Ma rro w I s I n d iv id u a lly Co n t ro lle d 24 , Most white blood cells function in tissues other than the blood. The blood simply transports them to

22 Ma rro w I s I n d iv id u a lly Co n t ro lle d 24 , Most white blood cells function in tissues other than the blood. The 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 signaling molecules produced locally by mast cells, nerve endings, platelets, and white blood cells, as well as by the activation of complement (discussed in Chapter 23). Some of these signaling molecules act on nearby capillaries, causing the endothelial cells to adhere less tightly to one another but 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 signaling molecules produced locally by mast cells, nerve endings, platelets, and white blood cells, as well as by the activation of complement (discussed in Chapter 23). Some of these signaling molecules act on nearby capillaries, causing the endothelial cells to adhere less tightly to one another but

Other signaling 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, if one goes to live at high altitude, where oxygen is scarce. Thus blood cell formation (hemopoiesis) necessarily involves complex controls in 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 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. On the other hand, the hemopoietic cells have a nomadic life-style that makes them more accessible to experimental study in other ways. Dispersed hemopoietic cells can be easily transferred, without damage, from one animal to another, and the proliferation and differentiation of individual cells and their progeny can be observed and analyzed in culture. 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.

B o n e Ma rro w Co n t a in s He m o p o ie t ic S t e m Ce lls 22 , 25

The different types of blood cells and their immediate precursors can be recognized in the bone marrow by their distinctive appearances (Figure 22-27). They are intermingled with one another, as well as with fat cells and other stromal cells (connective-tissue cells) that 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 mm), 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- 28).

Because of the complex arrangement of the cells in bone marrow, it is difficult to identify 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 there is no visible feature by which the ultimate stem cells can be recognized. To identify and characterize the stem cells, one needs a functional test, 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.

If an animal is exposed to a large dose of x-irradiation, 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 small numbers (about 1 cell in 10,000) that can colonize the irradiated host and permanently reequip it with hemopoietic tissue. One of the tissues where colonies develop is the spleen, which in a normal mouse is an important additional site of hemopoiesis. When the spleen of an irradiated mouse is examined a week or two after the transfusion of cells from a healthy donor, a number of distinct nodules are seen in it, each of which is found to contain a colony of myeloid cells (Figure 22-29); after 2 weeks some colonies may contain more than a million cells. The discreteness of the nodules suggests that each might be a clone of cells descended from a single founder cell, like a bacterial colony on

a culture plate; and with the help of genetic markers, it can be established that this is indeed the case.

The founder of such a colony is called a colony-forming cell, or CFC (also known as a colony- forming unit, CFU). The colony-forming cells are heterogeneous. Some give rise to only one type of myeloid cell, while others give rise to mixtures. Some go through many division cycles and form large colonies, while others divide less and form small colonies. Most of the colonies die out after generating a restricted number of terminally differentiated blood cells. A few of the colonies, however, are capable of extensive self-renewal and produce new colony-forming cells in addition to terminally differentiated blood cells. The founders of such self-renewing colonies are assumed to be the hemopoietic stem cells in the transfused bone marrow.

A P lu rip o t e n t S t e m Ce ll Giv e s Ris e t o All Cla s s e s o f B lo o d