Abstract
BMP-induced heterotopic bone formation in mice was studied by light and electron microscopy. A BMP-preparation from bovine bone was enclosed in a gelatin capsule and implanted into the thigh muscles of AKR-strain mice. Tissues from implanted areas were examined on 5, 7, 10, 14, 21, and 28 days after implantation. By 7 days of implantation, undifferentiated spindle-shaped cells migrated into the BMP, proliferated by mitosis, and differentiated into chondrocytes, their cell membranes adhering to the fine granular BMP material; a homogenous cartilage was thus formed. Endochondral as well as perichondral ossification occurred in the BMP-induced cartilage following 14 days of implantation. The process of endochondral ossification was similar to that observed in the embryonic bone rudiments. In the perichondral ossification, the cells in the transitional region between the capsular tissue and the periphery of the cartilage became osteoblast-like cells. The extracellular matrix of these cells became calcified, and contributed to the formation of the cortical part of the new bone. A spongy bone with trabeculae lined by osteoblasts and osteoclasts was formed after 21 days of implantation containing actively proliferating bone marrow, which was mostly replaced by adipose tissue after 28 days.
