Evidence for phosphoprotein microspheres in bone

Bone sialoprotein and osteopontin are 'bone-specific' phosphoproteins, but their function is uncertain and their ultrastructural associations remain unclear. Insight into their role was sought by special attention to their general distribution and specific morphology under the high-power optical microscope. Their extracellular staining characteristics were examined in cryosections of adult rat skeletal tissues using two immunohistochemical methods. The two proteins were clearly evident in immature woven bone of endochondral and intramembranous origin (although cartilage was negative, even when calcified). In mature lamellar bone, bone sialoprotein remained ubiquitous, while osteopontin was confined to cement lines and other relatively discrete sites of past and present resorption activity, particularly near blood vessels. In neither case was the distribution of the stain structureless and diffuse. Invariably (except when non-specific), it was sharply defined and had the form of microspheres measuring approximately 1 microm in diameter. In both immature and mature regions, these objects appeared in sheets, chains or groups in a pattern that was evidently coincident with a similar structural arrangement found within the inorganic phase of bone. It was concluded that phosphoproteins are not randomly located throughout the collagenous matrix but are apparently integral to calcified microsphere populations, and it is suggested that these structures are well placed to control the chemical state of the mineral over their surfaces and influence remodelling.