The decrease in bone mineral density (BMD) with age is a universal phenomenon that affects all races and cultures; non-pathological by itself but it is the background to development of osteoporosis (OP). In 1941, Albright described the postmenopausal OP, and discussed the special role of the estrogen (E2) lack. Others that prevented the OP with hormonal therapy confirmed this hypothesis. Later, Riggs and Melton proposed a classification of involutional OP on type I and type II. The type I OP appears in the first 10 years after menopause and is secondary to the deficit of E2. However, it is not clear the mechanisms of action of E2 on bone tissue: which are the mediators and also the target cells. There is no E2 receptors identified on osteoclast. Probably, the E2 effects are indirectly through osteoblast or by bone marrow stromal cells. The citokines, specially the interleucine 6 are candidate to be the mediators of E2 actions, but the data are still controversy in the literature. Type II or senile OP is defined to occur after 65 years old. In this age, others factors further than E2 deficiency determine the OP, and the secondary hyperparathyroidism play a special role. Recently, Riggs and Melton proposed a return to Albright's idea suggesting a unitary model of the involutional OP and placing the E2 as the central ethiological factor of bone loss. This distinction in OP type I or type II is theoretical, but shows the heterogeneous and multifactorial aspects of this disease.
Osteoporosis; Estrogen; Citokine; Physiopathology
