Diminution of pharyngeal segmentation and the evolution of the amniotes

Abstract

BackgroundThe pharyngeal arches are a series of bulges found on the lateral surface of the head of vertebrate embryos, and it is within these segments that components of the later anatomy are laid down. In most vertebrates, the post-otic pharyngeal arches will form the branchial apparatus, while in amniotes these segments are believed to generate the larynx. It has been unclear how the development of these segments has been altered with the emergence of the amniotes.ResultsIn this study, we examined the development of pharyngeal arches in amniotes and show that the post-otic pharyngeal arches in this clade are greatly diminished. We find that the post-otic segments do not undergo myogenesis or skeletogenesis, but are remodelled before these processes occur. We also find that nested DLX expression, which is a feature of all the pharyngeal arches in anamniotes, is associated with the anterior segments but less so with the posterior arches in amniotes. We further show that the posterior arches of the mouse embryo fail to properly delineate, which demonstrates the lack of function of these posterior segments in later development.ConclusionIn amniotes, there has been a loss of the ancestral “branchial” developmental programme that is a general feature of gnathostomes; myogenesis and skeletogenesis This is likely to have facilitated the emergence of the larynx as a new structure not constrained by the segmental organisation of the posterior pharyngeal region.

Cardiopharyngeal mesoderm origins of musculoskeletal and connective tissues in the mammalian pharynx

“…S8A) (McCauley and Bronner-Fraser, 2006;Abrial et al, 2017;Adachi et al, 2018). Our finding that arch 6 develops differently to anterior arches is consistent with a recent study by Poopalasundaram et al (2019) showing that posterior PA development in amniotes is markedly distinct from that in other vertebrate clades. However, although Poopalasundaram et al suggest that myogenesis is suppressed in posterior PAs, we observed MYOD protein accumulation in posterior CPM in the anlagen of branchiomeric muscles derived from arches 4-6 from early developmental stages.…”

A reappraisal and revision of the numbering of the pharyngeal arches

“…Thus, there are three features that are associated with the pharyngeal pouches: 1they form during the segmental phase of pharyngeal development; 2they contact the overlying ectoderm; 3they express definitive molecular markers such as PAX1/9, TBX1, etc. (Poopalasundaram et al 2019). We can conclude that in amniotes, including humans, there are five arches and four pouches, and their relationship is as follows: pouch 1 lies between arches 1 and 2; pouch 2 between arches 2 and 3; pouch 3 between arches 3 and 4; and pouch 4 between arches 4 and the terminal arch.…”

“…; Poopalasundaram et al. ). There have, of course, been fewer studies of pharyngeal pouch formation in human embryos, but again morphology and analysis of PAX9 expression demonstrate the formation of four pouches (Farley et al.…”

“…The formation of the pharyngeal pouches can be followed both morphologically and via the expression of genes that are expressed in these structures, such as PAX1 and PAX9, and studies in the two major models for understanding amniote development, chick and mouse, have shown that four pharyngeal pouches form (Okubo et al 2011;Poopalasundaram et al 2019). There have, of course, been fewer studies of pharyngeal pouch formation in human embryos, but again morphology and analysis of PAX9 expression demonstrate the formation of four pouches (Farley et al 2013;Poopalasundaram et al 2019).…”

“…(Poopalasundaram et al. ). We can conclude that in amniotes, including humans, there are five arches and four pouches, and their relationship is as follows: pouch 1 lies between arches 1 and 2; pouch 2 between arches 2 and 3; pouch 3 between arches 3 and 4; and pouch 4 between arches 4 and the terminal arch.…”