The effects of continuous estradiol therapy on cortical bone remodeling activity in the spayed beagle.

Ribs from 6 oophorectomized and 4 17 beta-estradiol-supplemented spayed, 4-year-old Beagle dams were subjected to histomorphometric analyses to determine what effects continuous estradiol treatment of 9 months duration had on the Basic Multicellular Unit (BMU) of cortical bone remodeling. The findings of this study suggest that 17 beta-estradiol has a two-step mode of action. First, this estrogen directly suppresses the formation of new BMUs. Secondly, it causes uncoupling of the Resorption/Formation (R/F) mechanism within each BMU, together with the creation of an approximate 1:1 balance between bone resorption and bone formation. This balance is probably responsible for the preservation of cortical bone mass seen to occur with continuous estrogen replacement therapy.