Tartrate-resistant acid phosphatase activity in rat osteoblasts and osteocytes.

Osteonectin was immunolocalized in human fetal and calf neonatal developing bone using newly developed monoclonal antibodies. The protein was localized to the cytoplasm of osteoblasts and young osteocytes. In bone matrix, strong reactivity was found in newly laid down osteoid. Bone matrix immunoreactivity was enhanced by pretreatment of sections with proteases, possibly because of an unmasking of epitopes engaged in protein-protein interactions. Osteonectin immunoreactivity was also found in preosteoblasts in all types of human fetal osteogenesis (membranous, endochondral, subperiosteal, and mantellar (Meckel's cartilage) ossification), and in some chondrocytes of metaphyseal growth plate, possibly modulating towards an osteoblastic phenotype.