The reversal phase of the bone-remodeling cycle: cellular prerequisites for coupling resorption and formation

The reversal phase couples bone resorption to bone formation by generating an osteogenic environment at remodeling sites. The coupling mechanism remains poorly understood, despite the identification of a number of 'coupling' osteogenic molecules. A possible reason is the poor attention for the cells leading to osteogenesis during the reversal phase. This review aims at creating awareness of these cells and their activities in adult cancellous bone. It relates cell events (i) on the bone surface, (ii) in the mesenchymal envelope surrounding the bone marrow and appearing as a canopy above remodeling surfaces and (iii) in the bone marrow itself within a 50-μm distance of this canopy. When bone remodeling is initiated, osteoprogenitors at these three different levels are activated, likely as a result of a rearrangement of cell-cell and cell-matrix interactions. Notably, canopies are brought under the osteogenic influence of capillaries and osteoclasts, whereas bone surface cells become exposed to the eroded matrix and other osteoclast products. In several diverse pathophysiological situations, including osteoporosis, a decreased availability of osteoprogenitors from these local reservoirs coincides with decreased osteoblast recruitment and impaired initiation of bone formation, that is, uncoupling. Overall, this review stresses that coupling does not only depend on molecules able to activate osteogenesis, but that it also demands the presence of osteoprogenitors and ordered cell rearrangements at the remodeling site. It points to protection of local osteoprogenitors as a critical strategy to prevent bone loss.