A possible contribution of undifferentiated cells to post-traumatic osteogenesis.

The contribution of undifferentiated cells to callus formation is difficult to observe during fracture healing and is therefore debated. In adult mongrel dogs these cells can easily be observed in a narrow space separating a freshly inserted screw from the surrounding compact bone. Undifferentiated connective tissue cells invade these spaces from the medullary and, less so, from the periosteal site. They seem to resemble histologically and more so ultrastructurally B-type lymphocytes. They migrate to the site of repair or are brought there by the blood stream. Locally these cells proliferate and differentiated into osteoblasts. The authors prefer to call these cells lymphocyte-like cells.