Deconstructing the pharyngeal metamere

A prominent feature of all vertebrate embryos is the presence of a series of bulges on the lateral surface of the head, the pharyngeal arches. These structures constitute a metameric series, with each arch forming a similar set of derivatives. Significantly, the development of the pharyngeal arches is complex as it involves interactions between disparate embryonic cell types: ectoderm, endoderm, mesoderm and neural crest. It is becoming increasingly apparent that the development of the pharyngeal metamere revolves around the pharyngeal endoderm. The segmentation of this tissue is central to the generation of the arches. The pharyngeal endoderm also provides positional cues for the neural crest, and is involved in the induction of a number of components of the pharyngeal metamere. The segmentation of the pharyngeal endoderm has also been key to the evolution of pharyngeal metamerism. It is likely that endodermal segmentation is a deuterostome characteristic and that this basic pattern was sequentially modified and over time the more complex pharyngeal metamere of vertebrates emerged.