The phenotypic interdependence of the musculoskeletal characters of the mandibular arch in mice.

Evidence from studies of craniofacial anomalies and the evolutionary transition from reptiles to mammals suggests that the temporomandibular joint (TMJ), bony zygomatic arch, middle ear ossicles and mandibular muscle pattern may form a correlated suite of characters. To test the degree of phenotypic interdependence among these features, mandibular arch defects were analysed in prenatal mice. Retinoic palmitate was administered to pregnant mice on day 8.7 to produce test foetuses with malformations of the mandibular arch. A rating scale was developed for each of the four characters so that numerical values could be assigned to each phenotype encountered. Control animals were used to establish normal phenotypes for each character which were assigned a value of 1. Data from each test age, 16, 18 and 19 days postconception, were pooled and Spearmann rank correlation coefficients between each of the traits were calculated. Coefficients (R) range from a high of 0.87, between the TMJ and zygomatic arch, to a low of 0.67 between the zygomatic arch and the mandibular musculature showing highly significant correlations (P less than 0.0001) among all characters. Therefore, the data suggest that the musculoskeletal features of the mandibular arch are phenotypically interdependent during development.