Growth and differentiation in the hemopoietic system.

Hemopoiesis is regulated by a complex series of interactions, including interactions among hemopoietic cells themselves, hemopoietic cells and the extracellular matrix, hemopoietic cells and marrow stromal cells, and hemopoietic cells and growth factors. In vitro culture systems have allowed a reductionist approach to the solution of these various problems and have facilitated experiments at the mechanistic level. The hemopoietic system is organized hierarchically with multipotential self-renewing stem cells, committed progenitor cells, and mature cells. The various stimuli necessary for growth and development of these cells are rapidly being elucidated. The nature of commitment (or differentiation) remains an enigma, but model systems have been developed in which various aspects of this problem can be investigated. In this respect, growth and differentiation factors obviously have a major role to play. Now that many of these factors have been molecularly cloned (and pure target cell populations are available) their role in vivo and their mode of action can be examined.