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Cell_Biology_Alberts. FibRoblaStS and theiR tRanSFoRmationS: the ConneCtive-tiSSue Cell Family From epithelia, with their varied patterns of renewal and their enormous variety of protective, absorptive, secretory, sensory, and biosynthetic functions, we turn now to connective tissues. Connective tissues typically consist of cells dispersed in extracellular matrix that they themselves secrete, as discussed in Chapter 19. They originate from the mesodermal (middle) layer of the early embryo, sandwiched between ectoderm and endoderm (see Chapter 21, Figure 21–3).


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Cell_Biology_Alberts. In the adult body, virtually all epithelia are supported by a connective-tissue bed, or stroma; and specialized types of connective tissue, such as bone, cartilage, and tendon, form the supporting framework of the body as a whole. No less important than its mechanical role, connective tissue also contains the blood vessels that bring the oxygen and nourishment on which all cells depend. Cells of the immune system roam through connective tissue, passing in and out of blood vessels and lymphatics, and providing defence against infection; and through the meshes of connective tissue run peripheral nerves. Also embedded in connective tissue are the muscles that enable us to move. In these many ways, the cells that form connective tissue and synthesize its various types of extracellular matrix contribute to the support and repair of almost every tissue and organ.


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Cell_Biology_Alberts. Connective-tissue cells belong to a family of cell types that are related by origin, and they are often remarkably interconvertible. The family includes fibroblasts, cartilage cells, and bone cells, all of which are specialized for the secretion of collagenous extracellular matrix and are jointly responsible for the architectural framework of the body. The connective-tissue family also includes fat cells (adipocytes) and smooth muscle cells. Figure 22–11 illustrates these cell types and the interconversions that are thought to occur between them. The adaptability of the differentiated character of connective-tissue cells is an important feature of responses to many types of damage. Fibroblasts Change their Character in Response to Chemical and physical Signals


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Cell_Biology_Alberts. Fibroblasts Change their Character in Response to Chemical and physical Signals Fibroblasts seem to be the least specialized cells in the connective-tissue family. They are dispersed in connective tissue throughout the body, where they secrete a nonrigid extracellular matrix that is rich in type I or type III collagen, or both, as discussed in Chapter 19. When a tissue is injured, the fibroblasts nearby proliferate, migrate into the wound (Movie 22.1), and produce large amounts of collagenous matrix that helps to isolate and repair the damaged tissue. Their ability to thrive in the face of injury, together with their solitary lifestyle, may explain why fibroblasts are the easiest of cells to grow in culture—a feature that has made them a favorite subject for cell biological studies.


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Cell_Biology_Alberts. Figure 22–11 The family of connective-tissue cells. arrows show the interconversions that are thought to occur within the family. For simplicity, the fibroblast is shown as a single cell type, but it is uncertain how many types of fibroblasts exist and whether the differentiation potential of different types is restricted in different ways.


Cell_Biology_Alberts

Cell_Biology_Alberts. A class of connective-tissue cells in the bone marrow, called bone marrow stromal cells, provides an example of radical connective-tissue versatility. These cells, which can be regarded as a kind of fibroblast, can be isolated from the bone marrow and propagated in culture. Large clones of progeny can be generated in this way from single ancestral stromal cells. Depending on the culture conditions, the members of such a clone either can continue proliferating to produce more cells of the same type, or can differentiate as fat cells, cartilage cells, or bone cells. The fate of the cells depends on physical as well as chemical signals: embedded in a stiff, unyielding matrix, they tend to turn into bone cells, whereas in a softer, more elastic matrix, they tend to turn into fat cells. This effect is mediated by an intracellular pathway that responds to tension in actin–myosin bundles and relays a signal to specific transcription regulators in the nucleus (Figure 22–12). Because of their


Cell_Biology_Alberts

Cell_Biology_Alberts. is mediated by an intracellular pathway that responds to tension in actin–myosin bundles and relays a signal to specific transcription regulators in the nucleus (Figure 22–12). Because of their self-renewing, multipotent character, the bone marrow stromal cells, and other cells with similar properties, are referred to as mesenchymal stem cells.


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Cell_Biology_Alberts. Cartilage and bone are tissues of very different character; but they are closely related in origin, and the formation of the skeleton depends on an intimate partnership between them.


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Cell_Biology_Alberts. Cartilage tissue is structurally simple, consisting of cells of a single type— chondrocytes—embedded in a more or less uniform, highly hydrated matrix consisting of proteoglycans and type II collagen (discussed in Chapter 19). The cartilage matrix is deformable, and the tissue grows by expanding as the chondrocytes divide and secrete more matrix (Figure 22–13). Bone, by contrast, is dense and rigid; it grows by apposition—that is, by deposition of additional matrix on free surfaces. Like reinforced concrete, the bone matrix is predominantly a mixture of tough fibers (type I collagen fibrils), which resist pulling forces, and solid particles (calcium phosphate as hydroxylapatite crystals), which resist compression. The bone matrix is secreted by osteoblasts that lie at the surface of the existing matrix and deposit fresh layers of bone onto it. Some of the osteoblasts remain free at the surface, while others gradually become embedded in their own secretion. This freshly formed material


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Cell_Biology_Alberts. matrix and deposit fresh layers of bone onto it. Some of the osteoblasts remain free at the surface, while others gradually become embedded in their own secretion. This freshly formed material (consisting chiefly of type I collagen) is rapidly converted into hard bone matrix by the deposition of calcium phosphate crystals in it.


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Cell_Biology_Alberts. Once imprisoned in hard matrix, the original bone-forming cell, now called an osteocyte, has no opportunity to divide, although it continues to secrete additional matrix in small quantities around itself. The osteocyte, like the chondrocyte, occupies a small cavity, or lacuna, in the matrix, but unlike the chondrocyte Figure 22–12 Control of fibroblast differentiation by the physical properties of the extracellular matrix. on a stiff matrix, the cells form strong adhesions, spread out, and tend to turn into bone cells. on a soft matrix, where the cells are unable to form strong anchorages, they fail to spread and tend to differentiate as fat cells. these effects depend on transcription regulators (yap and taZ proteins) that move into the cell nucleus in response to tension developed in actin–myosin bundles in the cytoplasm. (based on S. dupont et al., Nature 474:179–183, 2011.) Figure 22–13 The growth of cartilage.


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Cell_Biology_Alberts. Figure 22–13 The growth of cartilage. the tissue expands as the chondrocytes divide and make more matrix. the freshly synthesized matrix with which each cell surrounds itself is shaded dark green. Cartilage may also grow by recruiting fibroblasts from the surrounding tissue and converting them into chondrocytes. osteoblast osteoid (new, uncalcifed bone matrix) old, calcifed bone matrix it is not isolated from its fellows. Tiny channels, or canaliculi, radiate from each lacuna and contain cell processes from the resident osteocyte, enabling it to form gap junctions with adjacent osteocytes (Figure 22–14). Blood vessels and nerves run through the tissue, keeping the bone cells alive and reacting when the bone is damaged.


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Cell_Biology_Alberts. A mature bone has a complex and beautiful architecture, in which dense plates of compact bone tissue enclose spaces spanned by light frameworks of trabecular bone—a filigree of delicate shafts and flying buttresses of bone tissue, with soft marrow in the interstices (Figure 22–15). The creation, maintenance, and repair of this structure depend not only on the cells of the connective-tissue family that synthesize matrix, but also on a separate class of cells called osteoclasts that degrade it, as we explain below. bone is Continually Remodeled by the Cells within it


Cell_Biology_Alberts

Cell_Biology_Alberts. bone is Continually Remodeled by the Cells within it For all its rigidity, bone is by no means a permanent and immutable tissue. Running through the hard extracellular matrix are channels and cavities occupied by living cells, which account for about 15% of the weight of compact bone. These cells are engaged in an unceasing process of remodeling: while osteoblasts deposit new bone matrix, osteoclasts demolish old bone matrix. This mechanism provides for continuous turnover and replacement of the matrix in the interior of the bone. Osteoclasts (Figure 22–16) are large, multinucleated cells that originate, like macrophages, from hematopoietic stem cells in the bone marrow (discussed later


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Cell_Biology_Alberts. Osteoclasts (Figure 22–16) are large, multinucleated cells that originate, like macrophages, from hematopoietic stem cells in the bone marrow (discussed later Figure 22–14 Deposition of bone matrix by osteoblasts. osteoblasts lining the surface of bone secrete the organic matrix of bone (osteoid) and are converted into osteocytes as they become embedded in this matrix. the matrix calcifies soon after it has been deposited. the osteoblasts themselves are thought to derive from osteogenic stem cells that are closely related to fibroblasts.


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Cell_Biology_Alberts. Figure 22–15 Trabecular and compact bone. (a) low-magnification scanning electron micrograph of trabecular bone in a vertebra of an adult man. the soft marrow tissue has been dissolved away. (b) a slice through the head of the femur, with bone marrow and other soft tissue likewise dissolved away, reveals the compact bone of the shaft and the trabecular bone in the interior. because of the way in which bone tissue remodels itself in response to mechanical load, the trabeculae become oriented along the principle axes of stress within the bone. (a, courtesy of alan boyde; b, from J.b. kerr, atlas of Functional histology. mosby, 1999.) to matrix matrix of osteoclast


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Cell_Biology_Alberts. Figure 22–16 Osteoclasts. (a) drawing of an osteoclast in cross section. this giant, multinucleated cell erodes bone matrix. the “ruffled border” is a site of secretion of acids (to dissolve the bone minerals) and hydrolases (to digest the organic components of the matrix). osteoclasts vary in shape, are motile, and often send out processes to resorb bone at multiple sites. they develop from monocytes and can be viewed as specialized macrophages. (b) an osteoclast on bone matrix, seen by scanning electron microscopy. the osteoclast has been crawling over the matrix, eating it away, and leaving a trail of pits where it has done so. (a, from R.v. krstic,´ ultrastructure of the mammalian Cell: an atlas. berlin: Springer-verlag, 1979; b, courtesy of alan boyde.) in this chapter). The precursor cells are released into the bloodstream and collect at sites of bone resorption, where they fuse to form the multinucleated osteoclasts, which cling to surfaces of the bone matrix and eat it away.


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Cell_Biology_Alberts. cells are released into the bloodstream and collect at sites of bone resorption, where they fuse to form the multinucleated osteoclasts, which cling to surfaces of the bone matrix and eat it away. Osteoclasts are capable of tunneling deep into the substance of compact bone, forming cavities that are then invaded by other cells. A blood capillary grows down the center of such a tunnel, and the walls of the tunnel become lined with a layer of osteoblasts (Figure 22–17). These osteoblasts lay down concentric layers of new matrix, which gradually fill the cavity, leaving only a narrow canal surrounding the new blood vessel. At the same time as some tunnels are filling up with bone, others are being bored by osteoclasts, cutting through older concentric systems.


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Cell_Biology_Alberts. Figure 22–17 The remodeling of compact bone. osteoclasts acting together in a small group excavate a tunnel through the old bone, advancing at a rate of about 50 μm per day. osteoblasts enter the tunnel behind them, line its walls, and begin to form new bone, depositing layers of matrix at a rate of 1–2 μm per day. at the same time, a capillary sprouts down the center of the tunnel. the tunnel eventually becomes filled with concentric layers of new bone, with only a narrow central canal remaining. each such canal, besides providing a route of access for osteoclasts and osteoblasts, contains one or more blood vessels that transport the nutrients the bone cells require for survival. typically, about 5–10% of the bone in a healthy adult mammal is replaced in this way each year. (after Z.F.G. Jaworski, b. duck and G. Sekaly, J. Anat. 133:397–405, 1981. with permission from blackwell publishing.) osteoclasts are Controlled by Signals From osteoblasts


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Cell_Biology_Alberts. G. Sekaly, J. Anat. 133:397–405, 1981. with permission from blackwell publishing.) osteoclasts are Controlled by Signals From osteoblasts The osteoblasts that make the matrix also produce the signals that recruit and activate the osteoclasts to degrade it. Disturbance of the balance can lead to osteoporosis, where there is excessive erosion of the bone matrix and weakening of the bone, or to the opposite condition, osteopetrosis, where the bone becomes excessively thick and dense. Hormonal signals have powerful effects on this balance. Chronic use of corticosteroid drugs, for example, can cause osteoporosis as a side effect; but this can be treated by other drugs that redress the balance, including agents that block the factors that osteoblasts secrete to recruit osteoclasts.


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Cell_Biology_Alberts. Local controls allow bone to be deposited in one place while it is resorbed in another. Through such controls over the process of remodeling, bones are endowed with a remarkable ability to adjust their structure in response to longterm variations in the load imposed on them. It is this that makes orthodontics possible, for example: a steady force applied to a tooth with a brace will cause it to move gradually, over many months, through the bone of the jaw, by remodeling of the bone tissue ahead of it and behind it.


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Cell_Biology_Alberts. Bone can also undergo much more rapid and dramatic reconstruction when the need arises. Some cells capable of forming new cartilage persist in the connective tissue that surrounds a bone. If the bone is broken, the cells in the neighborhood of the fracture repair it by a process that resembles the way bones develop in the embryo: cartilage is first laid down to bridge the gap and is then replaced by bone. The capacity for self-repair, so strikingly illustrated by the tissues of the skeleton, is a property of living structures that has no parallel among present-day man-made objects.


Cell_Biology_Alberts

Cell_Biology_Alberts. The family of connective-tissue cells includes fibroblasts, cartilage cells, bone cells, fat cells, and smooth muscle cells. Some classes of fibroblasts, such as the mesenchymal stem cells of bone marrow, seem to be able to transform into any of the other members of the family. These transformations of connective-tissue cell type are regulated by the composition of the surrounding extracellular matrix, by cell shape, and by hormones and growth factors. Cartilage and bone both consist of cells and solid matrix that the cells secrete around themselves—chondrocytes in cartilage, osteoblasts in bone (osteocytes being osteoblasts that have become trapped within the bone matrix). The matrix of cartilage is deformable so that the tissue can grow by swelling, whereas bone is rigid and can grow only by apposition. While osteoblasts secrete bone matrix, they also produce signals that recruit monocytes from the circulation to become osteoclasts, which degrade bone matrix. Through the activities


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Cell_Biology_Alberts. by apposition. While osteoblasts secrete bone matrix, they also produce signals that recruit monocytes from the circulation to become osteoclasts, which degrade bone matrix. Through the activities of these antagonistic classes of cells, bone undergoes a perpetual remodeling through which it can adapt to the load it bears and alter its density in response to hormonal signals. Moreover, adult bone retains an ability to repair itself if fractured, by reactivation of the mechanisms that governed its embryonic development: cells in the neighborhood of the break convert into cartilage, which is later replaced by bone.


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Cell_Biology_Alberts. The term “muscle” includes many cell types, all specialized for contraction but in other respects dissimilar. As noted in Chapter 16, all eukaryotic cells possess a contractile system involving actin and myosin, but muscle cells have developed this apparatus to a high degree. Mammals possess four main categories of cells specialized for contraction: skeletal muscle cells, heart (cardiac) muscle cells, smooth muscle cells, and myoepithelial cells (Figure 22–18). These differ in function, structure, and development. Although all of them generate contractile forces by using organized filament systems based on actin and myosin II, the actin and myosin molecules employed have somewhat different amino acid sequences, are bundle of smooth muscle cells differently arranged in the cell, and are associated with different sets of proteins that control contraction.


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Cell_Biology_Alberts. We focus in this section on skeletal muscle cells, which are responsible for practically all movements that are under voluntary control. These cells can be very large (2–3 cm long and 100 μm in diameter in an adult human) and are often called muscle fibers because of their highly elongated shape. Each one is a syncytium, containing many nuclei within a common cytoplasm. In an intact muscle, they are bundled tightly together, with fibroblasts (and some fat cells) in the interstices between them and blood vessels and nerve fibers running through the tissue. The mechanisms of muscle contraction were discussed in Chapter 16. Here we consider the unusual strategy by which the multinucleate skeletal muscle cells are generated and maintained. myoblasts Fuse to Form new Skeletal muscle Fibers


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Cell_Biology_Alberts. myoblasts Fuse to Form new Skeletal muscle Fibers During development, certain cells, originating from the somites of a vertebrate embryo at a very early stage, become determined as myoblasts, the precursors of skeletal muscle fibers. After a period of proliferation, the myoblasts undergo a dramatic change of state: they stop dividing, switch on the expression of a whole battery of muscle-specific genes required for terminal differentiation, and fuse with one another to form multinucleate skeletal muscle fibers (Figure 22–19). Once differentiation and cell fusion have occurred, the cells do not divide and the nuclei never again replicate their DNA.


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Cell_Biology_Alberts. Figure 22–18 The four classes of muscle cells of a mammal. (a) Schematic drawings (to scale). (b–e) Scanning electron micrographs. Skeletal muscle fibers (b, from a hamster) are giant cells with many nuclei and are formed by cell fusion. the other types of muscle cells are more conventional, generally having only a single nucleus. heart muscle cells (C, from a rat) resemble skeletal muscle fibers in that their actin and myosin filaments are aligned in very orderly arrays to form a series of contractile units called sarcomeres, so that the cells have a striated (striped) appearance. the arrows in (C) point to intercalated discs—end-toend junctions between the heart muscle cells; skeletal muscle cells in long muscles are joined end-to-end in a similar way. Smooth muscle cells (d, from the urinary bladder of a guinea-pig) are so named because they do not appear striated; they belong to the connective-tissue family and are closely related to fibroblasts. note that the smooth muscle is


Cell_Biology_Alberts

Cell_Biology_Alberts. urinary bladder of a guinea-pig) are so named because they do not appear striated; they belong to the connective-tissue family and are closely related to fibroblasts. note that the smooth muscle is shown here at a lower magnification than the other muscle types. the functions of smooth muscle vary greatly, from propelling food along the digestive tract to erecting hairs in response to cold or fear. myoepithelial cells (e, from a secretory alveolus of a lactating rat mammary gland) also have no striations, but unlike all other muscle cells they lie in epithelia and are derived from the ectoderm. they form the dilator muscle of the eye’s iris and serve to expel saliva, sweat, and milk from the corresponding glands. (b, courtesy of Junzo desaki; C, from t. Fujiwara, in Cardiac muscle in handbook of microscopic anatomy [e.d. Canal, ed.]. berlin: Springer-verlag, 1986; d, courtesy of Satoshi nakasiro; e, from t. nagato et al., Cell Tissue Res. 209:1–10, 1980. with permission from


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Cell_Biology_Alberts. in handbook of microscopic anatomy [e.d. Canal, ed.]. berlin: Springer-verlag, 1986; d, courtesy of Satoshi nakasiro; e, from t. nagato et al., Cell Tissue Res. 209:1–10, 1980. with permission from Springer-verlag.)


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Cell_Biology_Alberts. Some myoblasts persist as Quiescent Stem Cells in the adult Even though humans do not normally generate new skeletal muscle fibers in adult life, they still have the capacity to do so, and existing muscle fibers can resume growth when the need arises. Cells capable of serving as myoblasts are retained as small, flattened, and inactive cells lying in close contact with the mature muscle cell and contained within its sheath of basal lamina (Figure 22–20). If the muscle is damaged or stimulated to grow, these satellite cells are activated to proliferate, and their progeny can fuse to repair the damaged muscle or to allow muscle growth. Satellite cells, or some subset of the satellite cells, are thus the stem cells of adult skeletal muscle, normally held in reserve in a quiescent state but available when needed as a self-renewing source of terminally differentiated cells.


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Cell_Biology_Alberts. The process of muscle repair by means of satellite cells is, however, limited in what it can achieve. In one form of muscular dystrophy, for example, a genetic defect in the cytoskeletal protein dystrophin damages differentiated skeletal muscle cells. As a result, satellite cells proliferate to repair the damaged muscle fibers. This regenerative response is, however, unable to keep pace with the damage, and connective tissue eventually replaces the muscle cells, blocking any further possibility of regeneration. A decline of capacity for repair likewise contributes to the weakening of muscle in the elderly. satellite cell activated to divide Figure 22–19 Myoblast fusion in culture.


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Cell_Biology_Alberts. satellite cell activated to divide Figure 22–19 Myoblast fusion in culture. the culture is stained with a fluorescent antibody (green) against skeletal muscle myosin, which marks differentiated muscle cells, and with a dna-specific dye (blue) to show cell nuclei. (a) a short time after a change to a culture medium that favors differentiation, just two of the many myoblasts in the field of view have switched on myosin production and have fused to form a muscle cell with two nuclei (upper right). (b) Somewhat later, almost all the cells have differentiated and fused. (C) high-magnification view, showing characteristic striations (fine transverse stripes) in two of the multinucleate muscle cells. (Courtesy of Jacqueline Gross and terence partridge.) damage to muscle fber


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Cell_Biology_Alberts. Figure 22–20 Satellite cells repair skeletal muscle fibers. (a) the specimen is stained with an antibody (red) against a muscle cadherin, m-cadherin, which is present on both the satellite cell and the muscle fiber and is concentrated at the site where their membranes are in contact. the nuclei of the muscle fiber are stained green, and the nucleus of the satellite cell is stained blue. (b) Schematic of the repair of a damaged muscle fiber by proliferation and fusion of satellite cells. (a, courtesy of terence partridge.)


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Cell_Biology_Alberts. Skeletal muscle fibers are one of four main categories of vertebrate cells specialized for contraction, and they are responsible for all voluntary movement. Each skeletal muscle fiber is a syncytium and develops by the fusion of many myoblasts. Myoblasts proliferate extensively, but once they have fused, they can no longer divide. Fusion generally follows the onset of myoblast differentiation, in which many genes encoding muscle-specific proteins are switched on coordinately. Some myoblasts persist in a quiescent state as satellite cells in adult muscle; when a muscle is damaged, these cells are reactivated to proliferate and to fuse in order to replace the muscle cells that have been lost. They are the stem cells of skeletal muscle, and exhaustion of their regenerative capacity is responsible for some forms of muscular dystrophy as well as for the decline of muscle mass in old age. blood veSSelS, lymphatiCS, and endothelial CellS


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Cell_Biology_Alberts. blood veSSelS, lymphatiCS, and endothelial CellS Almost all tissues depend on a blood supply, and the blood supply depends on endothelial cells, which form the linings of the blood vessels. Endothelial cells have a remarkable capacity to adjust their number and arrangement to suit local requirements. They create an adaptable life-support system, extending by cell migration into almost every region of the body. If it were not for endothelial cells extending and remodeling the network of blood vessels, tissue growth and repair would be impossible. Cancerous tissue is as dependent on a blood supply as is normal tissue, and this has led to a surge of interest in endothelial cell biology, in the hope that it may be possible to block the growth of tumors by attacking the endothelial cells that bring them nourishment.


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Cell_Biology_Alberts. The largest blood vessels are arteries and veins, which have a thick, tough wall of connective tissue and many layers of smooth muscle cells (Figure 22–21). The inner wall is lined by an exceedingly thin single sheet of endothelial cells, the endothelium, separated from the surrounding outer layers by a basal lamina. The amounts of connective tissue and smooth muscle in the vessel wall vary according to the vessel’s diameter and function, but the endothelial lining is always present. In the finest branches of the vascular tree—the capillaries and sinusoids—the walls consist of nothing but endothelial cells and a basal lamina (Figure 22–22), together with a few scattered pericytes. Related to vascular smooth muscle cells, pericytes wrap themselves around the small vessels and strengthen them (Figure 22–23). basal lamina nucleus of endothelial cell basal lamina lumen of capillary tight junction 2 µm loose connective elastic lamina


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Cell_Biology_Alberts. basal lamina nucleus of endothelial cell basal lamina lumen of capillary tight junction 2 µm loose connective elastic lamina Figure 22–21 Diagram of a small artery in cross section. the endothelial cells form the endothelial lining, which although inconspicuous, is the fundamental component. Compare with the capillary in Figure 22–22. Figure 22–22 Capillaries. electron micrograph (left) of a cross section of a small capillary in the pancreas. the wall is formed by a single endothelial cell surrounded by a basal lamina, as seen most clearly in the drawing to the right. (From R.p. bolender, J. Cell Biol. 61:269– 287, 1974. with permission from the Rockefeller university press.)


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Cell_Biology_Alberts. Less obvious than the blood vessels are the lymphatic vessels. These carry no blood and have much thinner and more permeable walls than the blood vessels. They provide a drainage system for the fluid (lymph) that seeps out of the blood vessels, as well as an exit route for white blood cells that have migrated from blood vessels into the tissues. Less happily, they can also provide the path by which cancer cells escape from a primary tumor to invade other tissues. The lymphatics form a branching system of tributaries, all ultimately discharging into a single large lymphatic vessel, the thoracic duct, which opens into a large vein close to the heart. Like blood vessels, lymphatics are lined with endothelial cells.


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Cell_Biology_Alberts. Thus, endothelial cells line the entire blood and lymphatic vascular system, from the heart to the smallest capillary, and they control the passage of materials—and the transit of white blood cells—into and out of the bloodstream. Arteries, veins, capillaries, and lymphatics all develop from small vessels constructed primarily of endothelial cells and a basal lamina: connective tissue and smooth muscle are added later where required, under the influence of signals from the endothelial cells. To understand how the vascular system comes into being and how it adapts to the changing needs of tissues, we have to understand endothelial cells. How do they become so widely distributed, and how do they form channels that connect in just the right way for blood to circulate through the tissues and for lymph to drain back to the bloodstream?


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Cell_Biology_Alberts. Endothelial cells originate at specific sites in the early embryo from precursors that also give rise to blood cells. From these sites, the early embryonic endothelial cells migrate, proliferate, and differentiate to form the first rudiments of blood vessels—a process called vasculogenesis. Subsequent growth and branching of the vessels throughout the body occurs mainly by proliferation and movement of the endothelial cells of these first vessels, in a process called angiogenesis.


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Cell_Biology_Alberts. Angiogenesis occurs in a broadly similar way in the young organism as it grows and in the adult during tissue repair and remodeling. We can watch the behavior of the cells in naturally transparent structures, such as the cornea of the eye or the fin of a tadpole, or in tissue culture, or in the embryo. The embryonic retina, which blood vessels invade according to a predictable timetable, provides a convenient example for experimental study. Each new vessel originates as a capillary sprout from the side of an existing capillary or small venule (Figure 22–24). At the tip of the sprout, leading the way, is an endothelial cell with a distinctive character. This tip cell has a pattern of gene expression somewhat different from that of the endothelial stalk cells following behind it, and while they divide, it does not. The tip cell’s most striking feature is that it puts out many long filopodia, resembling


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Cell_Biology_Alberts. Figure 22–24 Angiogenesis. (a) a new blood capillary forms by the sprouting of an endothelial cell from the wall of an existing small vessel. an endothelial tip cell, with many filopodia, leads the advance of each capillary sprout. the endothelial stalk cells trailing behind the tip cell become hollowed out to form a lumen. (b) blood capillaries sprouting in the retina of an embryonic mouse that had a red dye injected into the bloodstream, revealing the capillary lumen opening up behind the tip cell (Movie 22.2). (b, from h. Gerhardt et al., J. Cell Biol. 161:1163–1177, 2003. with permission from the author.) pericytes clinging 10 µmto outer face of small blood vessel


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Cell_Biology_Alberts. Figure 22–23 Pericytes. the scanning electron micrograph shows pericytes wrapping their processes around a small blood vessel (a post-capillary venule) in the mammary gland of a cat. pericytes are also present around capillaries, but are much more sparsely distributed there. (From t. Fujiwara and y. uehara, Am. J. Anat. 170:39–54, 1984. with permission from wiley-liss.) those of a neuronal growth cone. The column of stalk cells behind it, meanwhile, becomes hollowed out to form a lumen.


Cell_Biology_Alberts

Cell_Biology_Alberts. The endothelial tip cells that pioneer the growth of normal capillaries not only look like neuronal growth cones, but also respond similarly to signals in the environment. In fact, many of the same guidance molecules are involved, including the netrins, slits, and ephrins mentioned in our account of neural development in the previous chapter. The corresponding receptors are expressed in the tip cells and guide the vascular sprouts along specific pathways in the embryo, often in parallel with nerves. Perhaps the most important of the guidance molecules for endothelial cells, however, is one that is chiefly dedicated to the control of vascular development: vascular endothelial growth factor, or VEGF.


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Cell_Biology_Alberts. Almost every cell, in almost every tissue of a vertebrate, is located within 50–100 μm of a blood capillary. What mechanism ensures that the system of blood vessels branches into every nook and cranny? How is it adjusted so perfectly to the local needs of the tissues, not only during normal development but also in pathological circumstances? Wounding, for example, induces a burst of capillary growth in the neighborhood of the damage, to satisfy the high metabolic requirements of the repair process (Figure 22–25). Local irritants and infections also cause a proliferation of new capillaries, most of which regress and disappear when the inflammation subsides. Less benignly, a small sample of tumor tissue implanted in the cornea, which normally lacks blood vessels, causes blood vessels to grow quickly toward the implant from the vascular margin of the cornea; the growth rate of the tumor increases abruptly as soon as the vessels reach it.


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Cell_Biology_Alberts. In all these cases, the invading endothelial cells respond to signals produced by the tissue that they invade. The signals are complex, but a key part is played by vascular endothelial growth factor (VEGF). The regulation of blood vessel growth to match the needs of the tissue depends on the control of VEGF production, through changes in the stability of its mRNA and in its rate of transcription. The latter control is relatively well understood. A shortage of oxygen, in practically any type of cell, causes an increase in the intracellular level of a transcription factor called hypoxia-inducible factor 1α (HIF1α). HIF1α stimulates transcription of Vegf (and of other genes whose products are needed when oxygen is in short supply). The VEGF protein is secreted, diffuses through the tissue, and acts on nearby endothelial cells, stimulating them to proliferate, to produce proteases to help them digest their way through the basal lamina of the parent capillary or venule, and to form


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Cell_Biology_Alberts. and acts on nearby endothelial cells, stimulating them to proliferate, to produce proteases to help them digest their way through the basal lamina of the parent capillary or venule, and to form sprouts. The tip cells of the sprouts detect the VEGF gradient and move toward its source. As the new vessels form, bringing blood to the tissue, the oxygen concentration rises. The HIF1α activity then declines, VEGF production is shut off, and angiogenesis comes to a halt (Figure 22–26).


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Cell_Biology_Alberts. Figure 22–25 New capillary formation in response to wounding. Scanning electron micrographs of casts of the system of blood vessels surrounding the margin of the cornea show the reaction to wounding. the casts are made by injecting a resin into the vessels and letting the resin set; this reveals the shape of the lumen, as opposed to the shape of the cells. Sixty hours after wounding, many new capillaries have begun to sprout toward the site of injury, which is just above the top of the picture. their oriented outgrowth reflects a chemotactic response of the endothelial cells to an angiogenic factor released at the wound. (Courtesy of peter C. burger.) Figure 22–26 The regulatory mechanism controlling blood vessel growth according to a tissue’s need for oxygen. lack of oxygen triggers the secretion of veGF, which stimulates angiogenesis. Signals from endothelial Cells Control Recruitment of pericytes and Smooth muscle Cells to Form the vessel wall


Cell_Biology_Alberts

Cell_Biology_Alberts. The vascular network is continually remodeled as it grows and adapts. A newly formed vessel may enlarge; or it may sprout side branches; or it may regress. Smooth muscle and other connective-tissue cells that pack themselves around the endothelium (see Figure 22–23) help to stabilize vessels as they enlarge. This process of vessel wall formation begins with recruitment of pericytes. Small numbers of these cells travel outward in company with the stalk cells of each endothelial sprout. The recruitment and proliferation of pericytes and smooth muscle cells to form a vessel wall depend on platelet-derived growth factor-B (PDGF-B) secreted by the endothelial cells and on PDGF receptors in the pericytes and smooth muscle cells. In mutants lacking this signal protein or its receptor, these vessel wall cells are missing in many regions. As a result, the embryonic blood vessels develop microaneurysms—microscopic pathological dilatations—that eventually rupture, as well as other abnormalities,


Cell_Biology_Alberts

Cell_Biology_Alberts. wall cells are missing in many regions. As a result, the embryonic blood vessels develop microaneurysms—microscopic pathological dilatations—that eventually rupture, as well as other abnormalities, reflecting the importance of signals exchanged in both directions between the exterior cells of the wall and the endothelial cells.


Cell_Biology_Alberts

Cell_Biology_Alberts. Endothelial cells are the fundamental elements of the vascular system. They form a single cell layer that lines all blood vessels and lymphatics and regulates exchanges between the bloodstream and the surrounding tissues. New vessels originate as endothelial sprouts from the walls of existing small vessels. A specialized motile endothelial tip cell at the leading edge of each sprout puts out filopodia that respond to gradients of guidance molecules in the environment, leading the growth of the sprout in much the same way as the growth cone of a neuron is led. The endothelial stalk cells following behind become hollowed out to form a capillary tube. Signals from endothelial cells organize the growth and development of the connective-tissue cells that form the surrounding layers of the vessel wall.


Cell_Biology_Alberts

Cell_Biology_Alberts. A homeostatic mechanism ensures that blood vessels permeate every region of the body. Cells that are short of oxygen increase their concentration of hypoxia-inducible factor 1α (HIF1α), which stimulates the production of vascular endothelial growth factor (VEGF). VEGF acts on endothelial cells, causing them to proliferate and invade the hypoxic tissue to supply it with new blood vessels. As new vessels enlarge, they recruit increasing numbers of pericytes—cells that cling to the outside of the endothelial tube and mature into the smooth muscle coat that is needed to give the vessel strength. a hieRaRChiCal Stem-Cell SyStem: blood Cell FoRmation


Cell_Biology_Alberts

Cell_Biology_Alberts. The function of blood vessels is to carry blood, and it is to blood itself that we now turn. Blood contains many types of cells, with functions that range from the transport of oxygen to the production of antibodies. Some of these cells stay within the vascular system, while others use the vascular system only as a means of transport and perform their function elsewhere. All blood cells, however, have certain similarities in their life history. They all have limited life-spans and are produced throughout the life of the animal. Most remarkably, they are all generated ultimately from a common stem cell, located (in adult humans) in the bone marrow. This hematopoietic (blood-making) stem cell is thus multipotent, giving rise to all the types of terminally differentiated blood cells as well as some other types of cells, such as the osteoclasts in bone, as mentioned earlier. The hematopoietic system is the most complex of the stem-cell systems in the mammalian body, and it is


Cell_Biology_Alberts

Cell_Biology_Alberts. as well as some other types of cells, such as the osteoclasts in bone, as mentioned earlier. The hematopoietic system is the most complex of the stem-cell systems in the mammalian body, and it is exceptionally important in medical practice.


Cell_Biology_Alberts

Cell_Biology_Alberts. Red blood Cells are all alike; white blood Cells Can be Grouped in three main Classes Blood cells can be classified as red or white. The red blood cells, or erythrocytes, remain within the blood vessels and transport O2 and CO2 bound to hemoglobin. The white blood cells, or leukocytes, combat infection and in some cases phagocytose and digest debris. Leukocytes, unlike erythrocytes, must make their way across the walls of small blood vessels and migrate into tissues to perform their tasks. In addition, the blood contains large numbers of platelets, which are not entire cells but small, detached cell fragments or “minicells” derived from the cortical cytoplasm of large cells called megakaryocytes. Platelets adhere specifically to the endothelial cell lining of damaged blood vessels, where they help to repair breaches and aid in blood clotting.


Cell_Biology_Alberts

Cell_Biology_Alberts. All red blood cells belong in a single class, following the same developmental trajectory as they mature, and the same is true of platelets; but there are many distinct types of white blood cells. White blood cells are traditionally grouped into three major categories—granulocytes, monocytes, and lymphocytes—based on their appearance in the light microscope.


Cell_Biology_Alberts

Cell_Biology_Alberts. Granulocytes contain numerous lysosomes and secretory vesicles (or granules) and are subdivided into three classes according to the morphology and staining properties of these organelles (Figure 22–27). The differences in staining reflect major differences of chemistry and function. Neutrophils (also called polymorphonuclear leukocytes because of their multilobed nucleus) are the most common type of granulocyte; they phagocytose and destroy microorganisms, especially bacteria, and thus have a key role in innate immunity to bacterial infection, as discussed in Chapter 24 (see Movie 16.1). Basophils secrete histamine (and, in some species, serotonin) to help mediate inflammatory reactions; they are closely related to mast cells, which reside in connective tissues but are also generated from the hematopoietic stem cells. Eosinophils help to destroy parasites and modulate allergic inflammatory responses.


Cell_Biology_Alberts

Cell_Biology_Alberts. Once they leave the bloodstream, monocytes (see Figure 22–27D) mature into macrophages, which, together with neutrophils, are the main “professional phagocytes” in the body. As discussed in Chapter 13, both types of phagocytic cells contain specialized lysosomes that fuse with newly formed phagocytic vesicles (phagosomes), exposing phagocytosed microorganisms to a barrage of enzymatically produced, highly reactive molecules of superoxide (O2–) and hypochlorite (ClO–, the active ingredient in bleach), as well as to attack by a concentrated mixture of lysosomal hydrolase enzymes that become activated in the phagosome. Macrophages, however, are much larger and longer-lived than neutrophils. They recognize and remove senescent, dead, and damaged cells in many tissues, and they are unique in being able to ingest large microorganisms such as protozoa.


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 22–27 White blood cells. (a–d) these electron micrographs show (a) a neutrophil, (b) a basophil, (C) an eosinophil, and (d) a monocyte. electron micrographs of lymphocytes are shown in Figure 24–14. each of the cell types shown here has a different function, which is reflected in the distinctive types of secretory granules and lysosomes it contains. there is only one nucleus per cell, but it has an irregular lobed shape, and in (a), (b), and (C) the connections between the lobes are out of the plane of section. (e) a light micrograph of a blood smear stained with the Romanowsky stain, which colors the white blood cells strongly. (Courtesy 2 µm of dorothy bainton.)


Cell_Biology_Alberts

Cell_Biology_Alberts. Romanowsky stain, which colors the white blood cells strongly. (Courtesy 2 µm of dorothy bainton.) Monocytes also give rise to dendritic cells. Like macrophages, dendritic cells are migratory cells that can ingest foreign substances and organisms, but they do not have as active an appetite for phagocytosis and instead have a crucial role as presenters of foreign antigens to lymphocytes to trigger an immune response. Dendritic cells in the epidermis (called Langerhans cells), for example, ingest foreign antigens and carry these trophies back from the skin to present to lymphocytes in lymph nodes.


Cell_Biology_Alberts

Cell_Biology_Alberts. There are two main classes of lymphocytes, both involved in immune responses: B lymphocytes make antibodies, while T lymphocytes kill virus-infected cells and regulate the activities of other white blood cells. In addition, there are lymphocyte-like cells called natural killer (NK) cells, which kill some types of tumor cells and virus-infected cells. The production of lymphocytes is a specialized topic discussed in detail in Chapter 24. Here we concentrate mainly on the development of the other blood cells, often referred to collectively as myeloid cells. Table 22–1 summarizes the various types of blood cells and their functions. the production of each type of blood Cell in the bone marrow is individually Controlled


Cell_Biology_Alberts

Cell_Biology_Alberts. Table 22–1 summarizes the various types of blood cells and their functions. the production of each type of blood Cell in the bone marrow is individually Controlled Most white blood cells function in tissues other than the blood; blood simply transports them to where they are needed. A local infection or injury in any tissue rapidly attracts white blood cells into the affected region as part of the inflammatory response, which helps fight the infection or heal the wound (Movie 22.3).


Cell_Biology_Alberts

Cell_Biology_Alberts. The inflammatory response is complex and is governed by many different signal molecules produced locally by mast cells, nerve endings, platelets, and white blood cells, as well as by the activation of complement (discussed in Chapter 24). Some of these signal molecules act on the endothelial lining of nearby capillaries, helping white blood cells to first stick and then make an exit from the bloodstream into the tissue where they are needed, as described in Chapter 19 (see Figure 19–28 and Movie 19.2). Damaged or inflamed tissues and local endothelial cells secrete other molecules called chemokines, which act as chemoattractants for specific types of white blood cells, causing them to become polarized and crawl toward the source of the attractant. As a result, large numbers of white blood cells enter the affected tissue (Figure 22–28).


Cell_Biology_Alberts

Cell_Biology_Alberts. Other signal molecules produced during an inflammatory response escape into the blood and stimulate the bone marrow to produce more leukocytes and release them into the bloodstream. The regulation tends to be cell-type specific: some bacterial infections, for example, cause a selective increase in neutrophils, while infections with some protozoa and other parasites cause a selective increase in eosinophils. (For this reason, physicians routinely use differential white blood cell counts to aid in the diagnosis of infectious and other inflammatory diseases.) In other circumstances, erythrocyte production is selectively increased—for example, in response to anemia (lack of hemoglobin) due to blood loss, and in the process of acclimatization when one goes to live at high altitude, where oxygen is scarce. Thus, blood cell formation, or hematopoiesis, necessarily involves complex controls, which regulate the production of each type of blood cell individually to meet changing needs.


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 22–28 Chemotaxis of white blood cells to damaged tissue. a chemoattractive signal released from a site of damage, which is toward the bottom of the page, causes white blood cells to exit from the capillary by crawling between adjacent endothelial cells, as shown. megakaryocyte process budding off platelets endothelial cell of sinus wall bone marrow Contains multipotent hematopoietic Stem Cells, able to Give Rise to all Classes of blood Cells


Cell_Biology_Alberts

Cell_Biology_Alberts. In the bone marrow, the developing blood cells and their precursors, including the stem cells, are intermingled with one another, as well as with fat cells and other stromal cells (connective-tissue cells), which produce a delicate supporting meshwork of collagen fibers and other extracellular matrix components. In addition, the whole tissue is richly supplied with thin-walled blood vessels, called blood sinuses, into which the new blood cells are discharged. Megakaryocytes are also present; these, unlike other blood cells, remain in the bone marrow when mature and are one of its most striking features, being extraordinarily large (diameter up to 60 μm) with a highly polyploid nucleus. They normally lie close beside blood sinuses, and they extend processes through holes in the endothelial lining of these vessels; platelets pinch off from the processes and are swept away into the blood (Figure 22–29 and Movie 22.4).


Cell_Biology_Alberts

Cell_Biology_Alberts. Because of the complex arrangement of the cells in bone marrow, it is difficult to identify in ordinary tissue sections any but the immediate precursors of the mature blood cells. There is no obvious visible characteristic by which we can recognize the ultimate stem cells. In the case of hematopoiesis, the stem cells were first identified by a functional assay that exploited the wandering lifestyle of blood cells and their precursors.


Cell_Biology_Alberts

Cell_Biology_Alberts. When an animal is exposed to a large dose of x-rays, most of the hematopoietic cells are destroyed and the animal dies within a few days as a result of its inability to manufacture new blood cells. The animal can be saved, however, by a transfusion of cells taken from the bone marrow of a healthy, immunologically compatible donor. Among these cells there are some that can colonize the irradiated host and permanently reequip it with hematopoietic tissue (Figure 22–30). Such experiments prove that the marrow contains hematopoietic stem cells. They also show how we can assay for the presence of hematopoietic stem cells and hence discover the molecular features that distinguish them from other cells.


Cell_Biology_Alberts

Cell_Biology_Alberts. For this purpose, cells taken from bone marrow are sorted (using a fluorescence-activated cell sorter) according to the surface antigens that they display, and the different fractions are transfused back into irradiated mice. If a fraction rescues an irradiated host mouse, it must contain hematopoietic stem cells. In this way, it has been possible to show that the hematopoietic stem cells are characterized by a specific combination of cell-surface proteins, and by appropriate sorting we can obtain virtually pure stem-cell preparations. The stem cells turn out to be a tiny fraction of the bone marrow population—about 1 cell in 50,000–100,000; but this is enough. A single such cell injected into a host mouse with defective hematopoiesis is sufficient to reconstitute its entire hematopoietic system, generating a complete set of blood cell types, as well as fresh stem cells. This and other experiments (using artificial lineage markers) show that the individual hematopoietic stem cell is


Cell_Biology_Alberts

Cell_Biology_Alberts. system, generating a complete set of blood cell types, as well as fresh stem cells. This and other experiments (using artificial lineage markers) show that the individual hematopoietic stem cell is multipotent and can give rise to the complete range of blood cell types, both myeloid and lymphoid, as well as to new stem cells like itself (Figure 22–31).


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 22–29 A megakaryocyte among other developing blood cells in the bone marrow. the megakaryocyte’s enormous size results from its having a highly polyploid nucleus. one megakaryocyte produces about 10,000 platelets, which split off from long processes that extend through holes in the walls of an adjacent blood sinus. mouse survives; the injected stem cells colonize its hematopoietic tissues and generate a steady supply of new blood cells Figure 22–30 Rescue of an irradiated mouse by a transfusion of bone marrow cells. an essentially similar procedure is used in the treatment of leukemia in human patients by bone marrow transplantation. Commitment is a Stepwise process


Cell_Biology_Alberts

Cell_Biology_Alberts. Commitment is a Stepwise process Hematopoietic stem cells do not jump directly from a multipotent state into a commitment to just one pathway of differentiation; instead, they go through a series of progressive restrictions. The first step, usually, is commitment to either a myeloid or a lymphoid fate. This is thought to give rise to two kinds of progenitor cells, one capable of generating large numbers of all the different types of myeloid cells, and the other giving rise to large numbers of all the different types of lymphoid cells. Further steps give rise to progenitors committed to the production of just one cell type. The steps of commitment correlate with changes in the expression of specific transcription regulators, needed for the production of different subsets of blood cells. divisions of Committed progenitor Cells amplify the number of Specialized blood Cells


Cell_Biology_Alberts

Cell_Biology_Alberts. Hematopoietic progenitor cells generally become committed to a particular pathway of differentiation long before they cease proliferating and terminally differentiate. The committed progenitors go through many rounds of cell division to amplify the ultimate number of cells of the given specialized type. In this way, a single stem-cell division can lead to the production of thousands of differentiated progeny, which explains why the number of stem cells is such a small fraction of the total population of hematopoietic cells. For the same reason, a high rate of blood cell production can be maintained even though the stem-cell division rate is low. The smaller the number of division cycles that the stem cells themselves have to undergo in the course of a lifetime, the lower the risk of generating stem-cell mutations, which would give rise to persistent mutant clones of cells in the body—a particular danger in the hematopoietic system where, as discussed in Chapter 20, a relatively small


Cell_Biology_Alberts

Cell_Biology_Alberts. stem-cell mutations, which would give rise to persistent mutant clones of cells in the body—a particular danger in the hematopoietic system where, as discussed in Chapter 20, a relatively small accumulation of mutations can be sufficient to cause cancer. A low rate of stem-cell division also slows the process of replicative cell senescence (discussed in Chapter 17).


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 22–31 A tentative scheme of hematopoiesis. the multipotent stem cell normally divides infrequently to generate either more multipotent stem cells, which are self-renewing, or committed progenitor cells, which are limited in the number of times that they can divide before differentiating to form mature blood cells. as they go through their divisions, the progenitors become progressively more specialized in the range of cell types that they can give rise to, as indicated by the branching of this cell-lineage diagram. in adult mammals, all of the cells shown develop mainly in the bone marrow—except for t lymphocytes, which as indicated develop in the thymus, and macrophages and osteoclasts, which develop from blood monocytes. Some dendritic cells may also derive from monocytes.


Cell_Biology_Alberts

Cell_Biology_Alberts. The stepwise nature of commitment means that the hematopoietic system can be viewed as a hierarchical family tree of cells. Multipotent stem cells give rise to committed progenitor cells, which are specified to give rise to only one or a few blood cell types. The committed progenitors divide rapidly, but only a limited number of times, before they terminally differentiate into cells that divide no further and die after several days or weeks. Figure 22–31 depicts the hematopoietic family tree. It should be noted, however, that variations are thought to occur: not all stem cells generate the identical patterns of progeny via precisely the same sequence of steps.


Cell_Biology_Alberts

Cell_Biology_Alberts. Like the stem cells of other tissues, hematopoietic stem cells depend on signals from their niche, in this case created by the specialized connective tissue of the bone marrow. (This is the site in adult humans; during development, and in nonhuman mammals such as the mouse, hematopoietic stem cells can also make their home in other tissues—notably liver and spleen.) When they lose contact with their niche, the hematopoietic stem cells tend to lose their stem-cell potential (Figure 22–32). Evidently the loss of potency is not absolute or instantaneous, however, since the stem cells can still survive journeys via the bloodstream to colonize other sites in the body.


Cell_Biology_Alberts

Cell_Biology_Alberts. While the stem cells depend on contact with bone marrow stromal cells for longterm maintenance, their committed progeny do not, or at least not to the same degree. These cells can thus be dispersed and cultured in a semisolid matrix of dilute agar or methylcellulose, and factors derived from other cells can be added artificially to the medium. The semisolid matrix inhibits migration, so that the progeny of each isolated precursor cell remain together as an easily distinguishable colony. A single committed neutrophil progenitor, for example, may give rise to a clone of thousands of neutrophils. Such culture systems have provided a way to assay for the factors that support hematopoiesis and hence to purify them and explore their actions. These substances are glycoproteins and are usually called colony-stimulating factors (CSFs). Some of these factors circulate in the blood and act as hormones, while others act in the bone marrow as secreted local mediators; still others take the form of


Cell_Biology_Alberts

Cell_Biology_Alberts. colony-stimulating factors (CSFs). Some of these factors circulate in the blood and act as hormones, while others act in the bone marrow as secreted local mediators; still others take the form of membrane-bound signals that act through cell– cell contact.


Cell_Biology_Alberts

Cell_Biology_Alberts. An important example of the latter is a protein called Steel or Stem Cell Factor (SCF ). This is expressed both in the bone marrow stroma (where it helps to define the stem-cell niche) and along pathways of migration, and it occurs both in a membrane-bound and a soluble form. It binds to a receptor tyrosine kinase called Kit, and it is required during development for guidance and survival not only of hematopoietic cells but also of other migratory cell types—specifically, germ cells and pigment cells. erythropoiesis depends on the hormone erythropoietin The best understood of the CSFs that act as hormones is the glycoprotein erythropoietin, which is produced in the kidneys and regulates erythropoiesis, the formation of red blood cells, to which we now turn.


Cell_Biology_Alberts

Cell_Biology_Alberts. The erythrocyte is by far the most common type of cell in the blood (see Table 22–1). When mature, it is packed full of hemoglobin and contains hardly any of the usual cell organelles. In an erythrocyte of an adult mammal, even the nucleus, endoplasmic reticulum, mitochondria, and ribosomes are absent, having been extruded from the cell in the course of its development (Figure 22–33). The erythrocyte therefore cannot grow or divide, and it has a limited life-span—about 120 days in humans or 55 days in mice. Worn-out erythrocytes are phagocytosed and digested by macrophages in the liver and spleen, which remove more than 1011


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 22–32 Dependence of hematopoietic stem cells on contact with stromal cells. the contact-dependent interaction between the kit receptor and its ligand is one of several signaling mechanisms thought to be involved in hematopoietic stem-cell maintenance. the real system is certainly more complex. moreover, the dependence of hematopoietic cells on contact with stromal cells cannot be absolute, since small numbers of the functional stem cells can be found free in the circulation. SCF, stem-cell factor. Figure 22–33 A developing red blood cell (erythroblast). the cell is shown extruding its nucleus to become an immature erythrocyte (a reticulocyte), which then leaves the bone marrow and passes into the bloodstream. the reticulocyte will lose its mitochondria and ribosomes within a day or two to become a mature erythrocyte. erythrocyte clones develop in the bone marrow on the surface of a macrophage, which phagocytoses and digests the nuclei discarded by the erythroblasts.


Cell_Biology_Alberts

Cell_Biology_Alberts. senescent erythrocytes in each of us each day. Young erythrocytes actively protect themselves from this fate: they have a protein on their surface that binds to an inhibitory receptor on macrophages and thereby prevents their phagocytosis. A lack of oxygen or a shortage of erythrocytes stimulates specialized cells in the kidney to synthesize and secrete increased amounts of erythropoietin into the bloodstream. The erythropoietin, in turn, boosts the production of erythrocytes. The effect is rapid: the rate of release of new erythrocytes into the bloodstream rises steeply 1–2 days after an increase in erythropoietin levels in the bloodstream. Clearly, the hormone must act on cells that are close precursors of the mature erythrocytes.


Cell_Biology_Alberts

Cell_Biology_Alberts. The cells that respond to erythropoietin can be identified by culturing bone marrow cells in a semisolid matrix in the presence of erythropoietin. In a few days, colonies of about 60 erythrocytes appear, each founded by a single committed erythroid progenitor cell. This progenitor depends on erythropoietin for its survival as well as its proliferation. It does not yet contain hemoglobin, and it is derived from an earlier type of committed erythroid progenitor whose survival and proliferation are governed by other factors.


Cell_Biology_Alberts

Cell_Biology_Alberts. The two classes of cells dedicated to phagocytosis, neutrophils and macrophages, develop from a common progenitor cell called a granulocyte/macrophage (GM) progenitor cell. Like the other granulocytes (eosinophils and basophils), neutrophils circulate in the blood for only a few hours before migrating out of capillaries into the connective tissues or other specific sites, where they survive for only a few days. They then die by apoptosis and are phagocytosed by macrophages. Macrophages, in contrast, can persist for months or perhaps even years outside the bloodstream, where they can be activated by local signals to resume proliferation.


Cell_Biology_Alberts

Cell_Biology_Alberts. At least seven distinct CSFs that stimulate neutrophil and macrophage colony formation in culture have been defined, and some or all of these are thought to act in different combinations to regulate the selective production of these cells in vivo. These CSFs are synthesized by various cell types—including endothelial cells, fibroblasts, macrophages, and lymphocytes—and their concentration in the blood typically increases rapidly in response to bacterial infection in a tissue, thereby increasing the number of phagocytic cells released from the bone marrow into the bloodstream. The CSFs not only operate on the precursor cells to promote the production of differentiated progeny, they also activate the specialized functions (such as phagocytosis and target-cell killing) of the terminally differentiated cells. CSFs can be synthesized artificially and are now widely used in human patients to stimulate the regeneration of hematopoietic tissue and to boost resistance to infection.


Cell_Biology_Alberts

Cell_Biology_Alberts. the behavior of a hematopoietic Cell depends partly on Chance CSFs are defined as factors that promote the production of colonies of differentiated blood cells. But precisely what effect does a CSF have on an individual hematopoietic cell? The factor might control the rate of cell division or the number of division cycles that the progenitor cell undergoes before differentiating; it might act late in the hematopoietic lineage to facilitate differentiation; it might act early to influence commitment; or it might simply increase the probability of cell survival (Figure 22–34). By monitoring the fate of isolated individual hematopoietic cells in culture, it has been possible to show that a single CSF, such as granulocyte/ 1. Frequency of stem-cell division 2. Probability of stem-cell death 3. Probability that stem-cell daughter will become a committed progenitor cell of the given type 4. Division cycle time of committed progenitor cell 5. Probability of progenitor-cell death 6.


Cell_Biology_Alberts

Cell_Biology_Alberts. Probability that stem-cell daughter will become a committed progenitor cell of the given type 4. Division cycle time of committed progenitor cell 5. Probability of progenitor-cell death 6. Number of committed progenitorcell divisions before terminal differentiation 7. Lifetime of differentiated cells macrophage CSF, can exert all these effects, although it is still not clear which are most important in vivo.


Cell_Biology_Alberts

Cell_Biology_Alberts. Studies in vitro indicate, moreover, that there is a large element of chance in the way a hematopoietic cell behaves—a reflection, presumably, of “noise” in the genetic control system, as discussed in Chapters 7 and 8. If two sister cells are taken immediately after a cell division and cultured apart under identical conditions, they frequently give rise to colonies that contain different types of blood cells or the same types of blood cells in different numbers. Thus, both the programming of cell division and the process of commitment to a particular path of differentiation seem to involve random events at the level of the individual cell, even though the behavior of the multicellular system as a whole is regulated in a reliable way. The sequence of cell fate restrictions shown earlier, in Figure 22–31, conveys the impression of a program executed with computer-like logic and precision. Individual cells may be more varied, quirky, and erratic, and may sometimes progress by other


Cell_Biology_Alberts

Cell_Biology_Alberts. in Figure 22–31, conveys the impression of a program executed with computer-like logic and precision. Individual cells may be more varied, quirky, and erratic, and may sometimes progress by other decision pathways from the stem-cell state toward terminal differentiation.


Cell_Biology_Alberts

Cell_Biology_Alberts. Regulation of Cell Survival is as important as Regulation of Cell proliferation The default behavior of hematopoietic cells in the absence of CSFs is death by apoptosis (discussed in Chapter 18), and the control of cell survival plays a central part in regulating the numbers of blood cells. The amount of apoptosis in the vertebrate hematopoietic system is enormous: billions of neutrophils die in this way each day in an adult human, for example. In fact, most neutrophils produced in the bone marrow die there without ever functioning. This futile cycle of production and destruction presumably serves to maintain a reserve supply of cells that can be promptly mobilized to fight infection whenever it flares up, or phagocytosed and digested for recycling when all is quiet. Compared with the life of the organism, the lives of cells are cheap.


Cell_Biology_Alberts

Cell_Biology_Alberts. Too little cell death can be as dangerous to the health of a multicellular organism as too much proliferation. As noted in Chapter 18, mutations that inhibit cell death by causing excessive production of the intracellular apoptosis inhibitor Figure 22–34 Some of the parameters through which the production of blood cells of a specific type might be regulated. Studies in culture suggest that various colony-stimulating factors (CSFs) can affect all of these aspects of hematopoiesis. Bcl2 promote the development of cancer in B lymphocytes. Indeed, the capacity for unlimited self-renewal is a dangerous property for any cell to possess. Many cases of leukemia arise through mutations that confer this capacity on committed hematopoietic precursor cells that would normally be fated to differentiate and die after a limited number of division cycles.


Cell_Biology_Alberts

Cell_Biology_Alberts. The many types of blood cells, including erythrocytes, lymphocytes, granulocytes, and macrophages, all derive from a common multipotent stem cell. In the adult, hematopoietic stem cells are found mainly in bone marrow, and they depend on signals from the marrow stromal (connective-tissue) cells to maintain their stem-cell character. The stem cells are few and far between, and they normally divide infrequently to produce more stem cells (self-renewal) and various committed progenitor cells (transit amplifying cells), each able to give rise to only one or a few types of blood cells. The committed progenitor cells divide extensively under the influence of various protein signal molecules (colony-stimulating factors, or CSFs) and then terminally differentiate into mature blood cells, which usually die after several days or weeks.


Cell_Biology_Alberts

Cell_Biology_Alberts. Studies of hematopoiesis have been greatly aided by in vitro assays in which stem cells or committed progenitor cells form clonal colonies when cultured in a semisolid matrix. The progeny of stem cells seem to make their choices between alternative developmental pathways in a partly random manner. Cell death by apoptosis, controlled by the availability of CSFs, also plays a central part in regulating the numbers of mature differentiated blood cells.


Cell_Biology_Alberts

Cell_Biology_Alberts. As we have seen, many of the tissues of the body are not only self-renewing but also self-repairing, and this is largely thanks to stem cells and the feedback controls that regulate their behavior and maintain homeostasis. There are, however, limits to what these natural repair mechanisms can achieve. In most parts of the human brain, for example, nerve cells that die, as in Alzheimer’s disease, are not replaced. Likewise, when heart muscle dies for lack of oxygen, as in a heart attack, it is replaced by scar tissue rather than new heart muscle.


Cell_Biology_Alberts

Cell_Biology_Alberts. Some animals do far better than humans and can regenerate entire organs, such as whole limbs, after amputation. Among the invertebrates, there are some species that can even regenerate all the tissues of the body from a single somatic cell. These phenomena encourage the hope that human cells might be coaxed by artificial measures into similar feats of repair and regeneration, so as to replace the skeletal muscle fibers that degenerate in victims of muscular dystrophy, the nerve cells that die in patients with Parkinson’s disease, the insulin-secreting cells that are lacking in type 1 diabetics, the heart muscle cells that die in a heart attack, and so on. As we learn more about the basic cell biology, these goals, once only a dream, are beginning to seem attainable.


Cell_Biology_Alberts

Cell_Biology_Alberts. In this section, we start with some examples of the remarkable regenerative abilities of some animal species, as an indication of what is possible in principle. We shall then discuss how we can improve upon the natural repair processes of the human body and treat disease by exploiting the properties of the various types of stem cells found in human tissues. In the final section of the chapter, we shall see how a deeper understanding of the molecular biology of cell differentiation and of stem cells has revealed ways to convert one type of cell into another, opening up radically new possibilities.


Cell_Biology_Alberts

Cell_Biology_Alberts. Schmidtea mediterranea is a small freshwater flatworm, or planarian, just under a centimeter long when grown to full size (Figure 22–35). It has an epidermis, a gut, 0.2 mm pharynx a brain, a pair of primitive eyes, a peripheral nervous system, musculature, and excretory and reproductive organs—most of the basic body parts familiar in other animals, although all relatively simple by vertebrate standards and built from about 20–25 distinct differentiated cell types. For more than a century, planarians such as Schmidtea have intrigued biologists because of their extraordinary capacity for regeneration: a small tissue fragment taken from almost any part of the body will reorganize itself and grow to form a complete new animal. This property goes with another: when the animal is starved, it gets smaller and smaller, by reducing its cell numbers while maintaining essentially normal body proportions. This behavior is called degrowth, and it can continue until the animal is as little as


Cell_Biology_Alberts

Cell_Biology_Alberts. it gets smaller and smaller, by reducing its cell numbers while maintaining essentially normal body proportions. This behavior is called degrowth, and it can continue until the animal is as little as one-twentieth or even a smaller fraction of its full size. Supplied with food, it will grow back to full size again. Cycles of degrowth and growth can be repeated indefinitely, without impairing survival or fertility.


Cell_Biology_Alberts

Cell_Biology_Alberts. Underlying this behavior is a process of continual cell turnover. Along with the differentiated cells, which do not divide, there is a population of small, apparently undifferentiated dividing cells called neoblasts. The neoblasts constitute about 20% of the cells in the body and are widely distributed within it; by cell division, they serve as stem cells for the production of new differentiated cells. Differentiated cells, meanwhile, are continually dying by apoptosis, allowing their corpses to be phagocytosed and digested by neighboring cells. Through this cell cannibalism, the constituents of the dying cells can be efficiently recycled. Cell birth continues in a dynamic balance with cell death and cell cannibalism, no matter whether the animal is fed or starved. In conditions of starvation, the balance is evidently tilted toward cell cannibalism, and in conditions of plenty, toward cell birth.


Cell_Biology_Alberts

Cell_Biology_Alberts. A high dose of x-rays halts all cell division, puts a stop to cell turnover, and destroys the capacity for regeneration. The result is death after a delay of several weeks. The animal can be rescued, however, by injecting into it a single neoblast isolated from an unirradiated donor (Figure 22–36). In a certain proportion of cases, the injected cell divides to form a clone of progeny that eventually repopulate the entire body, creating a healthy regenerative individual with an apparently complete set of differentiated cell types as well as dividing neoblasts. Genetic markers prove that these are all derived from the single neoblast that was injected. It follows that at least some neoblasts are totipotent (or at least highly pluripotent) stem cells; that is, cells able to give rise to all (or at least almost all) of the cell types that make up the body of a flatworm, including more neoblasts like themselves.


Cell_Biology_Alberts

Cell_Biology_Alberts. One might think that such powers of regeneration would be a prerogative of small, simple, primitive animals. But some vertebrates, too, especially fish and amphibians, show remarkable regenerative abilities. A newt, for example, can regenerate


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 22–35 The planarian worm, Schmidtea mediterranea. (a) external view. (b) immunostaining with three different antibodies, revealing the internal anatomy. (a, courtesy of a. Sánchez alvarado; b, from a. Sánchez alvarado, BMC Biol. 10:88, 2012.) injection of a single regeneration of healthy neoblast complete animal a whole amputated limb. In this process, differentiated cells seem to revert to an embryonic character by first forming on the amputation stump a blastema—a small bud resembling an embryonic limb bud. The blastema then grows and its cells differentiate to form a correctly patterned replacement for the limb that has been lost, in what looks like a recapitulation of embryonic limb development (Figure 22–37). A large contribution to the blastema comes from the skeletal muscle cells in the limb stump. These multinucleate cells re-enter the cell cycle, dedifferentiate, and break up into mononucleated cells, which then proliferate within the blastema, before eventually


Cell_Biology_Alberts

Cell_Biology_Alberts. cells in the limb stump. These multinucleate cells re-enter the cell cycle, dedifferentiate, and break up into mononucleated cells, which then proliferate within the blastema, before eventually redifferentiating. But do they redifferentiate only into muscle, or do they behave like neoblasts in the planarian and give rise to the full range of cell types needed to reconstruct the missing part of the limb? Careful lineage tracing, using genetic markers, shows (contrary to previous belief ) that the cells are restricted according to their origins: muscle-derived cells give rise only to muscle, connective-tissue cells only to connective tissues, epidermal cells only to epidermal cells. The cells in the adult vertebrate body are, after all, less adaptable than the cells of the flatworm: by working in concert, they can replace the lost structure, but each cell type is far from totipotent.


Cell_Biology_Alberts

Cell_Biology_Alberts. Why a newt can regenerate a whole limb—as well as many other body parts— but a mammal cannot remains a profound mystery. Stem Cells Can be used artificially to Replace Cells that are diseased or lost: therapy for blood and epidermis Earlier in this chapter, we saw how mice can be irradiated to kill off their hematopoietic cells, and then rescued by a transfusion of new stem cells, which repopulate the bone marrow and restore blood cell production (see Figure 22–30). In the same way, patients with some forms of leukemia or lymphoma can be irradiated or chemically treated to destroy their cancerous cells along with the rest of their hematopoietic tissue, and then can be rescued by a transfusion of healthy, noncancerous hematopoietic stem cells. In favorable cases, these can be sorted out from samples of the patient’s own hematopoietic tissue before it is ablated. They are then transfused back afterward, avoiding problems of immune rejection.


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Cell_Biology_Alberts. Figure 22–36 Regeneration of a planarian from a single somatic cell. (a) the distribution of dividing cells (neoblasts, blue) in the adult body. irradiation blocks all cell division and prevents regeneration, but (b) a single unirradiated neoblast cell injected into the irradiated animal is able to reconstitute all tissues. this eventually produces a complete animal that consists entirely of the progeny of this one cell and can regenerate. (adapted from e.m. tanaka and p.w. Reddien, Dev. Cell 21:172–185, 2011.) Figure 22–37 Newt limb regeneration. the time-lapse sequence shows the stages of regeneration after amputation at the level of the humerus. the sequence spans the events of wound healing, dedifferentiation of stump tissues, blastema formation, and redifferentiation. (Courtesy of Susan bryant and david Gardiner.)


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Cell_Biology_Alberts. Figure 22–38 The continuing production of neurons in an adult mouse brain. the brain is viewed from above, in a cut-away section, to show the region lining the ventricles of the forebrain where neural stem cells are found. these cells continually produce progeny that migrate to the olfactory bulb, where they differentiate as neurons. the constant turnover of neurons in the olfactory bulb is presumably linked in some way to the turnover of the olfactory receptor neurons that project to it from the olfactory epithelium, as mentioned earlier. in adult humans, there is a continuing turnover of neurons in the hippocampus, a region specially concerned with learning and memory. (adapted from b. barres, Cell 97:667–670, 1999. with permission from elsevier.)


Cell_Biology_Alberts

Cell_Biology_Alberts. Another example of the use of stem cells is in the repair of the skin after extensive burns. By culturing cells from undamaged regions of the burned patient’s skin, it is possible to obtain epidermal stem cells quite rapidly in large numbers. These can then be used (through rather long and complicated procedures) to repopulate the damaged body surface. neural Stem Cells Can be manipulated in Culture and used to Repopulate the Central nervous System The central nervous system (the CNS) is the most complex tissue in the body, at an opposite extreme from the epidermis. And yet fish and amphibians can regenerate large parts of the brain, spinal cord, and eyes after they have been cut away. In adult mammals, however, these tissues have very little capacity for self-repair, and stem cells capable of generating new neurons are hard to find—so hard to find, indeed, that for many years they were thought to be absent.


Cell_Biology_Alberts

Cell_Biology_Alberts. We now know, however, that neural stem cells that generate both neurons and glial cells do persist in certain parts of the adult mammalian brain (Figure 22–38). Neuronal turnover occurs on a dramatic scale in certain songbirds’ brains, where large numbers of neurons die each year and are replaced by newborn neurons as part of a process by which the birds refine their song for each new breeding season. In the adult human brain, there is a continuing turnover of neurons in the hippocampus, a region specially concerned with learning and memory. Here, plasticity of adult function is associated with turnover of a specific subset of neurons. About 1400 fresh neurons in this class are generated every day, giving a turnover of 1.75% of the population per year.


Cell_Biology_Alberts

Cell_Biology_Alberts. Fragments taken from self-renewing regions of the adult brain, or from the brain of a fetus, can be dissociated and used to establish cell cultures, where they give rise to floating “neurospheres”—clusters consisting of a mixture of neural stem cells with neurons and glial cells derived from the stem cells. These neurospheres can be propagated through many cell generations, or their cells can be taken at any time and implanted back into the brain of an intact animal. Here they will produce differentiated progeny, in the form of neurons and glial cells.


Cell_Biology_Alberts

Cell_Biology_Alberts. Using slightly different culture conditions, with the right combination of growth factors in the medium, the neural stem cells can be grown as a monolayer and induced to proliferate as an almost pure stem-cell population without attendant differentiated progeny. By a further change in the culture conditions, these cells can be induced at any time to differentiate to give either a mixture of neurons and glial cells (Figure 22–39), or just one of these two cell types, according to the composition of the culture medium.


Cell_Biology_Alberts

Cell_Biology_Alberts. Neural stem cells, whether derived as above or from pluripotent stem cells as described in the next section, can be grafted into an adult brain. Once there, they show a remarkable ability to adjust their behavior to match their new location. Stem cells from the mouse hippocampus, for example, when implanted in the mouse olfactory-bulb-precursor pathway (see Figure 22–38), give rise to neurons that become correctly incorporated into the olfactory bulb. This capacity of neural stem cells and their progeny to adapt to a new environment in animals suggests applications in the treatment for diseases where neurons degenerate, and pure culture of neural stem cells (B) mixture (C) of differentiated neurons (red) and glial cells (green); cell nuclei dissociate cells and dissociate and switch to are blueculture in suspension culture as monolayer medium 3in medium 1 in medium 2 for injuries of the central nervous system. For example, might it be possible to use injected neural stem cells to


Cell_Biology_Alberts

Cell_Biology_Alberts. are blueculture in suspension culture as monolayer medium 3in medium 1 in medium 2 for injuries of the central nervous system. For example, might it be possible to use injected neural stem cells to replace the neurons that die in Parkinson’s disease or to repair accidents that sever the spinal cord?


Cell_Biology_Alberts

Cell_Biology_Alberts. Animals vary in their capacity for regeneration. At one extreme, planarian worms contain stem cells (neoblasts) that support continual turnover of all cell types, and an entire worm can be regenerated from practically any small body fragment or even from a single neoblast cell. Newts can regenerate limbs and other large body parts after amputation, but the cells remain restricted according to their origins: muscle cells in the regenerate derive from muscle, epidermis from epidermis, and so on. In mammals, regeneration is more limited. Nevertheless, it is becoming possible to go beyond the natural limits of wound healing by exploiting stem-cell biology. Thus, certain regions of the nervous system contain stem cells that support production of neurons in these sites throughout life. Neural stem cells can be obtained from these sites or from fetal brains, grown in culture, and then grafted back into other sites in the brain, where they are able to generate neurons appropriate to the new


Cell_Biology_Alberts

Cell_Biology_Alberts. stem cells can be obtained from these sites or from fetal brains, grown in culture, and then grafted back into other sites in the brain, where they are able to generate neurons appropriate to the new location.


Cell_Biology_Alberts

Cell_Biology_Alberts. When cells are transplanted from one site in the mammalian body to another or are removed from the body and maintained in culture, they remain largely faithful to their origins. Each type of specialized cell has a memory of its developmental history and seems fixed in its specialized fate. Some limited transformations can certainly occur, as we saw in our account of the connective-tissue cell family, and some stem cells can generate a variety of differentiated cell types, but the possibilities are restricted. Each type of stem cell serves for the renewal of one particular type of tissue, and the whole pattern of self-renewing and differentiated cells in the adult body is amazingly stable. What, at a fundamental molecular level, is the nature of these stable differences between cell types? Is there any way to override the cell memory mechanisms and force a switch from one state to another that is radically different?


Cell_Biology_Alberts

Cell_Biology_Alberts. We have already discussed these fundamental questions from a general standpoint in Chapter 7. Here we consider them more closely in the context of stem-cell biology, where there has been a recent revolution in our understanding and in our ability to manipulate states of cell differentiation. With further research, these advances would seem to have important practical consequences. Figure 22–39 Neural stem cells. Shown are the steps leading from fetal brain tissue, via neurospheres (a), to a pure culture of neural stem cells (b). these stem cells can be kept proliferating as such indefinitely, or, through a change of medium, can be caused to differentiate (C) into neurons (red) and glial cells (green). neural stem cells with the same properties can also be derived, via a similar series of steps, from embryonic stem (eS) or induced pluripotent stem (ipS) cells (discussed later in this chapter). (micrographs from l. Conti et al., PLoS Biol. 3:1594–1606, 2005.)


Cell_Biology_Alberts

Cell_Biology_Alberts. If we cannot switch the basic character of a specialized cell by changing its environment, can we do so by interfering with its inner workings in a more direct and drastic way? An extreme treatment of this sort is to take the nucleus of the cell and transplant it into the cytoplasm of a large cell of a different type. If the specialized character is defined and maintained by cytoplasmic factors, the transplanted nucleus should switch its pattern of gene expression to conform with that of the host cell. In Chapter 7, we described a famous experiment of this sort, using the frog Xenopus. In this experiment, the nucleus of a differentiated cell (a cell from the lining of a tadpole’s gut) was used to replace the nucleus of an oocyte (an egg-cell precursor arrested in prophase of the first meiotic division, in readiness for fertilization). The resulting hybrid cell went on, in a certain fraction of cases, to develop into a complete normal frog (see Figure 7–2A). This was crucial evidence


Cell_Biology_Alberts

Cell_Biology_Alberts. division, in readiness for fertilization). The resulting hybrid cell went on, in a certain fraction of cases, to develop into a complete normal frog (see Figure 7–2A). This was crucial evidence for what is now a central principle of developmental biology: the cell nucleus, even that of a differentiated cell, contains a complete genome, capable of supporting development of all normal cell types. At the same time, the experiment showed that cytoplasmic factors can indeed reprogram a nucleus: the oocyte cytoplasm can drive the gut cell nucleus back to an early embryonic state, from which it can then step through the changing patterns of gene expression that lead all the way to a complete adult organism.


Cell_Biology_Alberts

Cell_Biology_Alberts. The full story, however, is not quite so simple. First, the reprogramming in such experiments is not perfect. When the transplanted nucleus is taken from a gut cell, for example, a gene that is normally specific to the gut is found to be expressed persistently, even in the muscle cells of the final animal. Second, the experiment succeeds in only a limited proportion of cases, and this success rate becomes lower and lower, the more mature the animal from which the transplanted nucleus is taken: very large numbers of transplantations must be done to score a single success if the nucleus comes from a differentiated cell of an adult frog.


Cell_Biology_Alberts

Cell_Biology_Alberts. Nuclear transplantation can be done in mammals too, with basically similar results. Thus, a nucleus taken from a differentiated cell in the mammary gland of an adult sheep and transplanted into an enucleated sheep’s egg was able to support development of an apparently normal sheep—the famous Dolly. Again, the success rate is low: many transplantations have to be done to obtain one such individual. Reprogramming of a transplanted nucleus involves drastic epigenetic Changes


Cell_Biology_Alberts

Cell_Biology_Alberts. In a typical fully differentiated cell, there seem to be mechanisms maintaining the pattern of gene expression that cytoplasmic factors cannot easily override. An obvious possibility is that the stability of the pattern of gene expression in an adult cell may depend, in part at least, on self-perpetuating modifications of chromatin, as discussed in Chapter 4. As explained in Chapter 7, the phenomenon of X-inactivation in mammals provides a clear example of such epigenetic control. Two X chromosomes exist side by side in each female cell, exposed to the same chemical environment, but while one remains active, the other persists from one cell generation to the next in a condensed inactive state; cytoplasmic factors cannot be responsible for the difference, which must instead reflect mechanisms intrinsic to the individual chromosome. Elsewhere in the genome also, controls at the level of chromatin act in combination with other forms of regulation to govern the expression of each gene.


Cell_Biology_Alberts

Cell_Biology_Alberts. intrinsic to the individual chromosome. Elsewhere in the genome also, controls at the level of chromatin act in combination with other forms of regulation to govern the expression of each gene. Genes can be shut down completely, or switched on constitutively, or maintained in a labile state where they can be readily switched on or off according to changing circumstances.


Cell_Biology_Alberts

Cell_Biology_Alberts. The reprogramming of a nucleus transplanted into an oocyte involves dramatic changes in chromatin. The nucleus swells, increasing its volume 50-fold as the chromosomes decondense; there is a wholesale alteration in patterns of methylation of DNA and histones; the standard histone H1 (the histone that links adjacent nucleosomes) is replaced by a variant form that is peculiar to the oocyte and early embryo; and the preexisting type of histone H3 is also replaced at many sites by a distinct isoform. Evidently, the egg contains factors that reset the state of the chromatin in the nucleus, wiping out old histone modifications on chromatin and imposing new ones. Reprogrammed in this way, the genome becomes competent once again to initiate embryonic development and to give rise to the full range of differentiated cell types. embryonic Stem (eS) Cells Can Generate any part of the body


Cell_Biology_Alberts

Cell_Biology_Alberts. embryonic Stem (eS) Cells Can Generate any part of the body A fertilized egg, or an equivalent cell produced by nuclear transplantation, is a remarkable thing: it can generate a whole new multicellular individual, and that means that it can give rise to every normal type of specialized cell, including even egg or sperm cells for production of the next generation. A cell in such a state is said to be totipotent, and it is said to be pluripotent if it can give rise to most cell types but not absolutely all. Nevertheless, such a progenitor is not a stem cell: it is not self-renewing, but is instead dedicated to a program of progressive differentiation. If it were the only available starting point for study and exploitation of pluripotent cells, the enterprise would require a continual supply of fresh fertilized eggs or fresh nuclear transplantation procedures—an awkward requirement for studies in experimental animals, and unacceptable for practical applications in humans.


Cell_Biology_Alberts

Cell_Biology_Alberts. Here, however, nature has been unexpectedly kind to scientists. It is possible to take an early mouse embryo, at the blastocyst stage, and through cell culture to derive from it a class of stem cells called embryonic stem cells, or ES cells. These originate from the inner cell mass of the early embryo (the cluster of cells that give rise to the body of the embryo proper, as opposed to extraembryonic structures), and they have an extraordinary property: given suitable culture conditions, they will continue proliferating indefinitely and yet retain an unrestricted developmental potential. Their only limitation is that they do not give rise to extraembryonic tissues such as those of the placenta. Thus they are classified as pluripotent, rather than totipotent. But this is a minor restriction. If ES cells are put back into a blastocyst, they become incorporated into the embryo and can give rise to all the tissues and cell types in the body, integrating perfectly into whatever site they


Cell_Biology_Alberts

Cell_Biology_Alberts. If ES cells are put back into a blastocyst, they become incorporated into the embryo and can give rise to all the tissues and cell types in the body, integrating perfectly into whatever site they may come to occupy, and adopting the character and behavior that normal cells would show at that site. They can even give rise to germ cells, from which a new generation of animals can be derived (Figure 22–40).


Cell_Biology_Alberts

Cell_Biology_Alberts. ES cells let us move between cell culture, where we can use powerful techniques for genetic transformation and selection, and the intact organism, where we can discover how such genetic manipulations affect development and physiology. Thus, ES cells have opened the way to efficient genetic engineering in mammals, leading to a revolution in our understanding of mammalian molecular biology. Cells with properties similar to those of mouse ES cells can now be derived from early human embryos and from human fetal germ cells, and even, as we shall explain below, from differentiated cells taken from adult mammalian tissues. In this way, one can obtain a potentially inexhaustible supply of pluripotent