Fig. 3

The bone remodeling compartment (BRC). a The BRC provides the correct microenvironment to link bone formation and resorption through local signaling. The bone marrow envelope (BME) is a layer of cells of mesenchymal origin and a reservoir for osteoprogenitors that covers the layer of bone lining cells (BLCs). Once remodeling is initiated, osteoclasts lift the BME from the BLC, inducing BME cells to form a structure called the canopy. The canopy separates the remodeling site from the remainder of the bone marrow to allow osteoclast and osteoblast precursors to enter the blood compartment. After resorption, osteoclasts either undergo apoptosis or dedifferentiate into osteomorphs. The resorbed surface is then colonized by secondary osteoclasts and reversal cells. Reversal cells are osteoblast progenitors that digest fibrillar collagen remnants, similar to BLCs. Secondary osteoclasts and reversal cells provide the basis for the recruitment and expansion of osteoblastic pools (b) except under certain counteracting conditions, such as glucocorticoid or alendronate treatment, myeloma, or postmenopausal osteoporosis