Mammalian collagenase predisposes bone surfaces to osteoclastic resorption

The cell-free endocranial surface of young adult rat parietal bones was used as a substrate for bone cell-derived mammalian collagenase. Incubation of parietal bones in a concentration of enzyme comparable to that secreted by osteoblastic cells in vitro caused destruction of surface osteoid, and resulted in exposure of mineral onto the bone surface. Bones so pre-treated were considerably more susceptible to osteoclastic resorption than bones pre-incubated in the absence of collagenase. These results are consistent with the view that the osteoid layer which covers bone surfaces acts as a barrier to osteoclastic contact with underlying, resorption-stimulating bone mineral; and that cells of the osteoblastic lineage induce osteoclastic resorption through collagenase secretion which, by digestion of the surface osteoid, exposes bone mineral to osteoclastic contact.