Adhesion molecules in skeletogenesis: I. transient expression of neural cell adhesion molecules (NCAM) in osteoblasts during endochondral and intramembranous ossification

We report that neural cell adhesion molecules (NCAM) are expressed transiently in developing chicken osteoblasts during osteogenesis using immunostaining on cryostat sections. NCAM is strongly expressed in most osteoblasts along bone trabeculae that coincide with the presence of collagen I and alkaline phosphatase activity. In endochondral ossification, NCAM is highly expressed in osteogenic buds as seen in the epiphysis and diaphysis of tibia and vertebrae. In intramembranous ossification, NCAM is seen in osteogenic condensation of calvaria and in the periosteum of tibial diaphysis. The expression is transient because NCAM is not expressed in mesenchymal cells before osteogenic condensation and NCAM expression is lost in osteocytes in later stages. The staining pattern suggests that NCAM is present on the cell membrane of osteoblasts. Using a specific monoclonal antibody, the osteoblast NCAM is shown to contain polysialic acid, which is enriched in embryonic brain. Northern blot analysis using chicken brain NCAM cDNA as probes showed two major sizes of mRNA at 6.4 and 4.2 kb in calvarial mRNA as opposed to bands at 7.2, 6.4, and 4.2 kb in the brain. An immunoblot showed major proteins at Mr 165 and 110 kd, unlike brain NCAM, which are 180, 140, and 120 kD. That NCAM is involved in bone morphogenesis is consistent with the general hypothesis that NCAM plays pivotal roles in mesenchymal condensation, as shown in the formation of muscle, kidney, skin, and cartilage. The results establish NCAM as a cell surface molecule expressed transiently during osteoblast lineage. The implication that NCAM may mediate osteoblast interaction and regulate skeletal morphogenesis is discussed.