Osteoclast function in healthy subjects and in patients with Paget's disease

Osteoclasts, the bone-resorbing cells, are important in normal bone remodeling. Resorption of bone by osteoclasts is coupled to osteoblastic bone formation. An increase in osteoclastic bone resorption relative to bone formation is found in many conditions, which lead to a net loss of bone resulting in osteoporosis. Like osteoporosis, Paget's disease of the bone is also characterized by an increase in osteoclastic resorption. It is an important clinical entity in our aging population, and the osteoclasts are the primary cells responsible for the characteristic bone lesions. Unlike osteoporosis, the profound increase in bone resorption is followed by excessive new bone formation. The study of the factors responsible for normal osteoclastic differentiation, maturation and activation has been invaluable in understanding the pathophysiology of osteoporosis and Paget's disease. This review will focus on normal osteoclastic morphology, the factors that affect osteoclast function and formation, and then contrast these with abnormalities of osteoclasts in Paget's disease.