Regulation of hemopoiesis.

The development of in vitro cloning systems for populations of hemopoietic and lymphoid cells has enabled a family of specific regulatory macromolecules to be detected and characterized. These macromolecules control proliferation and differentiation in hemopoietic cell populations. Studies have demonstrated the complexity of the spectrum of molecules which are involved in the regulation of each group of hemopoietic cells. Heterogeneous subpopulations of molecules exist for each regulatory function and more than one molecular form exists for the various regulator molecules. While the cellular origin of many of these regulators has not been clearly demonstrated, it is apparent that hemopoietic populations themselves can be significant sources of both stimulatory and inhibitory regulators. In particular, there are now clear examples of regulatory interactions which occur in both directions between hemopoietic and lymphoid populations. While in no case a complete analysis of the control systems has been achieved, maintenance of homeostasis in hemopoietic systems is now better understood than for most other cell populations in the body. Further, with existing techniques, and in particular, the exploitation of the full potential of the semi-solid cloning systems, it is realistic to expect that a complete analysis of hemopoietic regulation can now be accomplished.