Transformation of fetal secondary cartilage into embryonic bone in organ cultures of human mandibular condyles

Mandibular condyles of human fetuses, 14–21 weeks in utero, were kept in an organ culture system for up to 60 days. After 6 days in culture, the cartilage of the mandibular condyle appeared to have maintained its inherent structural characteristics, including all its various layers: chondroprogenitor, chondroblastic, and hypertrophic. After 12 days in culture, no chondroblasts could be seen; instead, the entire cartilage was occupied by hypertrophic chondrocytes. At the same time, the mesenchymal cells in the vicinity of the chondroprogenitor zone differentiated into osteoblast-like cells that produced type I collagen. The progenitor cells were still actively incorporating 3H-thymidine. The newly formed osteoid-like tissue lacked both metachromatic reactivity and a response to antibodies against chondroitin sulfate. Instead, the tissue reacted positively for osteocalcin (bone gla-protein). The process of new bone formation further progressed and, by the 20th day in culture, the new bone reacted positively for type I collagen, osteonectin, and to a lesser extent for chondroitin sulfate. The osteoid also underwent mineralization as revealed by both the von Kossa stain and vital staining with tetracycline. The above feature appeared even more intense in 40-day-old cultures. After 60 days, the newly formed bone contained osteoblasts and osteocytes, whereas the extracellular matrix revealed a high degree of matrix polarization. The results of the present study recapitulate findings reported for organ cultures of mice mandibular condyles. However, the in vitro process of de novo bone formation in human specimens requires a 6-fold longer culture time than that needed for mice condyles.