Evidence for the Presence of the Fifth Pharyngeal Arch During the Chick Development

The pharyngeal arch (PA) is a transient structure of vertebrate embryos.Their number varies from species to species; nine in lampreys, seven in teleosts, and six in amniotes.Each pharyngeal arch contains a nerve and an artery as well as skeletomuscular elements.There has been a long-standing controversy about the formation of the fifth PA.Here, we investigated the formation of the fifth PA by tracking the distribution of neural crest cells in pharyngeal arches in chick embryos.Fertilized chick and quail eggs were used.Neural crest cells were detected by using quail-chick cell tracing technique and in ovo electroporation of EGFP constructs.Our results showed that neural crest cells from the post-otic level contributed to all post-otic pharyngeal arches (PA3-6).In contrast, neural crest cells from the somite 1-3 level avoided to invade the third pharyngeal arch, although they gave rise post-otic pharyngeal arches.Based on the trajectory of neural crest cells, the fifth pharyngeal arch was visible between the fourth and sixth pharyngeal arch, although it was considerably smaller compared to other arches.The fifth PA forms during early development at least in avian species.It is closer to the sixth PA than the fourth PA.This is in line with the formation of the fifth PAA, which can form a bypass and even fuse with the sixth PAA.