Rapid improvements in cortical bone dynamics and structure after lactation in established breeder rats.

The mineral requirements for milk production during lactation usually result in a decrease in maternal skeletal mass during this period. The purpose of this study was to characterize changes in cortical bone formation, resorption, and structure after lactation in established breeder rats. Rats were taken at the end of the second pregnancy, second lactation, and at two, four, and six weeks after the pups were weaned. Age-related, nulliparous groups were included for comparison. Cortical bone structure and bone formation and resorption were measured at the tibiofibular junction using histomorphometric methods. As expected, there were decreases in cortical bone area, width, and minimum cortical thickness with an increase in marrow cavity area during lactation. Bone formation rates were essentially zero on the periosteal and endocortical surfaces at the end of lactation, while eroded (resorption) endocortical surface was greatly increased compared with the end of pregnancy. At or immediately after weaning, there was a rapid reversal of resorption to formation surface on the endocortical envelope similar to the events of true bone remodeling. Likewise, there was a commencement of bone formation on the periosteal surface. The volume- and surface-referent bone rates measured on the endocortical surface were substantially elevated over those measured at the end of the second pregnancy or in the nulliparous animals. Peak bone formation rates were observed on both the endocortical and periosteal surfaces by four weeks after weaning. These results show that the postlactation period is profoundly anabolic for cortical bone in the established breeder rat. The rapid and dramatic increases in bone formation likely serve to restore bone lost during lactation and to prepare the maternal skeleton for the next reproductive cycle.