Immunolocalization of gamma-carboxyglutamic acid containing proteins in developing rat bones

The localization of a gamma-carboxyglutamic acid (Gla)-containing protein, BGP (also called osteocalcin) was examined in developing calvaria, alveolar bones and long bones of newborn rats by immunostaining with the peroxidase-antiperoxidase method. In undemineralized tissues osteoblasts of intramembranous bones stained positive; osteoid was negative, whereas young mineralized bone stained weakly. After a mild demineralization all bones stained positive; no staining was found in cartilage, muscles or soft connective tissues. In addition to the osteoblasts, osteocytes were also immunoreactive. Osteoclasts, identified by a subsequent staining for acid phosphatase activity, demonstrated no immunostaining for BGP. These data support the hypothesis that BGP is synthesized by osteoblasts and osteocytes and is subsequently deposited in the mineralizing bone.