The developmental features of marrow stroma in ectopic bone marrow implants.

Implantation of bits of marrow in ectopic sites is followed by reorganization of tissue and the formation of a hemopoietic nodule surrounded by a shell of bone. This regenerative process is reminiscent of marrow ontogeny and the model can serve to study marrow ontogeny in a relatively short period of time. Early events during this regeneration were studied by scanning (SEM) and transmission electron microscopy (TEM). Within 24 hours the implant elicited an angiogenic reaction and new vessels penetrated the implant. Intense circulation, thus established, divested the implant from hemopoietic cells, leaving the stroma behind. Stromal cells proliferated and the impetus for this proliferation appeared to result from an impulse caused by the presence of bony fragments outside and within the stromal cells. Previous studies of this model have not appreciated the presence of non-viable bone in the implant, although the fact that non-viable bone can trigger osteogenesis and new bone marrow formation is well-known. This experimental model lends itself to the study of the interrelationship of hemopoietic cells and their supporting stroma as well as the interrelationship of bone and hemopoiesis.