Role of mineralizing cartilage in osteoclast and osteoblast recruitment

Periost-free, live and/or devitalized cartilaginous long bone rudiments of fetal mice were transplanted under the renal capsule of adult syngeneic mice to study the role of cells and intercellular matrix in the recruitment and formation of osteoclasts and osteoblasts, both identified by means of enzyme- and immunohistochemical methods. Live bone rudiments recruited host-derived osteoclasts within 5 days after transplantation. Osteoblasts developed as rapidly as osteoclasts and participated in the modeling of the rudiments into hemopoietic bone marrow containing ossicles. Devitalized bone rudiments, killed before osteoclastic invasion had occurred, did not recruit osteoclasts or osteoblasts, and were not resorbed up till 35 days after transplantation. Co-transplantation of live and devitalized bone rudiments however resulted in osteoclastic resorption of the killed rudiments, starting 9 days after transplantation. Again the live rudiments were modeled into ossicles. Devitalized bone rudiments which had been invaded by osteoclasts before killing and transplantation, did recruit host osteoclasts, but at a slower rate than live rudiments, and depending on the number of resorption sites at the time of transplantation. Osteoblasts were not formed. These data suggest that in developing long bones chondrocyte activity is involved in the recruitment of osteoclasts as well as osteoblasts. Matrix components diffusing from resorbing surfaces seem to be involved in osteoclast recruitment.