Further studies on skull growth in achondroplasic (cn) mice

The morphology of the basioccipital, basisphenoid and mandibular bones in achondroplasic (cn/cn) mice was compared with that of normal siblings. The two bones of the cranial base were markedly reduced in length but not in width. The percentage reduction in the basisphenoid was twice that in the basioccipital bone and of the same magnitude as that previously observed in the long bones of the limbs. This difference may arise because the basisphenoid, like the long bones, grows in length from two cartilaginous growth sites while the basioccipital grows from one cartilaginous and one periosteal site. The mandible of the (cn/cn) mice was also reduced in size, although to a lesser extent than were the cranial bones and without the ensuing disproportion. The scale of the mandibular changes suggests that they are largely attributable to regulatory responses to the shortened cranium. The finding that the condylar cartilage of the cn/cn mice is reduced in thickness indicates, however, that the cn gene may have a direct effect on condylar chondrocytes.