Craniofacial integration, plasticity and biomechanics in the mouse masticatory system

Baverstock, Hester

February 2014

Thesis or dissertation

© 2014 Hester Baverstock. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

The craniomandibular skeleton is a complex, dynamic structure, housing many vital tissues and required to perform critical functions. This region is however subject to substantial morphological change during development, and required to adapt to its environment and individual variance. The capacity of this region to maintain correlated form and appropriate functional performance despite these challenges is not fully understood. The sample consists of three strains of mice; a wild-type strain and two mutant strains from the same genetic background strain. Both mutations selectively affect chondrocranial growth, and thus influence of both are limited to the crania. The brachymorph mutant phenotype is characterised by a shortened cranium, while the pten is elongated. This sample therefore allows exploration of a potential plastic response in terms of the mandible, the masticatory lever system, and in turn mechanical advantage, when cranial length and the out-lever are varied. Three dimensional landmarks were applied to micro-CT scans and partial-least-squares analysis carried out to determine covariance between crania and mandibles. Mechanical advantage was calculated as a ratio of muscle in-lever and jaw out-lever for three key masticatory muscles. A common pattern of both variance and covariance was found among all three strains, with mandibular morphology in each strain covarying with cranial phenotypes. Jaw out-lever lengths were found to be significantly different in all three strains, and yet little significant difference between strains was found in mechanical advantage for any muscles. This maintenance of mechanical advantage is attributed to plastic adaptation in regions influencing muscle in-lever length, the latter which were found to be significantly different in the three strains. These results show the potential of the craniomandibular complex to plastically adapt to maintain both correlated form and functionality when variation occurs in one region, and thus these results have significant implications for the evolvability of the complex.

Hull York Medical School, The University of Hull and the University of York
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