The origin of the hematopoietic microenvironment in continuous bone marrow culture

Marrow-derived adherent cells (MDAC) have been shown to provide a microenvironment which supports the proliferation of hematopoietic stem cells in continuous bone marrow culture. A study was undertaken to investigate the origin of MDAC. Normal CBA mice were transplanted with marrow cells from CBA donors bearing two T6 chromosomes. Five weeks after transplantation, the mice were sacrificed, their marrow cells explanted in liquid culture and the numbers of T6 positive and T6 negative mitoses were monitored during cultivation of MDAC. After 6 weeks, established MDAC monolayers were recharged with second explants of marrow cells and their capacity to support HSC proliferation in continuous marrow culture was assessed. The number of T6 positive mitoses declined steadily during the period of MDAC cultivation and none were detected after 4 weeks. The MDAC monolayers were able to support in vitro proliferation of CFU for periods of at least 5 weeks. These results suggest that the in vitro hematopoietic microenvironment was of recipient origin and therefore stromal.