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The effects of ovariectomy and 17β‐estradiol on cortical bone histomorphometry in growing rats

The effects of ovariectomy for four weeks and of 17β‐estradiol for three weeks on histomorphometry of the tibial diaphysis were determined in young rats. The effects of ovariectomy on histomorphometry of subcutaneous implants of demineralized bone matrix were also examined. Groups of young female rats were either ovariectomized or sham operated. After surgery, the animals were weight matched and pair fed. Despite the same caloric intake, ovariectomized rats grew more rapidly than pair‐fed, sham‐operated controls but were significantly heavier at sacrifice in only one of three experiments. Ovariectomy did not change mean serum calcium, phosphate, 25‐hydroxyvitamin D (25‐OHD), or 1,25‐dihydroxyvitamin D [1,25(OH)2D] but significantly lowered mean serum magnesium. Serum estradiol was not detectable in ovariectomized animals. 17β‐Estradiol in ovariectomized animals significantly increased mean serum estradiol and lowered mean serum phosphate but did not change mean serum calcium, magnesium, 25‐OHD, or 1,25(OH)2D, as compared to values in sham‐operated controls. Bone formation rate was significantly enhanced in ovariectomized animals at both the endosteal and periosteal surfaces of the tibial diaphysis as compared to values in sham‐operated controls. The increase in bone formation rate was reversed by 17β‐estradiol at the periosteal but not endosteal surface. Ovariectomy increased the bone apposition rate, mineralization rate, and osteoid thickness of the tibial diaphysis. These increases were reversed by 17β‐estradiol. In implants, ovariectomy increased the resorption of implant matrix and enhanced the formation of new matrix. Ovariectomy resulted in increases in forming surface and resorbing surface in the implants. The results of these short‐term studies indicate (a) that ovariectomy in growing rats leads to increases in both bone formation and resorption, and (b) that these increases are reversed by treatment with 17β‐estradiol.