Studies on bone formation by cartilage reconstructed by isolated epiphyseal chondrocytes, transplanted syngeneically or across known histocompatibility barriers in mice.

Calcified cartilage transplants induce bone formation. This process may be inhibited if the recipient is immunized by the transplant. To study the relation between the degree of antigen incompatibility between the donor and recipient and bone formation in more detail, chondrocytes isolated from cartilaginous epiphyses of five-day-old mice were transplanted within a fully compatible syngeneic system and across weak (H-Y, non-H-2) and strong (H-2) histocompatibility barriers. Reconstruction of cartilage occurred in all cases. In these transplants, which did not evoke immunologic reaction (fully compatible system, transplants across H-Y barrier in nonrejector strain), reconstructed cartilage hypertrophied, calcified, yielded to resorption by mesenchyme, and finally, was replaced by bone. When (independently of the degree of antigenic difference) cartilage was surrounded by mononuclear infiltration, bone formation did not occur or was delayed. The presence of infiltrations around transplants led to the degeneration of chondrocytes as well as of matrix in peripheral regions of cartilage. Immunologic infiltration may prevent endochondral osteogenesis by inhibiting cartilage invasion by vascularized mesenchyme, and/or interfering with matrix mineralization. The function of the chondrocyte is not yet defined, but in endochondral ossification, it plays more than a passive role.