Effects of estrogen and progesterone on tibia histomorphometry in growing rats

The present study was performed to evaluate possible interactions between e strogen and progesterone on peak cancellous bone mass. Ovariectomized (OVX) growing rats were treated with 17 beta-estradiol (4.8 mu g/day), progester one (4.8 mg/day), a combination of the two sex steroids, or with vehicle fo r 14 days beginning 7 days after OVX. The tibiae were removed for histomorp hometric analysis of the proximal metaphysis. OVX and growth each resulted in net resorption of cancellous bone at a sampling site adjusted for longit udinal bone growth. Estradiol and progesterone treatment each antagonized b one loss by inhibiting the decrease in trabecular number. Estradiol increas ed but progesterone had no effect on trabecular thickness, Progesterone did not influence either osteoclast number or the resorption of the pretreatme nt fluorochrome label. Estradiol reduced osteoclast number and inhibited la bel resorption, the latter change being accentuated by combination treatmen t. Estradiol reduced and progesterone enhanced the mineral apposition and b one formation rates. The results indicate that estradiol and progesterone h ave independent activities on cancellous bone turnover during growth. Where as estradiol reduced bone turnover, progesterone had a stimulatory effect o n bone formation. These findings suggest that progesterone has a role in es tablishing and maintaining peak cancellous bone volume during growth.