Development, morphogenesis and evolution of pharyngeal segmentation in vertebrates

Abstract

Pharyngeal arches are bulges found on the lateral surface of the head of vertebrate embryos. They are lined externally by ectoderm and internally by endoderm, with a mesenchymal core of neural crest cells and mesoderm. Lateral expansion of pockets of endoderm form pharyngeal pouches at specific locations along the pharynx. Each one of these aligns with invaginating portions of overlying ectoderm to form the anterior and posterior border of each pharyngeal arch. Current studies suggest endoderm plays a prominent role in patterning the arches, but little is known about how this tissue develops and is organised. Investigation of pharyngeal pouch morphogenesis revealed morphological differences between anterior and posterior pouches. These region-based differences are also evident during epithelial interaction at each ectoderm/endoderm interface, where the first interface does not sustain direct contact in contrast to those posteriorly. This results in the fusion and subsequent breakdown of their basement membranes and cell death of the overlying ectoderm. I have revealed that this morphogenetic program for posterior pouch development is conserved in vertebrates and outpocketing of the pouch endoderm represents an early conserved stage of ‘gill’ development. To molecularly characterise the differences between anterior and posterior pharyngeal regions I examined Hox gene expression revealing alignment with specific pouches, thereby separating the pharynx into anterior and posterior regions. Furthermore, the most posterior pouch is demarcated by Hoxb1 expression, so as new pouches form this expression ‘moves’ posteriorly. This dynamic expression pattern is conserved and therefore may underlie how pouch number is controlled within each species. Moreover, a general trend toward a reduction in the number of pharyngeal arches has occurred with vertebrate evolution and I have localised this reduction to the posterior pharyngeal region. By using morphogenetic, molecular and comparative anatomical data I have characterised pharyngeal development and highlighted key differences in anterior and posterior regions reflecting the two main functions of the pharynx: feeding and respiration. As vertebrates transitioned from water to land, their method of respiration was adapted and this is reflected by the reduction in posterior pharyngeal segments in tetrapod species.

Date of Award	2014
Original language	English
Awarding Institution	King's College London
Supervisor	Anthony Graham (Supervisor)