| Estrogens are a series of bioactive steroid hormone, existing in many statuses. 17β estradiol plays the major physiological role. In premenopausal women, the ovaries are the principle source of estrogen, which functions as a circulating hormone to act on distal target tissues. However, in postmenopausal women the ovaries cease to produce estrogen. Instead, estrogens are produced in a number of extragonadal sites and act locally at these sites as a paracine or even intracirne factor but no longer a solely endocrine factor. Estrogen is closely associated with bone metabolism and maintains the balance of bone microenvironment in many ways. Estrogens are not only involved in the physiological process of osteoblasts and osteoclasts in order to maintain the dynamic equilibrium between bone formation and bone resorption, but also involved in the differentiation of osteoblasts by bone mesenchymal stem cells. Furthermore, estrogen deficiency is associated with biological activities of PTH, GH, and IGF-1, leading to bone resorption over bone formation and the onset of osteoporosis. So estrogen replacement therapy (ERT) is effective in treating postmenopausal osteoporosis. However, it is still controversial whether long-term use of ERT would bring side effects. This review summarizes the sources of estrogen and their importance in bone metabolism, and explores the possibility of extragonadal source of estrogen and the long-term use of ERT. |