Induction of proliferation of haemopoietic stem cells in culture.

The behavior of the pluripotent haemopoietic stem cells (HSC) in liquid culture was investigated by determining CFU-S number and cell cyclinc status (by in vitro thymidine suicide) at regular intervals. The influence of fibroblast feeder cells and fibroblast conditioned medium on these parameters was investigated. Evidence was obtained for self replication of HSC in these cultures. Proliferation of HSC was found to be dependent on a humoral factor, released into the medium by fibroblasts. Separation experiments show that this stem cell activating factor (SAF) is distinct from the colony stimulating factor (CSF). Comparison of SAF with a lysate preparation of red blood cells, a product able to sustain an elevated recovery of stem cells in culture, showed a different mode of action on the persistence of HSC. The system offers an approach to the in vitro study of regulation at the level of HSC self generation.