog egg undergoes rapid cleavage divisions that result in an embryo consisting of thousands of cells. Patterning must therefore be mediated by extracellular signal molecules Figure 21–28 Blastula fate map in a frog embryo. the endoderm derives from the most vegetal blastomeres (yellow), the ectoderm from the most animal (blue), and the mesoderm from a middle set (green) that contributes also to endoderm and ectoderm. Different cell types derive from different positions along the dorsoventral axis. Figure 21–29 Induction of a secondary axis by the Organizer. an amphibian embryo receives a graft of a small cluster of cells taken from a specific site, called the Organizer region, on the dorsal side of another embryo at the same stage. Signals from the graft organize the behavior of neighboring cells of the host embryo, causing development of a pair of conjoined (Siamese) twins. See Movie 21.4. [after


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Cell_Biology_Alberts. J. holtfreter and V. hamburger, in analysis of Development (B.h. Willier, p.a. Weiss and V. hamburger, eds), pp. 230–296. philadelphia: Saunders, 1955.] 1168 Chapter 21: Development of Multicellular Organisms that diffuse through the embryo from cell to cell, not by transcription regulators that move through the cytoplasm of a syncytium. Not surprisingly, the Organizer is now known to be a major source of secreted protein signals. a competition Between Secreted Signaling proteins patterns the Vertebrate embryo


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Cell_Biology_Alberts. The signal molecules that pattern the frog embryo along the animal-vegetal (A-V) axis belong to the TGFβ family: they are secreted by a signaling center at the vegetal pole and form concentration gradients along the A-V axis. The Nodal protein acts over a relatively short range: cells near the vegetal pole are exposed to high levels of it and respond by switching on genes that promote the development of endoderm; cells further away are exposed to lower levels and activate genes that promote the formation of mesoderm. The cells at the vegetal pole that produce Nodal also produce a more rapidly diffusing TGFβ-like protein called Lefty, which antagonizes Nodal. The result is a high ratio of Lefty to Nodal at the animal pole, where Lefty predominates and Nodal signaling is blocked; this causes the cells there to develop as ectoderm (Figure 21–30A). Thus, a mid-range activation by Nodal, combined with a long-range inhibition by Lefty, sets up the pattern of progenitors along the A-V axis


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Cell_Biology_Alberts. the cells there to develop as ectoderm (Figure 21–30A). Thus, a mid-range activation by Nodal, combined with a long-range inhibition by Lefty, sets up the pattern of progenitors along the A-V axis for the three germ layers—endoderm, mesoderm, and ectoderm.


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Cell_Biology_Alberts. The frog’s dorsal signaling system uses a different set of secreted signals from that of the vegetal signaling system to subdivide the germ-layer territories according to location along the D-V axis of the embryo. It exerts its influence by secreting two inhibitory signal proteins, called Chordin and Noggin. These antagonize the action of bone morphogenetic proteins (BMPs; members of yet another subclass of the TGFβ family), which themselves are secreted throughout the embryo. In this way, Chordin and Noggin form a dorsal-to-ventral gradient that blocks BMP signaling on the dorsal side but allows it to remain high on the ventral side (Figure 21–30B). Ectodermal cells that experience high levels of BMP signaling are driven to epidermal fates, whereas cells that experience little or no BMP signaling remain neural.


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Cell_Biology_Alberts. Knowing the signals that specify the three germ layers and various tissue types of the vertebrate body, one can reproduce this specification in a culture dish. Frog cells taken from the animal-pole region of the embryo, for example, Figure 21–30 How Nodal and bone morphogenic protein (BMP) signaling pattern the embryonic axes. Nodal and its antagonist Lefty pattern the animal-vegetal axis, while BMp and its antagonists chordin and Noggin pattern the dorsoventral axis. In the animal pole region, where Nodal levels are low relative to Lefty, Lefty blocks Nodal from binding to its receptors. In the vegetal region, there is an excess of Nodal, resulting in Nodal pathway activation. along the dorsoventral axis, BMp is widely present but chordin and Noggin are concentrated at the dorsal side: there, they bind to BMp and block its binding to receptors. the resulting patterns of Nodal and BMp activity are illustrated at the bottom of the figure.


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Cell_Biology_Alberts. will differentiate into blood (a ventral mesodermal tissue) when diverted from their original fate by exposure to intermediate concentrations of Nodal and high concentrations of BMP. Similarly, mouse or human embryonic stem cells can be coaxed to generate specific cell types by exposing them in culture to appropriate combinations of signal molecules. In this way, the insights gained through studies of animal development can be used to generate the cell types needed for regenerative medicine, as we discuss in the next chapter. the Insect Dorsoventral axis corresponds to the Vertebrate Ventral-Dorsal axis


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Cell_Biology_Alberts. the Insect Dorsoventral axis corresponds to the Vertebrate Ventral-Dorsal axis The signaling systems that pattern the D-V axis in Drosophila and in vertebrates are similar. In Drosophila, as we saw, Dpp and its inhibitor Sog are responsible, whereas in vertebrates, BMP and its inhibitors Chordin and Noggin do the job. Dpp is a member of the BMP family, while Sog is a homolog of Chordin. Both in flies and frogs, high levels of the inhibitors define the region that is neurogenic, and high levels of BMP/Dpp activity define the region that is not. These and other molecular parallels strongly suggest that this aspect of body patterning has been conserved in evolution from insects to vertebrates. Curiously, however, the axis is inverted: dorsal in the fly corresponds to ventral in the vertebrate (Figure 21–31). At some point in evolution, it seems that the ancestor of one of these classes of animals took to living life upside-down. Hox Genes control the Vertebrate a-p axis


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Cell_Biology_Alberts. Hox Genes control the Vertebrate a-p axis The conservation of developmental mechanisms between Drosophila and vertebrates extends beyond the D-V signaling system. Hox genes are found in almost every animal species studied, where they are often grouped in complexes similar to the insect Hox complex. In mice and humans, for example, there are four such complexes—called the HoxA, HoxB, HoxC, and HoxD complexes—each on a different chromosome. Individual genes in each complex can be recognized by their sequences as counterparts of specific members of the Drosophila set. Indeed, mammalian Hox genes can function in Drosophila as partial replacements for the corresponding Drosophila Hox genes. It appears that each of the four mammalian Hox complexes is, roughly speaking, the equivalent of one complete insect Hox complex (that is, an Antennapedia complex plus a Bithorax complex) (Figure 21–32).


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Cell_Biology_Alberts. The ordering of the genes within each vertebrate Hox complex is essentially the same as in the insect Hox complex, suggesting that all four vertebrate complexes originated by duplications of a single primordial complex and have preserved its basic organization. Most tellingly, when the expression patterns of the Hox genes are examined in the vertebrate embryo, it turns out that the members of each complex are expressed in a head-to-tail series along the axis of the body, just as they are in Drosophila. As in Drosophila, vertebrate Hox gene expression patterns are often aligned with vertebrate segments. This alignment is especially clear in the hindbrain (see Figure 21–32), where the segments are called rhombomeres.


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Cell_Biology_Alberts. The products of the vertebrate Hox genes, the Hox proteins, specify positional values that control the A-P pattern of parts in the hindbrain, neck, and trunk (as well as some other parts of the body). As in Drosophila, when a posterior Hox gene is artificially expressed in an anterior region, it can convert the anterior tissue to


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Cell_Biology_Alberts. Figure 21–31 The vertebrate body plan as a dorsoventral inversion of the insect body plan. Note the correspondence with regard to the circulatory system as well as the gut and nervous system. In insects, the circulatory system is represented by a tubular heart and a main dorsal blood vessel, which pumps blood out into the tissue spaces through one set of apertures and receives blood back from the tissues through another set. Unlike vertebrates, insects have no system of capillary vessels to contain the blood as it percolates through the tissues. Nevertheless, heart development depends on homologous genes in vertebrates and insects, reinforcing the relationship between the two body plans. (after e.L. Ferguson, Curr. Opin. Genet. Dev. 6:424–431, 1996. With permission from elsevier.)


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Cell_Biology_Alberts. Bcd, Lab Pb a posterior character. Conversely, loss of posterior Hox genes allows the posterior tissue where they are normally expressed to adopt an anterior character (Figure 21–33). Because of a redundancy between genes in the four Hox gene clusters, the transformations observed in mouse Hox mutants are not always so straightforward as those in the fly, and they are often incomplete. Nonetheless, it seems clear that the fly and the mouse use essentially the same molecular machinery to impart individual characteristics to successive regions along at least a part of the A-P axis. Just as there are genes that regulate pattern formation and segmental identity, there are genes whose products act as triggers for the development of a specific cell type or even a specific organ, initiating and coordinating the whole complex program of gene expression that is required. An example is the MyoD/myogenin


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Cell_Biology_Alberts. Figure 21–32 The Hox complexes of an insect and a mammal, compared and related to body regions. the genes of the Antennapedia and Bithorax complexes of Drosophila are shown in their chromosomal order in the top line. the corresponding genes of the four mammalian Hox complexes are shown below, also in chromosomal order. the gene expression domains in fly and mammal are indicated in a simplified form by color in the cartoons of animals above and below. there is a remarkable parallelism. however, the details of the patterns depend on developmental stage and vary somewhat from one mammalian Hox complex to another. also, in many cases, genes shown here as expressed in an anterior domain are also expressed more posteriorly, overlapping the domains of more posterior Hox genes.


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Cell_Biology_Alberts. the complexes are thought to have evolved as follows: first, in some common ancestor of worms, flies, and vertebrates, a single primordial homeotic selector gene underwent repeated duplication to form a series of such genes in tandem—the ancestral Hox complex. In the Drosophila sublineage, this single complex became split into separate Antennapedia and Bithorax complexes. Meanwhile, in the lineage leading to the mammals, the whole complex was repeatedly duplicated to give four Hox complexes. the parallelism is not perfect because apparently some individual genes have been duplicated and others lost. Still others have been co-opted for different purposes (genes in parentheses in the top line) over the time that has elapsed since the complexes diverged. (Based on a diagram courtesy of William McGinnis.)


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Cell_Biology_Alberts. Hox gain of function Hox loss of function family of transcription regulators that we encountered in Chapter 7. These proteins drive cells to differentiate into muscle, expressing muscle-specific actins and myosins and all the other specialized cytoskeletal, metabolic, and membrane proteins that a muscle cell needs. Analogously, members of the Achaete/Scute family of transcription regulators drive cells to become neural progenitors. In both these examples, the proteins belong to the basic helix–loop–helix (bHLH) class of transcription regulators (see p. 377), and the same is true for many of the other proteins that induce the differentiation of particular cell types. These master transcription regulators exert their powerful differentiation-inducing activity by binding to many different regulatory sites in the genome and thereby controlling the expression of large numbers of downstream target genes. In one well-studied case, that of an Achaete/Scute family member called Atonal homolog


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Cell_Biology_Alberts. sites in the genome and thereby controlling the expression of large numbers of downstream target genes. In one well-studied case, that of an Achaete/Scute family member called Atonal homolog 1 (Atoh1), the number of direct target genes in the mouse genome is more than 600. It is important to note, however, that even such powerful drivers of cell differentiation can have radically different effects according to the context and history of the cells in which they act: Atoh1, for example, drives differentiation of certain classes of neurons in the brain, of sensory hair cells in the inner ear, and of secretory cells in the lining of the gut.


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Cell_Biology_Alberts. Other genes encoding transcription regulators can drive the formation and assembly of the multiple cell types that constitute an entire organ. A famous example is the transcription regulator Eyeless. When it is artificially expressed in a patch of cells in the leg precursors of Drosophila, a well-organized eye-like organ develops on the leg, with the various eye cell types correctly arranged (see Figure 7–35B); conversely, loss of the Eyeless gene results in flies that lack eyes. Moreover, loss of the Eyeless homolog Pax6 in vertebrates likewise leads to loss of eye structures. Similar organ-selector proteins are known for foregut, heart, pancreas, and other organs. They are all master transcription regulators that directly regulate hundreds of target genes, the products of which then specify and construct the different elements of the appropriate organ. However, as in the example of Atoh1, they usually exert their specific effect only in combination with the right partners, which are


Cell_Biology_Alberts

Cell_Biology_Alberts. and construct the different elements of the appropriate organ. However, as in the example of Atoh1, they usually exert their specific effect only in combination with the right partners, which are only expressed in cells that were appropriately primed during their earlier development.


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Cell_Biology_Alberts. After the establishment of the basic body plan and the generation of organ precursors, many further steps of pattern refinement are required to achieve the adult pattern of terminally differentiated cells in tissues and organs. As we discussed Figure 21–33 Control of anteroposterior pattern by Hox genes in the mouse. (a,B) a normal mouse (wild type) has about 65 vertebrae, differing in structure according to their position along the body axis: 7 cervical (neck), 13 thoracic (with ribs), 6 lumbar [bracketed by yellow asterisks in (B)], 4 sacral [bracketed by red asterisks in (B)], and about 35 caudal (tail). shows a side view and (B) shows a dorsal view; for clarity, the limbs have been removed in each picture.


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Cell_Biology_Alberts. shows a side view and (B) shows a dorsal view; for clarity, the limbs have been removed in each picture. the HoxA10 gene is normally expressed in the lumbar region (together with its paralogs HoxC10 and HoxD10); here it has been artificially expressed in the developing vertebral tissue all along the body axis. as a result, the cervical and thoracic vertebrae are all converted to a lumbar character. (D) conversely, when HoxA10 is removed along with HoxC10 and HoxD10, vertebrae that should normally have a lumbar or sacral character take on a thoracic character instead. (a and c, from M. carapuço et al., Genes Dev. 19:2116–2121, 2005. With permission from cold Spring harbor Laboratory press; B and D, from D.M. Wellik and M.r. capecchi, Science 301:363–367, 2003.) earlier, lateral inhibition mediated by Notch signaling is crucial for both cell diversification and fine-grained patterning in an enormous variety of tissues in all animals.


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Cell_Biology_Alberts. One example is the development of sensory bristles in Drosophila, most easily seen on the fly’s back, but also present on most of its other exposed surfaces. Each of these is a miniature sense organ, consisting of a sensory neuron and a small set of supporting cells. Some bristles respond to chemical stimuli, others to mechanical stimuli, but they are all constructed in a similar way (Figure 21–34). The pro-neural genes Achaete and Scute mentioned earlier mark the patches of epidermis within which bristles will form. Mutations that eliminate the expression of these genes at some of their usual sites block development of bristles at just those sites, and mutations that cause expression in abnormal sites cause bristles to develop there.


Cell_Biology_Alberts

Cell_Biology_Alberts. The initial cells expressing the proneural genes are called proneural cells, and they are primed to take the neurosensory pathway of differentiation, but which of the cells will actually do so depends on competitive interactions among them. In the first round of these interactions, a single cell within each small group of proneural cells is picked to serve as the progenitor of the bristle. This single cell is called the sensory mother cell. It becomes distinct from the other cells of the cluster through lateral inhibition mediated by the Notch signaling pathway. This operates in the way we discussed earlier. The cells in the proneural cluster initially all express both the transmembrane receptor Notch and its transmembrane ligand Delta, along with proteins that regulate the signaling activity of Delta. Wherever Delta activates Notch, an inhibitory signal is transmitted that diminishes the tendency of the Notch-activated cell to specialize as a sensory mother cell.At first, all the


Cell_Biology_Alberts

Cell_Biology_Alberts. of Delta. Wherever Delta activates Notch, an inhibitory signal is transmitted that diminishes the tendency of the Notch-activated cell to specialize as a sensory mother cell.At first, all the cells in the cluster inhibit one another. However, receipt of the signal in a given cell diminishes that cell’s ability to fight back by delivering the inhibitory Delta signal in return. This creates a competitive situation, from which a single cell in each cluster—the future sensory mother cell—eventually emerges as winner, sending a strong inhibitory signal to its immediate neighbors but receiving no such signal in return (Figure 21–35). If a cell that would normally become a sensory mother cell is genetically disabled from doing so, a neighboring proneural cell, freed from lateral inhibition, will become a sensory mother cell instead.


Cell_Biology_Alberts

Cell_Biology_Alberts. The sensory mother cell goes through a short program of further divisions to generate the set of cells that form the final bristle. Notch signaling acts repeatedly at successive stages in this program to drive the descendants of the sensory mother cell along different pathways and assign them to their various specialized fates. However, it does so in conjunction with additional mechanisms that bias the outcome of the competition mediated by lateral inhibition. Determinants that


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Cell_Biology_Alberts. Figure 21–34 The basic structure of a mechanosensory bristle. the lineage of the four cells of the bristle—all descendants of a single sensory mother cell—is shown on the left. the sensory mother cell, once it is specified, generates this set of cells through a short program of division cycles. In each generation of the progeny, lateral inhibition operates again to drive the newborn cells toward different fates: one of the ultimate progeny will become the neuron; another, the shaft of the bristle; others, supporting cells of various sorts. as the sensory mother cell and its progeny divide, certain proteins are allocated preferentially to one of each pair of newborn sister cells, biasing the outcome of the lateral-inhibition competition mediated by Notch signaling. are asymmetrically localized inside the dividing cells have this role in sensory bristle development. They are also important in other contexts, as we now discuss.


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Cell_Biology_Alberts. are asymmetrically localized inside the dividing cells have this role in sensory bristle development. They are also important in other contexts, as we now discuss. Cell diversification does not always have to depend on extracellular signals: in some cases, sister cells are born different as a result of an asymmetric cell division, during which some significant set of molecules is divided unequally between them. This asymmetrically segregated molecule (or set of molecules) then acts as a determinant for one of the cell fates by directly or indirectly altering the pattern of gene expression within the daughter cell that receives it (see Figure 21–12). We have already encountered the asymmetric segregation of molecules in the context of the early frog embryo: VegT RNA is localized in the vegetal region of the fertilized egg. Following cell division, only vegetal daughter cells will inherit VegT RNA.


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Cell_Biology_Alberts. Asymmetric divisions often occur at the beginning of development, but they are also encountered at some later stages. As mentioned for the sensory bristle, they can set the scene for an exchange of Notch signals between the daughter cells, with the signaling occurring after the cells have become separate and reinforcing the differences between them. In the central nervous system, asymmetric divisions have a key role in generating the very large numbers of neurons and glial cells that are needed. A special class of cells becomes committed as neural precursors, but instead of differentiating directly as neurons or glial cells, these undergo a long series of asymmetric divisions through which a succession of additional neurons and glial cells are added to the population. The process is best understood in Drosophila, although there are many hints that something similar occurs also in vertebrate neurogenesis.


Cell_Biology_Alberts

Cell_Biology_Alberts. In the embryonic central nervous system of Drosophila, the nerve-cell precursors, or neuroblasts, are initially singled out from the neurogenic ectoderm by a typical lateral-inhibition mechanism that depends on Notch. Each neuroblast then divides repeatedly in an asymmetric fashion (Figure 21–36). At each division, one daughter remains as a neuroblast, while the other, which is much


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Cell_Biology_Alberts. Figure 21–35 Lateral inhibition. (a) the basic mechanism of Notch-mediated competitive lateral inhibition, illustrated for just two interacting cells. In this diagram, the absence of color on proteins or effector lines indicates inactivity. (B) the outcome of the same process operating in a larger patch of cells. at first, all cells in the patch are equivalent, expressing both the transmembrane receptor Notch and its transmembrane ligand Delta. each cell has a tendency to specialize (as a sensory mother cell), and each sends an inhibitory signal to its neighbors to discourage them from also specializing in that way. this creates a competitive situation. as soon as an individual cell gains any advantage in the competition, that advantage becomes magnified. the winning cell, as it becomes more strongly committed to differentiating as a sensory mother cell, also inhibits its neighbors more strongly. conversely, as these neighbors lose their capacity to differentiate as sensory mothers,


Cell_Biology_Alberts

Cell_Biology_Alberts. more strongly committed to differentiating as a sensory mother cell, also inhibits its neighbors more strongly. conversely, as these neighbors lose their capacity to differentiate as sensory mothers, they also lose their capacity to inhibit other cells from doing so. Lateral inhibition thus makes adjacent cells follow different fates.


Cell_Biology_Alberts

Cell_Biology_Alberts. although the interaction is thought to be normally dependent on cell–cell contacts, the future sensory mother cell may be able to deliver an inhibitory signal to cells that are more than one cell diameter away—for example, by sending out long protrusions to touch them.


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Cell_Biology_Alberts. smaller, becomes specialized as a ganglion mother cell. Each ganglion mother cell will divide only once, giving a pair of neurons, or a neuron plus a glial cell, or a pair of glial cells, with Notch-mediated interactions helping to drive the daughters along different paths. The neuroblast itself becomes smaller at each division, as it parcels out its substance into one ganglion mother cell after another. Eventually, typically after about 12 cycles, the process halts, presumably because the neuroblast becomes too small to pass the cell-size checkpoint in the cell-division cycle. Later, in the larva, neuroblast divisions resume, but now they are accompanied by cell growth, permitting the process to continue indefinitely and to generate the much larger numbers of neurons and glial cells required in the adult fly.


Cell_Biology_Alberts

Cell_Biology_Alberts. In the preceding sections, we have seen that animals contain the same essential cell types, have a similar collection of genes, and share many of the molecular mechanisms of pattern formation. But how can we square this with the radical differences that we see in the body structures of animals as diverse as a worm, a fly, a frog, and a mouse? We asserted earlier, in a general way, that these differences usually seem to reflect differences in the regulatory DNA that calls into play the components of the conserved basic kit of parts. We must now examine the evidence a little more closely.


Cell_Biology_Alberts

Cell_Biology_Alberts. When we compare animal species with similar basic body plans—different vertebrates, for example, such as fish, birds, and mammals—we find that corresponding genes usually have similar sets of regulatory elements: the regulatory DNA sequences have been well conserved and are recognizably homologous in the different animals. The same is true if we compare different species of nematode worms or insects. But, when we compare vertebrate regulatory regions with those of worms or flies, it is hard to see any such resemblance. The protein-coding sequences are unmistakably similar, but the corresponding regulatory DNA sequences appear mostly very different, suggesting that the differences in body plans mainly reflect differences in regulatory DNA. Although variations in the proteins themselves also contribute, differences in regulatory DNA would be enough to generate radically different tissues and body structures even if the proteins were the same.


Cell_Biology_Alberts

Cell_Biology_Alberts. It is not yet possible to trace the genetic steps that have led to all the spectacular diversity of animals. Their lineages have diverged over hundreds of millions of years, and in most cases too many changes have occurred for us to be able to say that this or that feature results from this or that mutation. The picture is clearer, however, for more recent evolutionary events. Studies of both closely related animal populations and plant populations whose members have different morphologies have revealed that dramatic developmental effects can result from subtle changes in regulatory DNA.


Cell_Biology_Alberts

Cell_Biology_Alberts. A well-studied example is the morphological diversity found in stickleback fish. After the last ice age ended about 10,000 years ago, marine sticklebacks colonized many newly formed freshwater streams and lakes. Marine sticklebacks extend sharp spines from their pelvic skeleton. These spines are thought to help protect the fish from soft-mouthed fish predators. In contrast, several populations of freshwater sticklebacks have lost these spines, usually in lakes that lack such predators. The different morphologies reflect differences in control of the expression of a transcription regulator called Pitx1. Whereas marine sticklebacks express the Pitx1 gene in the pelvic bone precursor cells that will form the spikes,


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Cell_Biology_Alberts. Figure 21–36 Neuroblasts and asymmetric cell division in the central nervous system of a fly embryo. the neuroblast originates as a specialized ectodermal cell. It is singled out by lateral inhibition and emerges from the basal (internal) face of the ectoderm. It then goes through repeated division cycles, dividing asymmetrically to generate a series of ganglion mother cells. each ganglion mother cell divides just once to give a pair of differentiated daughters (typically a neuron plus a glial cell). freshwater sticklebacks have lost this expression as a result of a change at the Pitx1 locus. These changes do not lie in the coding sequence. Instead, each is a small deletion of a block of adjacent regulatory DNA that controls Pitx1 expression specifically in the pelvic cells (Figure 21–37).


Cell_Biology_Alberts

Cell_Biology_Alberts. The Pitx1 protein has important functions elsewhere in the body, so that the DNA sequences that encode this protein must be retained. The regulatory DNA responsible for Pitx1 expression at these other sites is also unchanged in the two populations of sticklebacks. The evolution of pelvis development in sticklebacks shows how the modular nature of regulatory DNA elements that we encountered in Chapter 7 (see Figure 7–29) allows independent modification of the different parts of the body, even when formation of those body parts depends on the same proteins. In the recent evolution of plants, changes of body structure can be traced in a similar way to changes in regulatory DNA. For example, these account for a large part of the dramatic difference between the wild teosinte plant and its modern descendant, maize, through some 10,000 years of mutation and selection by Native Americans.


Cell_Biology_Alberts

Cell_Biology_Alberts. Drosophila has been the foremost model organism for the study of the genetics of animal development. Its embryonic pattern is initiated by the products of maternal-effect genes called egg-polarity genes, which operate by setting up graded distributions of transcription regulators in the egg and early embryo. The gradient of Bicoid protein along the A-P axis, for example, helps initiate the orderly expression of gap genes, pair-rule genes, and segment-polarity genes. These three classes of segmentation genes, through a hierarchy of interactions, become expressed in some regions of the embryo and not others, progressively subdividing the embryo along the A-P axis into a regular series of repeating modular units called segments.


Cell_Biology_Alberts

Cell_Biology_Alberts. Superimposed on the pattern of gene expression that repeats itself in every segment, there is a serial pattern of expression of Hox genes that confer on each segment a different identity. These genes are grouped in complexes and are arranged in a sequence that matches their sequence of expression along the A-P axis of the body. Although Hox gene expression is initiated in the embryo, it is subsequently maintained by the action of chromatin-binding proteins of the Polycomb and Trithorax group, which stamp the chromatin of the Hox complex with a heritable record of its embryonic state of repression or activation, respectively. Hox complexes homologous to that of Drosophila are found in virtually every type of animal, where they help pattern the A-P axis of the body.


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Cell_Biology_Alberts. Signaling gradients are also set up along the dorsoventral (D-V) axis. Initially, Toll signaling generates a nuclear gradient of Dorsal protein, which induces an extracellular signaling gradient of the TGFβ-family protein Dpp and its antagonist, Figure 21–37 Morphological diversity in stickleback fish is caused by changes in regulatory elements. (a–D) pelvic spines are present in marine (a) but not in freshwater (c) populations. correspondingly, Pitx1 is expressed in the pelvic area in marine (B) but not in freshwater (D) fish. the lack of expression in the pelvic area of freshwater populations is caused by mutations in an enhancer element. Other enhancers and sites of expression for Pitx1 are the same in marine and freshwater sticklebacks. (courtesy of Michael D. Shapiro.) Sog. This creates a gradient of Dpp activity that helps refine the assignment of different characters to cells at different positions along the D-V axis.


Cell_Biology_Alberts

Cell_Biology_Alberts. Sog. This creates a gradient of Dpp activity that helps refine the assignment of different characters to cells at different positions along the D-V axis. In Xenopus, the polarity of the egg and the site of sperm entry set up the embryonic axes. A gradient generated by the TGFβ-family protein Nodal induces different fates along the animal-vegetal axis, whereas BMP and Chordin—proteins homologous to Drosophila Dpp and Sog, respectively—control the patterning of the D-V axis. This axis is inverted, so that dorsal in the fly corresponds to ventral in the frog.


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Cell_Biology_Alberts. Transcription regulators control the formation of specific cell types. Members of the MyoD/myogenin family drive the process of muscle cell determination, coordinating the many components required, whereas Achaete/Scute transcription regulators control neural fate. Other genes encoding such master transcriptional regulators can regulate the formation of entire organs. Eyeless, for example, is both necessary and sufficient to generate eye structures in Drosophila.


Cell_Biology_Alberts

Cell_Biology_Alberts. To refine the anatomical pattern within such an organ, the cells interact locally, both by diffusible inductive signals and by short-range mechanisms. Often, the cells compete with one another by lateral inhibition. This process results in activation of the Notch signaling pathway in one cell and inhibition in its neighbors, generating two different cell types. Asymmetric cell divisions, in which daughter cells inherit different molecular determinants from the mother cell, provide an additional way to organize a fine-grained diversity of cell types. Evidence from recent evolutionary events indicates that anatomical changes are mostly driven by changes in regulatory DNA sequences that determine when and where developmental genes are expressed. How the striking diversity in body structures has evolved over longer times remains largely unknown, although it seems likely that similar principles apply.


Cell_Biology_Alberts

Cell_Biology_Alberts. Developmental events unfold over minutes, hours, days, weeks, months, or even years, with each organism following its own strict timetable. The cascades of inductive interactions and transcriptional regulatory events described earlier take time, as signals are transmitted and transcription regulators are synthesized and then bind to DNA to activate or repress their target genes. At the beginning of this chapter, we compared development with an orchestral performance. There are many players, and each must do the right thing at the right time; yet there is no leader or conductor to set the tempo and coordinate the timing of all the different events. Each developmental process must thus occur at an appropriate rate, tuned by evolution to fit with the timing of other processes in the embryo or in the environment. The control of timing is one of the most important problems in developmental biology, but also one of the least understood.


Cell_Biology_Alberts

Cell_Biology_Alberts. Developmental processes are complex, but they are built up from simple steps. A first challenge is to understand the timing of these steps. How long does it take, for example, to switch the expression of a gene on or off? This is not like throwing a light switch: it involves delays. First, it takes time to make an mRNA molecule: the RNA polymerase must travel the length of the gene, the primary RNA transcript must be spliced and otherwise processed, and the resulting mRNA must be exported from the nucleus and delivered to the site where it will be translated. This adds up to what one might call the gestation time of the individual molecule. Second, it takes time for the individual mRNA molecules to accumulate to their fully effective concentration; as explained in Chapter 15, this accumulation time is dictated by the average lifetime of the molecules—the longer they last, the higher their ultimate concentration, and the longer the time taken to attain it. Similar delays occur at the


Cell_Biology_Alberts

Cell_Biology_Alberts. time is dictated by the average lifetime of the molecules—the longer they last, the higher their ultimate concentration, and the longer the time taken to attain it. Similar delays occur at the next step, where the mRNA is translated into protein: synthesis of each individual protein molecule involves a gestation delay, and attainment of an effective concentration of protein molecules involves an accumulation delay that depends on the protein’s lifetime. The time for the whole gene switching process is just the sum of the gestation delays and the accumulation delays (basically, the molecular lifetimes) for both the mRNA and the protein molecules. Somewhat counterintuitively, it is the combined length of these delays, rather than the rate of molecular synthesis (the number of molecules synthesized per second), that chiefly determines the switching time.


Cell_Biology_Alberts

Cell_Biology_Alberts. The same additive principle applies to long cascades of gene switching, where gene A activates gene B, and gene B activates gene C, and so on. It also applies in other circumstances, such as in signaling pathways where one protein directly regulates the activation of the next. In all these cases, molecular lifetimes, along with gestation delays, play a key part in determining the pace of development. The lifetimes of mRNA and protein molecules are enormously variable, from a few minutes or hours to days or more, explaining much of the variation we see in the tempo of developmental events.


Cell_Biology_Alberts

Cell_Biology_Alberts. Gene switching delays, however, are not the be-all and end-all of developmental timing. Development involves many other kinds of delay that contribute to timing. Chromatin structure takes time to remodel. Inductive signals take time to diffuse across a field of cells (see Figure 21–9). Cells take time to move and rearrange themselves in space. Nevertheless, the timing of gene switching plays a fundamental part in developmental timing, as illustrated in an especially clear and striking way by a gene-expression oscillator that controls the segmentation of the vertebrate body axis, as we now explain. a Gene-expression Oscillator acts as a clock to control Vertebrate Segmentation


Cell_Biology_Alberts

Cell_Biology_Alberts. a Gene-expression Oscillator acts as a clock to control Vertebrate Segmentation The main body axis of all vertebrates has a repetitive, periodic structure, seen in the series of vertebrae, ribs, and segmental muscles of the neck, trunk, and tail. These segmental structures originate from the mesoderm that lies as a long slab on either side of the embryonic midline. This slab becomes broken up into a regular repetitive series of separate blocks, or somites—cohesive groups of cells, separated by clefts (Figure 21–38A). The somites form (as bilateral pairs) one after another, in a regular rhythm, starting in the region of the head and ending in the tail. Depending on the species, the final number of somites ranges from less than 40 (in a frog or a zebrafish) to more than 300 (in a snake).


Cell_Biology_Alberts

Cell_Biology_Alberts. The posterior, most immature part of the mesodermal slab, called the presomitic mesoderm, supplies the required cells: as the cells proliferate, this mesoderm of oscillation cycle of oscillation cycle tail moves back as new somites form Figure 21–38 Somite formation in the chick embryo. (a) a chick embryo at 40 hours of incubation. (B) how the temporal oscillation of gene expression in the presomitic mesoderm becomes converted into a spatial alternating pattern of gene expression in the formed somites. In the posterior part of the presomitic mesoderm, each cell oscillates with a cycle time of 90 minutes. as cells mature and emerge from the presomitic region, their oscillation is gradually slowed down and finally brought to a halt, leaving them in a state that depends on the phase of the cycle they happen to be in at the critical moment. In this way, a temporal oscillation of gene expression traces out an alternating spatial pattern. (a, from Y.J. Jiang, L. Smithers and


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Cell_Biology_Alberts. J. Lewis, Curr. Biol. 8:r868–r871, 1998. With permission from elsevier.) retreats tailward, extending the embryo (Figure 21–38B). In the process, it deposits a trail of somites formed from cells that group together into blocks as they emerge from the anterior end of the presomitic region. The special character of the presomitic mesoderm is maintained by a combination of fibroblast growth factor (FGF) and Wnt signals, produced by a signaling center at the tail end of the embryo, and the range of these signals seems to define the length of the presomitic mesoderm. The somites emerge with clocklike timing, but what determines the rhythm of the process?