Bone cells predispose bone surfaces to resorption by exposure of mineral to osteoclastic contact

The cell-free endocranial surface of young adult rat parietal bones was used as a substrate for osteoclastic bone resorption, either without prior treatment, or after incubation of the parietal bones with collagenase or neonatal rat calvarial cells. Untreated, the endocranial surface consisted of unmineralized organic fibres; incubation with calvarial cells or collagenase caused disruption and removal of these fibres, with extensive exposure of bone mineral on the endocranial surface, without morphologically detectable mineral dissolution. Neonatal rabbit osteoclasts resorbed bone to a greater extent from parietal bones pre-incubated with calvarial cells or collagenase than from untreated bones; mineral exposure and subsequent osteoclastic resorption were both increased if calvarial cells were incubated with parathyroid hormone; removal of bone mineral after incubation with calvarial cells removed the predisposition to osteoclastic resorption. These experiments demonstrate that calvarial cells are capable of osteoid destruction, and indicate that one mechanism by which osteoblasts induce osteoclastic bone resorption may be through digestion of the unmineralized organic material that covers bone surfaces, to expose the underlying resorption-stimulating bone mineral to osteoclastic contact.