Regulation of hemopoiesis by cytokines that restrict options for growth and differentiation.

Regulators of hemopoiesis are traditionally classified as belonging to two groups having antagonistic activities, that is, stimulators versus inhibitors. The majority of the so-called "inhibitory" molecules do not differ from stimulators in their biological activity since both are inducers of differentiation and consequent cell death. It is hypothesized that these cytokines operate during emergency situations (such as bacterial infections) in peripheral organs and bloodstream. Conversely, steady-state hemopoiesis in blood-forming organs is regulated by stromal factors that support stem cell renewal and direct cell positioning. Some of these stromal cell factors are lineage-specific inhibitors. It is proposed that hemopoietically active cytokines be called restrictions, since their function is to minimize the options available for the cell and thereby determine its growth and differentiation pathway.