Rapid inactivation and apoptosis of osteoclasts in the maternal skeleton during the bone remodeling reversal at the end of lactation.

There is a rapid reversal in maternal skeletal metabolism and bone remodeling from accelerated bone resorption during lactation to skeletal rebuilding after lactation. The purpose was to determine the changes that occur in maternal osteoclasts during the transition from lactation to postlactation. Skeletal samples were taken from female rats on days 10 and 19 of lactation and 1 and 7 days after lactation. The pups were weaned on day 20. There was a rapid change in the osteoclast population after weaning, resulting in less resorption surface. Osteoclasts detached from bone surfaces, lost their ruffled borders, and became fragmented with immunocytochemical evidence of apoptosis within 24 hr after lactation. Concomitant with the rapid regression in the osteoclast population was an over fivefold increase in maternal calcitonin (CT) levels at 24 hr after weaning. Serum calcium and estrogen (E2) increased, but prolactin (PRL) and PTH decreased after weaning. The hormone changes, particularly that of CT, are consistent with the rapid regression of the osteoclast population at the end of lactation. These changes are similar to a reversal phase of a bone remodeling cycle where bone formation commences when resorption ceases on bone surfaces and suggests that the fate of osteoclasts during bone remodeling is programmed cell death. These results also suggest that bone remodeling is well synchronized prior to, during, and after lactation to accommodate the mineral requirements of the offspring as well as the mother.