Deposition of cement at reversal lines in rat femoral bone.

Femora from young adult Wistar rats were prepared for both light and electron microscopy. Routinely processed wax sections showed the appearance of cement lines immediately proximal to the resorption surface formed by active osteoclasts and distal to the onset of lamella formation in femoral bone tissue. This early stage of extracellular matrix elaboration at reversal lines was then studied by scanning electron microscopy (SEM) of actively remodeling sites, mainly on trabecular and endosteal surfaces. The resorption surface was shown to comprise a decalcified collagenous mat with individual fibers running either parallel or perpendicular to the surface plane. By examining different, neighboring, areas of resorption lacunae, a temporal sequence of new extracellular matrix production could be established. Before the deposition of new collagen, globular accretions were deposited onto the resorption surface. In areas where individual collagen fibers were oriented perpendicular to the surface plane, this globular matrix was initially deposited on the exposed fiber tips. The globules increased in size and fused laterally to form a continuous cement layer, which not only interdigitated with the collagen mesh of the resorption surface but also provided anchorage for new collagen fibers, which themselves became mineralized. These morphologic results provide a mechanistic explanation of coupling at reversal lines.