Introduction

Bone tissue adapts itself continuously to changing demands during development and growth (modeling) and in response to stress or damage (remodeling). The modeling and remodeling processes involve degradation of bone tissue by large multinucleated cells, called osteoclasts, and synthesis and deposition of new bone by mononuclear cuboidal cells lining bone, called osteoblasts. A close anatomical and functional relationship exists between resorptive and formative cells at discrete remodeling sites called ‘basic multicellular units of bone remodeling’ or BMU (Frost, 1966). This is, in all likelihood, responsible for the phenomenon in which treatments of metabolic bone disease developed to inhibit resorption often result in simultaneous inhibition of formation. The mechanism(s) whereby the actions of the resorbing osteoclasts and the bone forming osteoblasts are co-ordinated are not yet clear. Nevertheless, striking progress has been made in our understanding of osteoblast—osteoclast interaction with regard to regulating osteoclast formation.

This chapter focuses on osteoclasts and osteoclastic bone resorption. Morphological characteristics of osteoclasts, the processes whereby osteoclasts degrade bone, the origin of osteoclasts and the regulation of osteoclast formation and activity will be reviewed here.

Morphological characteristics of osteoclasts

Osteoclasts are easily recognized in histological sections of bone tissue as large multinucleated cells with up to 25 nuclei and are found in close association with bone surfaces (Fig. 3.1).