![]() Assorted extracellular matrix proteins are involved in the formation and stabilization of pharyngeal pouches. Pouch formation is governed by regions of endoderm containing high amounts of actin fibers, directing pouch expansion towards ectoderm.Īdaptor proteins stimulate signal transduction pathways within the endoderm, driving protein complex formation. Pharyngeal pouches form in distinct positions along the anteroposterior axis of the developing embryo. Each of these tissues develops from their respective pharyngeal pouch, or pouches, in the case of the parathyroid gland. The resulting pouches give rise to the middle ear cavity, Eustachian tube, mastoid air cells, palatine tonsils, thymus, parathyroid, and parafollicular cells of the thyroid. The endodermal out-pockets migrate towards midline as a final position in the adult following the completion of pouch formation. The fifth pharyngeal arch disappears during embryonal development. Finally, the fourth pharyngeal pouch lies between the fourth arch and the terminal, sixth, arch. In the same fashion, the third pouch is in between the third and fourth arches. The second pharyngeal pouch sits between the second and third pharyngeal arches. The first pharyngeal pouch lies between arches one and two. Expression of transcription factors within the endoderm drives out pocketing and pouch formation in between each pharyngeal arch. Neural crest ablation experiments performed demonstrated endodermal pharyngeal pouch formation not being affected by neural crest ablation. ![]() Pharyngeal pouch development is hypothetically independent of neural crest migration towards the endoderm. Humans have four pharyngeal pouches, as the fifth and sixth pharyngeal pouches are comprised within the fourth pharyngeal pouch. There are six pharyngeal pouches in mammals and birds. The formation of pharyngeal pouches starts anteriorly, in the most posterior region of the pharynx, and proceeds sequentially in a posterior direction. The medial side invaginates, forming the pharyngeal clefts. As embryogenesis continues, the lateral side of the pharynx evaginates, creating an out pocket between each arch, which forms the pharyngeal pouches. Segmentation of pharyngeal arches is a controlled process of outward folding of endoderm towards the ectoderm, regulated by various transcription factors. The pharyngeal arches develop in the fourth week of embryological development in vertebrates and are composed of ectoderm externally, endoderm internally, and a mesoderm core containing both mesoderm and neural crest cells. Pharyngeal pouches emerge during the segmental phase of pharyngeal arch development, forming partitions between each pharyngeal arch.
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