Hematopoiesis Can Be Studied In Vitro

Cell-culture systems that can support the growth and differentiation of lymphoid and myeloid stem cells have made it possible to identify many hematopoietic growth factors. In these in vitro systems, bone-marrow stromal cells are cultured to form a layer of cells that adhere to a petri dish; freshly isolated bone-marrow hematopoietic cells placed on this layer will grow, divide, and produce large visible colonies (Figure 2-2). If the cells have been cultured in semisolid agar, their progeny will be immobilized and can be analyzed for cell types. Colonies that contain stem cells can be replated to produce mixed colonies that contain different cell types, including progenitor cells of different cell lineages. In contrast, progenitor cells, while capable of division, cannot be replated and produce lineage-restricted colonies.

Various growth factors are required for the survival, proliferation, differentiation, and maturation of hematopoietic cells in culture. These growth factors, the hematopoietic cytokines, are identified by their ability to stimulate the formation of hematopoietic cell colonies in bone-marrow cultures. Among the cytokines detected in this way was a family of acidic glycoproteins, the colony-stimulating factors (CSFs), named for their ability to induce the formation of distinct hematopoietic cell lines. Another important hematopoietic cytokine detected by this method was the glycoprotein erythropoietin (EPO). Produced by the kidney, this cytokine induces the terminal development of erythro-cytes and regulates the production of red blood cells. Further studies showed that the ability of a given cytokine to signal growth and differentiation is dependent upon the presence of a receptor for that cytokine on the surface of the target cell—commitment of a progenitor cell to a particular differentiation pathway is associated with the expression of membrane receptors that are specific for particular cy-tokines. Many cytokines and their receptors have since been shown to play essential roles in hematopoiesis. This topic is explored much more fully in the chapter on cytokines (Chapter 11).

Bone Marrow Stromal Cells

FIGURE 2-2

(a) Experimental scheme for culturing hematopoietic cells. Adherent bone-marrow stromal cells form a matrix on which the hematopoietic cells proliferate. Single cells can be transferred to semisolid agar for colony growth and the colonies analyzed for differentiated cell types. (b) Scanning electron micrograph of cells in long-term culture of human bone marrow. [Photograph from M. J. Cline and D. W. Golde, 1979, Nature 277:180; reprinted by permission; © 1979 Macmillan Magazines Ltd., micrograph courtesy of S. Quan.]

FIGURE 2-2

(a) Experimental scheme for culturing hematopoietic cells. Adherent bone-marrow stromal cells form a matrix on which the hematopoietic cells proliferate. Single cells can be transferred to semisolid agar for colony growth and the colonies analyzed for differentiated cell types. (b) Scanning electron micrograph of cells in long-term culture of human bone marrow. [Photograph from M. J. Cline and D. W. Golde, 1979, Nature 277:180; reprinted by permission; © 1979 Macmillan Magazines Ltd., micrograph courtesy of S. Quan.]

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