The Evolution and Development of The Mandibular Coronoid Process in Mammals

Abstract

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 processWe 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 patterning of the bone. In contrast other mouse mutants lacking the coronoid, such as the Tbx1 null, loss of the coronoid is a consequence of the loss of the muscle acting on the developing bone. Changes to this structure can therefore be influenced by intrinsic and extrinsic factors. The relative size of the coronoid process varies considerably across mammals, with herbivores, such as the guinea pig having a reduced or absent coronoid, while carnivores can have extremely large coronoid processes. The contribution of genes such as Pax9 and Tbx1 to changes in size of this processes on other mammals have therefore also been investigated. Our data implicates cooption of Pax9 in to the osteogenic program of the dentary enabled the development of the coronoid process, a key component in the emergence of the novel jaw.