THE EFFECTS OF MUSCULAR-DYSTROPHY ON CRANIOFACIAL GROWTH IN MICE - A STUDY OF HETEROCHRONY AND ONTOGENIC ALLOMETRY

Mechanical loading of muscles on bones at their sites of attachment ca n regulate skeletal morphology. The present study examined the effects of muscle degeneration on craniofacial growth, using two strains of m uscular dystrophic mice, Mus musculus, differing in pathological sever ity. We collected radiographic and weight data longitudinally and digi tized radiographs to obtain distances between anatomical landmarks in different functional regions of the skull. We then quantified heteroch ronic and allometric differences among genotypes and between sexes. Be cause growth is nonlinear with respect to time, we first used the Gomp ertz model to obtain heterochronic growth parameters, which were then tested with ANOVA. Ontogenetic allometric analyses examined the scalin g relationships between various measurements with linear regressions. For most measurements the severely dystrophic mice are significantly s maller in final size than both the control and the mildly dystrophic m ice, which are statistically indistinguishable. Measures of total grow th and the neurocranium exhibit more differences among groups in heter ochronic parameters of early ontogeny because growth in these regions is controlled primarily by brain expansion that ceases early in develo pment. In contrast, the face and mandible exhibit more differences in later growth parameters possibly because of the increased influence of muscles on these regions as growth progresses. The severely dystrophi c mice have flatter, more elongate skulls and mandibles than those of the other two genotypes, concurrent with an absence of muscular forces to stimulate growth in a superior-inferior direction. (C) 1998 Wiley- Liss, Inc.