BONE differs from reproductive tissues in that many aspects of skeletal growth, turnover, and function occur in the absence of gonadal hormones. Nevertheless, sex steroids have an important impact on bone physiology: they participate in the sexual dimorphism of the skeleton, play a role in maintenance of mineral homeostasis during reproduction, and are essential to maintaining bone balance in adults. Insufficient levels of certain sex steroids predispose the human skeleton to bone loss and to osteoporotic fractures. Estrogen appears to be the most important sex steroid in preventing osteoporosis in women and is the subject of this review.

Serious difficulties must be overcome to understand the action of estrogen on mineralized tissues. The skeleton is heterogeneous and contains many cell types. The cells that synthesize (osteoblasts), lie within (osteocytes), and degrade (osteoclasts) bone matrix are present in small numbers, are difficult to isolate as pure populations, and probably do not proliferate.