Restrictins: stromal cell associated factors that control cell organization in hemopoietic tissues.

Hemopoiesis is a sequence of events initiated by the self-renewal of pluripotent stem cells followed by a series of differentiation steps and completed in the formation of distinct tissue patterns. Differentiation and self-renewal are antagonistic processes. A mechanism that attenuates the differentiation flow is obligatory to prevent the exhaustion of the stem cell pool. We suggest that stromal cells from the bone marrow control stem cell renewal through a mechanism that does not require colony-stimulating factors. The organization of cells within the tissue and their specific localization is suggested to be directed by stromal cell activities other than differentiation inducers. These stromal cell activities restrict differentiation or accumulation of mature cells. They are therefore designated as 'Restrictins'.