Erythropoietic stimulation enhances, and erythropoietic inhibition suppresses, multidirectional differentiation in 4-day transient endogenous spleen colonies.

Transient endogenous spleen colonies were found to be composed of either erythroid, granuloid or megakaryocytic cells, or mixtures of these cell types. Independently of the directions of differentiation of the colonies their formation was uniformly stimulated by bleeding and almost completely prevented by hypertransfusion. It is suggested that cells which form these colonies constitute a separate class of pluripotential hemopoietic progenitors, whose differentiation in either direction passes the stage sensitive to erythropoiesis-modulating factors.