The Evolution and Development of The Mandibular Coronoid Process in Mammals

The coronoid process of the mammalian dentary bone is an evolutionary novel structure reflecting the new role of the dentary in mammals as an attachment site for the muscles of mastication. Previous studies have shown that the size and shape of the coronoid is correlated to the bite force required by the adult animal, and is dependent upon the mechanical force acting upon it during development. However little is known about the developmental programme leading to the specification of the process. We demonstrate that the paired homeobox transcription factor Pax9 is expressed in the ossifying front of the future coronoid process, and that Pax9 expressing cells contribute to the coronoid process, but not the other mandibular processes. In Pax9 mutants the coronoid is absent, however the temporalis muscle, which attaches to it, develops normally. Histological and gene expression studies indicated that the muscle attachment site remains. Loss of the coronoid is therefore driven by changes in the intrinsic patte...