Studies on congenital osteopetrosis in tl and ia rats using organ culture.

We studied bone resorption in rats bearing the ia and tl osteopetrotic mutant genes by measuring the release of previously incorporated 45Ca from the long bone shafts of 19-day fetuses. Fetuses from matings of ia/ia to ia/+ rats showed good bone resorptive responses. There was a significant increase in 45Ca release in control cultures compared to killed bones, with further stimulation by parathyroid hormone (PTH) and no bimodal distribution. Fetuses from matings of ia/ia rats to ia/ia rats showed similar resorptive responses. The responses were greater in media supplemented with serum than in media containing bovine serum albumin. Serum from ia/ia rats did not inhibit resorption of ia/ia or normal bones. In contrast, matings of tl/+ to tl/+ heterozygotes produced 18 of 71 fetuses which showed markedly decreased bone resorptive responses and were therefore considered to be tl/tl. These "non-responders" showed no significant cell-mediated control resorption compared with killed bones and no response to PTH, 1,25-dihydroxyvitamin D or prostaglandin E2. The remaining animals, which were presumptive heterozygotes or homozygous normals, showed significant control resorption and responded with further increases to all three stimulators. Morphologically the tibiae from the fetuses of non-responders were not grossly abnormal, but lacked typical multinucleated osteoclasts. Osteoclasts were found in most samples from responsive fetuses.