Regulation of haemopoietic stem cell proliferation in long term bone marrow cultures.

The development of a suitable bone marrow derived adherent cell population appears to be essential for the prolonged maintenance of haemopoietic stem cells in vitro. When established adherent layers are inoculated with freshly isolated bone marrow cells, proliferation of stem cells (CFU-S) regularly occurs both in the adherent layer and amongst the non-adherent cells. Furthermore, CFU-S present within the adherent layer are able to regenerate both themselves and the "non-adherent" CFU-S. One day after re-feeding the cultures (by removal of half the growth medium and addition of fresh medium) both the "non-adherent" and the "adherent" CFU-S are in a high cycling state (greater than 40% kill with 3HTdR). This proportion decreases with time of re-feeding and 5-7 days later the majority of "adherent" and "non-adherent" CFU-S are in a low cycling state ( less than 10% 3HTdR kill). Following a further re-feeding, CFU-S again enter a high cycling state.