Cell repopulation in vascularized bone grafts

Following vascularized bone autografts, osteocyte viability is largely maintained. Viable cells within a graft may be surviving graft-derived cells, their progeny, or host-derived cells from circulation or surrounding bone. This study was conducted to define the process of cell repopulation, within vascularized bone grafts. Using inbred Lewis rats, 30 female vascularized tibial bones were transplanted to syngeneic male recipients and 45 male grafts were transplanted to female recipients. Twenty-five female recipients were immunosuppressed with FK506 to prevent rejection caused by Y-chromosome related antigens. The grafts were assessed up to 24 weeks post-transplant by radiography, histology and semi-quantitative polymerase chain reaction (PCR) using both Y-chromosome and autosome gene-specific primers. The female to male isograft transplants were useful to measure low levels of repopulation with host-derived cells, while male to female transplants more accurately quantified higher levels of cellular replacement. No host-derived cells were detected in the transplanted bone before six weeks. Thereafter, the ratio of host-derived cells gradually increased. By 24 weeks only 0.1-1.0% of graft-derived cells remained in the transplanted tibias. This study demonstrates that Y-chromosome-specific PCR is a useful tool to detect the cell lineage and cell repopulation following rat sex-mismatch vascularized bone grafting. Our results showed that donor cells in vascularized bone grafts were gradually repopulated with recipient cells. Correlation with histologic findings suggests that the periosteal hypertrophy observed by six weeks post-transplant results from graft-derived cells, while later remodeling is associated with host-derived cells.