Ultrastructure of the tibial epiphyseal growth plate in microphthalmic (mi/mi) mice

Osteoclasts from growth plates of affected (mi/mi) and control (mi/+) microphthalmic mice, from newborn to 15 days old, were examined. Osteoclasts from unaffected mice were generally large, multinucleated cells with distinct ruffled borders and clear zones. Osteoclasts from the mi/mi litter mates were predominantly mononucleated, lacking in ruffled borders, clear zones and an abundance of organelles, features not normally associated with a functioning resorptive cell. The lamina limitans was more distinct in mi/mi mice. By 15 days, no evidence of bone resorption was seen in mi/mi mice. The immature appearance of osteoclasts and the lack of bone marrow cavity formation in mi/mi mice may result from either a fusion defect or from a limited life span of osteoclasts.