In vivo osteochondrogenic potential of cultured cells derived from the periosteum.

Periosteal cells were isolated from young chicks, introduced into cell culture, subcultured, and then inoculated into athymic, nude mice to test the in vivo osteochondrogenic potential of cultured periosteal cells. In monolayer cultures, the adherent periosteal cells showed fibroblastlike morphology and overtly expressed neither osteogenic nor chondrogenic phenotypes. Cultured cells inoculated heterotopically into nude mice eventually gave rise to bone tissue at the subcutaneous injection site. The process of bone formation occurred through two different mechanisms: intramembranous bone formation at the peripheral portion of the inoculum early and endochondral bone formation in the central portion later. Frozen, preserved periosteal cells also formed bone after introduction into nude mice in the same temporal histologic sequence as the unfrozen cells. Cultured chick muscle fibroblasts from donors that were the same age as controls did not form bone or cartilage when inoculated under identical conditions to those of cultured periosteal cells. These results suggest that periosteum of young chicks contains subsets of progenitor cells that possess the potential to differentiate directly into osteoblasts or chondrocytes when inoculated in vivo. Importantly, this potential is retained after enzymatic isolation, cell culture, subculturing, and freeze preservation.