Identification of a hypoxic population of bone marrow cells.

A technique using collagenase has been devised to release and separate, with reproducibility, hematopoietic cells (HC) from various microenvironments of mouse femurs. HC were assayed by an in vitro gel culture technique used traditionally to score granulocyte-macrophage precursor cells (CFU-C). CFU-C which resided in the medullary cavity and endosteal regions were sensitive to ionizing radiation and resistant to misonidazole (MISO) cytotoxicity. CFU-C which resided within the compact bone were resistant to ionizing radiation and sensitive to the cytotoxic action of MISO. These results suggest that HC which reside in the bone are hypoxic and retain clonogenic potential. When animals were exposed to various treatments with MISO followed by myelotoxic doses of cyclophosphamide (CTX) or total body irradiation (TBI), the LD50 of both agents was significantly reduced. This result suggests that a hypoxic component of HC could be important in the regenerative process within the marrow after such myelotoxic trauma.