Correspondence between the development of hemopoietic tissue and the time of colony formation by colony-forming cells.

: The development of a haemopoietic tissue and the time when colony-forming cells in it formed detectable colonies were studied with in vivo spleen colony-forming units (CFUs) and in vitro high-proliferation-potential colony-forming cells (HPP CFC). Cells that form colonies first are developmentally more mature than those doing so later. Marrow containing mature spleen colony-forming cells formed fewer cells in the femora of recipients than that which contained early colony-forming cells. The growth curve of developmentally early high-proliferation potential-colony-forming cells was steeper than that of later cells. The time period before colony-formation occurs is a property of the colony-forming cell and is not due to regulatory mechanisms in the animal or to regulatory cells in the haemopoietic stroma.