Developmental appearance and ultrastructural immunolocalization of a major 66 kDa phosphoprotein in embryonic and post-natal chicken bone.

Biochemical analyses and immunocytochemistry were used to examine the developmental appearance of a major approximately 66 kDa bone phosphoprotein (66 kDa BPP) in the mid-diaphyseal region of embryonic and post-natal chicken tibiae in vivo. Total protein and O-phosphoserine (Ser-P) and O-phosphothreonine (Thr-P) content of 8-, 12-, and 18-day embryonic, and 4-wk post-natal chicken tibiae were determined by amino acid analysis. Similar bone samples were carried through a wide variety of tissue-processing regimes including different protocols for fixation, decalcification, dehydration, and embedding prior to electron microscopy. For immunocytochemistry, tissue sections were incubated with a polyclonal antibody raised in rabbits against 66 kDa BPP, and the antigen was revealed by the high-resolution protein A-gold technique. Amino acid analysis, Western blotting, and immunocytochemistry all showed the presence and increasing concentration of bone phosphoprotein with advancing developmental age. Immunogold labeling was observed over osteoblasts and mineral deposits throughout the bone with the most intense reaction occurring at the mineralization front in embryonic tibiae. Electron probe X-ray microanalysis confirmed the association of 66 kDa BPP with mineral. The levels of phosphoprotein in the tissue were directly correlated with increasing degrees of mineralization. These observations are consistent with previous proposals suggesting that phosphoproteins may play a significant role in the calcification of bone matrix.