Establishment of the hematopoietic microenvironment in the marrow of matrix-induced endochondral bone

Implantation of demineralized diaphyseal bone matrix (DBM) into subcutaneous sites in allogeneic Long-Evans rats results in new endochondral bone formation accompanied by hematopoietic bone marrow differentiation in the newly formed ossicles. In the present study, we investigated the relationship between the time of appearance of hematopoietic stem cells (CFU-S) and those of certain postulated elements of the hematopoietic microenvironment, i.e., fibroblast colony-forming cells (CFC-F), colony-stimulating factors (CSF), and collagen types. CFU-S were first detected at 16 days' postimplantation in the developing ossicle. Their numbers increased exponentially until day 24, decreased slightly between days 27 and 33, and then slowly increased in number again until day 36. CFC-F were present at day 10, and their numbers increased exponentially until day 15, decreased dramatically until day 24, and then remained constant till day 35. Thus, the transient growth of CFC-F preceded the appearance and growth of CFU-S in the ossicle by two or three days.