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Progenitor cells in the periosteum-perichondrium of the posterior hook of the quadratojugal (QJ, a membrane bone) in the embryonic chick are bipotential for osteogenesis and chondrogenesis. These cells switch from osteogenesis to chondrogenesis between 10 to 11 days in normal (mobile) embryos but not in paralyzed (immobile) embryos. Expression of neural cell adhesion molecule (NCAM) was studied using a monoclonal antibody in QJ hooks from normal and paralyzed chick embryos between 10 and 21 days of incubation. NCAM is expressed in osteoprogenitor cells and osteoblasts but not in chondroprogenitor cells, chondroblasts, or chondrocytes. The switch of progenitor cell differentiation from an osteogenic to a chondrogenic pathway between 10 and 11 days of incubation coincides with down-regulation of NCAM expression. Both initiation of secondary chondrogenesis and down-regulation of NCAM depend on biomechanical stimulation. In embryos paralyzed at 9 days, secondary cartilage fails to form and progenitor cells remain positive for NCAM. Furthermore, paralysis influences NCAM expression in progenitor cells before secondary chondrogenesis morphologically begins, indicating that NCAM may play a role in the initiation of secondary chondrogenesis. In 15-day normal embryos, NCAM-positive cells accumulate between the perichondrium and secondary cartilage in a position that prevents further cartilage formation in the hook. In 19-day embryos, these cells lose their NCAM expression and restart chondrogenesis in a second phase of differentiation, forming an articular cartilage. Loss of NCAM expression in this cell layer and re-commencement of chondrogenesis do not occur in embryos paralyzed at 13 days, and therefore also require biomechanical stimulation. Hence, down-regulation of NCAM expression correlates with two phases of secondary chondrogenesis in embryonic life, both of which are dependent upon embryonic movement.

Differential expression of neural cell adhesion molecule (NCAM) during osteogenesis and secondary chondrogenesis in the embryonic chick.