The Three Main Categories of White Blood Cells: Granulocytes, Monocytes, and Lymphocytes

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—on the basis of their appearance in the light microscope.

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 22-30). The differences in staining reflect major differences of chemistry and function. Neutrophils (also called polymorphonuclear leucocytes 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 25. Basophils secrete histamine (and, in some species, serotonin) to help mediate inflammatory reactions; they are closely related in function to mast cells, which reside in connective tissues but are also generated from the hemopoietic stem cells. Eosinophils help to destroy parasites and modulate allergic inflammatory responses.

Figure 22-30

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 24-7. Each of the cell types shown here has a different function, (more...)

Once they leave the bloodstream, monocytes (see Figure 22-30D) 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 (HOCl, the active ingredient in bleach), as well as to a concentrated mixture of lysosomal hydrolases. Macrophages, however, are much larger and longer-lived than neutrophils. They are responsible for recognizing and removing senescent, dead, and damaged cells in many tissues, and they are unique in being able to ingest large microorganisms such as protozoa.

Monocytes also give rise to dendritic cells, such as the Langerhans cells scattered in the epidermis. 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 are instead specialized as presenters of foreign antigens to lymphocytes to trigger an immune response. Langerhans cells, for example, ingest foreign antigens in the epidermis and carry these trophies back to present to lymphocytes in lymph nodes.

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 lymphocytelike 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 shall concentrate mainly on the development of the other blood cells, often referred to collectively as myeloid cells.

The various types of blood cells and their functions are summarized in Table 22-1.

Table 22-1

Blood Cells.

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.

The inflammatory response is complex and is mediated by a variety of signal molecules produced locally by mast cells, nerve endings, platelets, and white blood cells, as well as by the activation of complement (discussed in Chapters 24 and 25). Some of these signal molecules act on nearby capillaries, causing the endothelial cells to adhere less tightly to one another but making their surfaces adhesive to passing white blood cells. The white blood cells are thus caught like flies on flypaper and then can escape from the vessel by squeezing between the endothelial cells and crawling across the basal lamina with the aid of digestive enzymes. The initial binding to endothelial cells is mediated by homing receptors called selectins, and the stronger binding required for the white blood cells to crawl out of the blood vessel is mediated by integrins (see Figure 19-30). Other molecules called chemokines are secreted by damaged or inflamed tissue and local endothelial cells; they act as chemoattractants for specific types of white blood cells, causing these cells 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 22-31).

Figure 22-31

The migration of white blood cells out of the bloodstream during an inflammatory response. The response is initiated by a variety of signal molecules produced locally by cells (mainly in the connective tissue) or by complement activation. Some of these (more...)

Other signal molecules produced in the course of an inflammatory response escape into the blood and stimulate the bone marrow to produce more leucocytes and release them into the bloodstream. The bone marrow is the key target for such regulation because, with the exception of lymphocytes and some macrophages, most types of blood cells in adult mammals are generated only in the bone marrow. 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 the process of acclimatization when one goes to live at high altitude, where oxygen is scarce. Thus, blood cell formation, or hemopoiesis, necessarily involves complex controls, by which the production of each type of blood cell is regulated individually to meet changing needs. It is a problem of great medical importance to understand how these controls operate, and much progress has been made in this area in recent years.

In intact animals, hemopoiesis is more difficult to analyze than is cell turnover in a tissue such as the epidermal layer of the skin. In the epidermis there is a simple, regular spatial organization that makes it easy to follow the process of renewal and to locate the stem cells. This is not true of the hemopoietic tissues. However, hemopoietic cells have a nomadic life-style that makes them more accessible to experimental study in other ways. Dispersed hemopoietic cells are easily obtained and can be readily transferred, without damage, from one animal to another. Moreover, the proliferation and differentiation of individual cells and their progeny can be observed and analyzed in culture, and numerous molecular markers distinguish the various stages of differentiation. Because of this, more is known about the molecules that control blood cell production than about those that control cell production in other mammalian tissues.

Bone Marrow Contains Hemopoietic Stem Cells

The different types of blood cells and their immediate precursors can be recognized in the bone marrow by routine staining methods (Figure 22-32). They 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 22-33).

Figure 22-32

Bone marrow. (A) A light micrograph of a stained section. The large empty spaces correspond to fat cells, whose fatty contents have been dissolved away during specimen preparation. The giant cell with a lobed nucleus is a megakaryocyte. (B) A low-magnification (more...)

Figure 22-33

A megakaryocyte among other cells in the bone marrow. Its 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 (more...)

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. The corresponding cells at still earlier stages of development, before any overt differentiation has begun, are confusingly similar in appearance, and although the spatial distribution of cell types has some orderly features, there is no obvious visible characteristic by which the ultimate stem cells can be recognized. To identify and characterize the stem cells, one needs a functional assay, which involves tracing the progeny of single cells. As we shall see, this can be done in vitro simply by examining the colonies that isolated cells produce in culture. The hemopoietic system, however, can also be manipulated so that such clones of cells can be recognized in vivo in the intact animal.

When an animal is exposed to a large dose of x-rays, most of the hemopoietic 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 hemopoietic tissue (Figure 22-34). Experiments of this sort prove that the marrow contains hemopoietic stem cells. They also show how one can assay for the presence of hemopoietic stem cells and hence discover the molecular features that distinguish them from other cells.

Figure 22-34

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.

For this purpose, cells taken from bone marrow are sorted (with the help of 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 hemopoietic stem cells. In this way, it has been possible to show that the hemopoietic stem cells are characterized by a specific combination of cell-surface proteins, and by appropriate sorting virtually pure stem cell preparations can be obtained. The stem cells turn out to be a tiny fraction of the bone-marrow population—about 1 cell in 10,000; but this is enough. As few as five such cells injected into a host mouse with defective hemopoiesis are sufficient to reconstitute its entire hemopoietic system, generating a complete set of blood-cell types, as well as fresh stem cells.

A Multipotent Stem Cell Gives Rise to All Classes of Blood Cells

To see what range of cell types a single hemopoietic stem cell can generate, one needs a way of tracing the fate of its progeny. This can be done by marking the individual stem cell genetically, so that the progeny can be identified even after they have been released into the bloodstream. Although several methods have been used for this, a specially engineered retrovirus (a retroviral vector carrying a marker gene) serves the purpose particularly well. The marker virus, like other retroviruses, can insert its own genome into the chromosomes of the cell it infects, but the genes that would enable it to generate new infectious virus particles have been removed. The marker is therefore confined to the progeny of the cells that were originally infected, and the progeny of one such cell can be distinguished from the progeny of another because the chromosomal sites of insertion of the virus are different. To analyze hemopoietic cell lineages, bone marrow cells are first infected with the retroviral vector in vitro and then are transferred into a lethally irradiated recipient; DNA probes can then be used to trace the progeny of individual infected cells in the various hemopoietic and lymphoid tissues of the host. These experiments show that the individual hemopoietic stem cell is multipotent and can give rise to the complete range of blood cell types, both myeloid and lymphoid, as well as new stem cells like itself (Figure 22-35).

Figure 22-35

A tentative scheme of hemopoiesis. 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 (more...)

The same methods that were developed for experimentation in mice can now be used for the treatment of disease in humans. Mice, we have seen, can be irradiated to kill off their hemopoietic cells, and then rescued by a transfusion of new stem cells. In the same way, patients with leukemia, for example, can be irradiated or chemically treated to destroy their cancerous cells along with the rest of their hemopoietic tissue, and then can be rescued by a transfusion of hemopoietic stem cells that are free of the cancer-causing mutations. These healthy stem cells can be obtained either from an immunologically matched donor, or by sorting from a sample of bone marrow previously taken from the leukemic patient himself or herself.

The same technology also opens the way, in principle, to one form of gene therapy: hemopoietic stem cells can be isolated in culture, genetically modified by DNA transfection or some other technique to introduce a desired gene, and then transfused back into a patient in whom the gene was lacking, to provide a self-renewing source of the missing genetic component.

Commitment Is a Stepwise Process

Hemopoietic 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 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, or perhaps of myeloid cells plus B lymphocytes, and the other to large numbers of all the different types of lymphoid cells, or at least T lymphocytes. Further steps give rise to progenitors committed to the production of just one cell type. The steps of commitment can be correlated with changes in the expression of specific gene regulatory proteins, needed for the production of different subsets of blood cells. These seem to act in a complicated combinatorial fashion: the GATA-1 protein, for example, is needed for the maturation of red blood cells, but is active also at much earlier steps in the hemopoietic pathway.

The meaning of “commitment” in molecular terms is still unclear, but it has at least two aspects: genes for the chosen mode of differentiation begin to be switched on, while access to other developmental pathways is shut off. These processes normally go hand-in-hand, but studies of mutants show that they are in principle distinct. Animals lacking the gene regulatory protein Pax5 have a block in the production of mature B lymphocytes: progenitor cells begin the process leading to B cell restriction but do not complete it. Normal progenitors that have taken the same initial step cannot be induced, no matter what their environment, to differentiate along any other pathway than that of a B lymphocyte. But the Pax5-defective cells show no such restriction: exposure to appropriate conditions can drive them to generate other blood cell types, including T lymphocytes, macrophages, and granulocytes. This is because the Pax5 protein is required not only to activate genes required for B cell development, but also to shut off genes required for development along other blood-cell pathways.

The Number of Specialized Blood Cells Is Amplified by Divisions of Committed Progenitor Cells

Hemopoietic 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 so small a fraction of the total population of hemopoietic cells. For the same reason, a high rate of blood cell production can be maintained even though the stem-cell division rate is low. Infrequent division is a common feature of stem cells in several tissues, including epidermis and gut, as well as the hemopoietic system. The amplifying divisions of the committed progenitors reduce the number of division cycles that the stem cells themselves have to undergo in the course of a lifetime, thereby reducing the risk of replicative senescence (discussed in Chapter 17) and damaging mutations.

The stepwise nature of commitment means that the hemopoietic 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. Many cells normally die at the earlier steps in the pathway as well. Studies in culture provide a way to find out how the proliferation, differentiation, and death of the hemopoietic cells are regulated.

Stem Cells Depend on Contact Signals From Stromal Cells

Hemopoietic cells can survive, proliferate, and differentiate in culture if, and only if, they are provided with specific signal proteins or accompanied by cells that produce these proteins. If deprived of such proteins, the cells die. For long-term maintenance, contact with appropriate supporting cells also seems to be necessary: hemopoiesis can be kept going for months or even years in vitro by culturing dispersed bone-marrow hemopoietic cells on top of a layer of bone-marrow stromal cells, which mimic the environment in intact bone marrow. Such cultures can generate all the types of myeloid cells, and their long-term continuation implies that stem cells, as well as differentiated progeny, are being continually produced.

There has to be some mechanism, in these cultures as well as in vivo, to guarantee that while some stem cell progeny become committed to differentiation, others remain as stem cells. The cell-fate choice seems to be controlled, in part at least, by specific signals generated at contacts with stromal cells. These signals seem to be needed by the hemopoietic stem cells to keep them in their uncommitted state, just as signals from contacts with the basal lamina are needed to maintain stem cells of the epidermis. In both systems, stem cells are normally confined to a particular niche, and when they lose contact with this niche, they tend to lose their stem-cell potential (Figure 22-36).

Figure 22-36

Dependence of hemopoietic stem cells on contact with stromal cells. The cartoon shows a dependence on two signaling mechanisms for which there is some evidence. The real system is certainly more complex; the dependence of hemopoietic cells on contact (more...)

The nature of the critical signals from the stromal cells is not yet certain, although several mechanisms have been implicated, involving both secreted and cell-surface-attached factors. For example, hemopoietic precursor cells, including stem cells, in the bone marrow display the transmembrane receptor Notch1 in their plasma membrane, while marrow stromal cells express Notch ligands, and there is evidence that Notch activation helps to keep the stem cells or progenitor cells from embarking on differentiation (see Chapter 15).

Another contact interaction that is important for the maintenance of hemopoiesis came to light through the analysis of mouse mutants with a curious combination of defects: a shortage of red blood cells (anemia), of germ cells (sterility), and of pigment cells (white spotting of the skin; see Figure 21-81). As discussed in Chapter 21, this syndrome results from mutations in either of two genes: one, called c-kit, codes for a receptor tyrosine kinase; the other, called Steel, codes for its ligand, stem-cell factor (SCF). The cell types affected by the mutations all derive from migratory precursors, and it seems that these precursors in each case must express the receptor (Kit) and be provided with the ligand (SCF) by their environment if they are to survive and produce progeny in normal numbers. Studies in mutant mice suggest that SCF must be membrane-bound to be effective, implying that normal hemopoiesis requires direct cell-cell contact between the hemopoietic cells that express Kit receptor protein and stromal cells that express SCF.

Factors That Regulate Hemopoiesis Can Be Analyzed in Culture

While stem cells depend on contact with stromal cells for long-term maintenance, their committed progeny do not, or not to the same degree. Thus, dispersed bone-marrow hemopoietic cells can be cultured in a semisolid matrix of dilute agar or methylcellulose, and factors derived from other cells can be added artificially to the medium. Because cells in the semisolid matrix cannot migrate, 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 hemopoiesis and hence to purify them and explore their actions. These substances are found to be glycoproteins and are usually called colony-stimulating factors (CSFs). Of the growing number of CSFs that have been defined and purified, some circulate in the blood and act as hormones, while others act in the bone marrow either as secreted local mediators or, like SCF, as membrane-bound signals that act through cell-cell contact. 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.

Erythropoiesis Depends on the Hormone Erythropoietin

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 practically none 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 22-37). The erythrocyte therefore cannot grow or divide; the only possible way of making more erythrocytes is by means of stem cells. Furthermore, erythrocytes have 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 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.

Figure 22-37

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 (more...)

A lack of oxygen or a shortage of erythrocytes stimulates cells in the kidney to synthesize and secrete increased amounts of erythropoietin into the bloodstream. The erythropoietin, in turn, stimulates the production of more erythrocytes. Since a change in the rate of release of new erythrocytes into the bloodstream is observed as early as 1–2 days after an increase in erythropoietin levels in the bloodstream, the hormone must act on cells that are very close precursors of the mature erythrocytes.

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 cell is known as an erythrocyte colony-forming cell, or CFC-E, (or colony-forming unit, CFU-E) and it gives rise to mature erythrocytes after about six division cycles or less. The CFC-Es do not yet contain hemoglobin, and they are derived from an earlier type of progenitor cell whose proliferation does not depend on erythropoietin. CFC-Es themselves depend on erythropoietin for their survival, as well as for proliferation: if erythropoietin is removed from the cultures, the cells rapidly undergo apoptosis.

A second CSF, called interleukin-3 (IL-3), promotes the survival and proliferation of the earlier erythroid progenitor cells. In its presence, much larger erythroid colonies, each comprising up to 5000 erythrocytes, develop from cultured bone marrow cells in a process requiring a week or 10 days. These colonies derive from erythroid progenitor cells called erythrocyte burst-forming cells, or BFC-Es (or burst-forming units, BFU-E). A BFC-E is distinct from a multipotent stem cell in that it has a limited capacity to proliferate and gives rise to colonies that contain erythrocytes only, even under culture conditions that enable other progenitor cells to give rise to other classes of differentiated blood cells. It is distinct from a CFC-E in that it is insensitive to erythropoietin, and its progeny must go through as many as 12 divisions before they become mature erythrocytes (for which erythropoietin must be present). Thus, the BFC-E is thought to be a progenitor cell committed to erythrocyte differentiation and an early ancestor of the CFC-E.

Multiple CSFs Influence the Production of Neutrophils and Macrophages

The two professional phagocytic cells, 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.

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. IL-3 is one of the least specific of the factors, acting on multipotent stem cells as well as on most classes of committed progenitor cells, including GM progenitor cells. Various other factors act more selectively on committed GM progenitor cells and their differentiated progeny (Table 22-2), although in many cases they act on certain other branches of the hemopoietic family tree as well.

Table 22-2

Some Colony-stimulating Factors (CSFs) That Influence Blood Cell Formation.

All of these CSFs, like erythropoietin, are glycoproteins that act at low concentrations (about 10^-12 M) by binding to specific cell-surface receptors, as discussed in Chapter 15. A few of these receptors are transmembrane tyrosine kinases but most belong to the large cytokine receptor family, whose members are usually composed of two or more subunits, one of which is frequently shared among several receptor types (Figure 22-38). 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. Proteins produced artificially from the cloned genes for these factors (sometimes referred to as recombinant proteins because they are made using recombinant DNA technology) are strong stimulators of hemopoiesis in experimental animals. They are now widely used in human patients to stimulate the regeneration of hemopoietic tissue and to boost resistance to infection—an impressive demonstration of how basic cell biological research and animal experiments can lead to better medical treatment.

Figure 22-38

Sharing of subunits among CSF receptors. Human IL-3 receptors and GM-CSF receptors have different α subunits and a common β subunit. Their ligands are thought to bind to the free α subunit with low affinity, and this triggers the (more...)

The Behavior of a Hemopoietic Cell Depends Partly on Chance

Up to this point we have glossed over a central question. 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 hemopoietic 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 hemopoietic lineage to facilitate differentiation; it might act early to influence commitment; or it might simply increase the probability of cell survival (Figure 22-39). By monitoring the fate of isolated individual hemopoietic cells in culture, it has been possible to show that a single CSF, such as GM-CSF, can exert all these effects, although it is still not clear which are most important in vivo.

Figure 22-39

Some of the parameters through which the production of blood cells of a specific type might be regulated. Studies in culture suggest that colony-stimulating factors (CSFs) can affect all of these aspects of hemopoiesis.

Studies in vitro indicate, moreover, that there is a large element of chance in the way a hemopoietic cell behaves. At least some of the CSFs seem to act by regulating probabilities, not by dictating directly what the cell shall do. In hemopoietic cell cultures, even if the cells have been selected to be as homogeneous a population as possible, there is a remarkable variability in the sizes and often in the characters of the colonies that develop. And 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.

Regulation of Cell Survival Is as Important as Regulation of Cell Proliferation

While such observations show that CSFs are not strictly required to instruct the hemopoietic cells how to differentiate or how many times to divide, CSFs are required to keep the cells alive: the default behavior of the cells in the absence of CSFs is death by apoptosis (discussed in Chapter 17). In principle, the CSFs could regulate the numbers of the various types of blood cells entirely through selective control of cell survival in this way, and there is increasing evidence that the control of cell survival plays a central part in regulating the numbers of blood cells, just as it does for hepatocytes and many other cell types, as we have already seen. The amount of apoptosis in the vertebrate hemopoietic system is enormous: billions of neutrophils die in this way each day in an adult human, for example. Indeed, the vast majority of 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.

Too little cell death can be as dangerous to the health of a multicellular organism as too much proliferation. In the hemopoietic system, mutations that inhibit cell death by causing excessive production of the intracellular apoptosis inhibitor Bcl-2 promote the development of cancer in B lymphocytes. Indeed, the capacity for unlimited self-renewal is a dangerous property for any cell to possess, and many cases of leukemia arise through mutations that confer this capacity on committed hemopoietic precursor cells that would normally be fated to differentiate and die after a limited number of division cycles.

Summary

The many types of blood cells, including erythrocytes, lymphocytes, granulocytes, and macrophages, all derive from a common multipotent stem cell. In the adult, hemopoietic stem cells are found mainly in bone marrow, and they depend on contact-mediated signals from the marrow stromal (connective-tissue) cells to maintain their stem-cell character. The stem cells 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.

Studies of hemopoiesis 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.