Degradation of Bone and the Role of Osteoclasts, Bone Lining Cells and Osteocytes

Abstract

Resorption of bone depends on the combined action of osteoblast-like bone lining cells and osteoclasts. The bone lining cell plays an essential role in modulating the formation of the osteoclasts and in the mean time prepares the bone surface for these multinucleated bone resorbing cells to attach to. The osteoclast attaches to the bone and the resorption starts with lowering the pH at the resorption site, a site that is secluded from the rest of the bone surface and the surrounding environment. The lowered pH results in dissolution of the mineral component, thus exposing the matrix. The bone matrix is subsequently digested by proteolytic enzymes, among which the cysteine proteinase cathepsin K appears to be essential. Recent findings indicate that not all osteoclasts are alike, different bones may harbour osteoclasts that differ in several aspects, among which the proteolytic enzymes used for resorption. In this chapter we will discuss the different steps that result in the degradation of bone, the onset of new bone matrix deposition and the presence of different osteoclasts at various bone sites.