Histologic evidence for osteopenia and increased bone turnover in ovariectomized rats.

Bone histomorphometry was performed in the proximal tibia and lumbar vertebra of female Sprague-Dawley rats subjected to either bilateral ovariectomy or sham surgery. Ovariectomized rats killed 10 weeks after surgery were characterized by osteopenia and histomorphometric indices of increased bone turnover at both skeletal sites. Osteoclast surface and number as well as osteoblast surface and number were significantly increased in ovariectomized rats. Tetracycline-based data were consistent with accelerated bone formation in response to ovariectomy. Active formation surface (double-labeled surface), calcification rate, and bone formation rate (tissue level, total surface referent) were all significantly elevated in the proximal tibia of ovariectomized rats. Ovariectomy also increased active formation surface and bone formation rate in the lumbar vertebra, but the vertebral calcification rate was nearly identical in control and ovariectomized rats. Osteopenia and dynamic tetracycline-based estimates of increased bone turnover in ovariectomized animals were more pronounced in the proximal tibia relative to the lumbar vertebra. Longitudinal bone growth in the proximal tibia and mineralization lag time at the tibial cortical endosteal surface were not affected by ovariectomy. These histomorphometric data indicate that ovariectomy induces osteopenia and accelerated skeletal metabolism in rats.