The decrease in bone mass associated with aging and menopause.

The human skeleton accumulates bone up to approximately age 30, after which bone is gradually lost. Although estrogen replacement therapy prevents postmenopausal bone loss, it is not certain that estrogen deficiency alone is responsible for the decrease in bone mass. Progesterone deficiency could also be a factor, and progesterone replacement therapy has been shown to prevent postmenopausal bone loss associated with ovarian dysfunction. This article reviews what is known about bone remodeling and bone loss as a function of age and gender, discusses evidence from studies in rats that progesterone plays an important role in regulating bone formation, and suggests directions for future studies in predicting the success or failure of implant therapy based on the number and kinds of osteoprogenitor cells present.