Bone is a complex tissue. Apart from the calcified matrix it contains a number of cell types, or rather a number of cell families, of which the boneforming and the bone-resorbing cell families are the most important. Each of these families consists of a number of differentiation stages. These stages within each family are in delicate equilibrium with each other. External factors such as hormones, drugs, or mechanical forces may disturb or change these cellular equilibria. Most studies on the physiology and endocrinology of bone metabolism have concentrated on the changes in activity of the most differentiated cell stages of the bone-forming and bone-resorbing cells: the osteoblasts and osteoclasts. This is understandable because osteoblasts and osteoclasts are easily recognizable in the tissue, whereas their precursors are not. Osteoblasts and osteoclasts are the cells primarily responsible for formation and resorption. However, effects on the size of precursor pools or on