Question

1. The fate of pharyngeal gill arches have featured prominently in our discussions of vertebrate evolution and in Neil Shubin’s Your Inner Fish.  What are pharyngeal gill arches and what structures do they give rise to in different vertebrate lineages including our own?  (15 pts)

Answer 1

The pharyngeal arches also called gill arches are a series of tissue bands lying in the anterior side of the embryo that give rise to the structures of the head and neck. In humans, five pharyngeal arches are present.

Each arch has an internal endodermal pouch, a mesenchymal core, a membrane and an ectoderm. Mesenchymal core contains blood vessel, nerve and cartilage.

1. The first pharyngeal arches is maxillary/mandibular arch from which Trigeminal CN, Maxillary artery, malleus.incus, mandible, maxillla and muscles and ligaments associated with this are formed.

2. The second pharyngeal arche is the hyoid arch from which the Facial cranial nerve, corticotympanic artery, stapis, styloid process, lesser cornu of hyoid, upper part of body of hyoid bone, muscles and ligaments associated with this are formed.

3. From third pharyngeal arches glossopharyngeal CN, common carotid, internal carotid arteries, greater cornu of hyoid, lower part of body of hyoid bone and muscles associated with this are formed.

4. From fourth pharyngeal arches superior laryngeal branch of vagus, part of aortic arch (left), part right subclavian artery (right), thyroid, cricoid, arytenoid, corniculate and cuneform cartilages and muscles associated with this structures are formed.

5. From sixth pharyngeal arches recurrent laryngeal branch of vagus, part of left pulmonary artery (left), part of right pulmonary artery (right), thyroid, cricoid, arytenoid, corniculate and cuneform cartilages and muscles associated with this structures are formed.

Humans have 5 arches - 1, 2, 3, 4, 6 (Arch 5 does not form or regresses rapidly) Face is formed mainly from arch 1 and 2, Arches 3 and 4 (arch 4 and 6 fuse)give rise to structures of neck.

Nonaquatic vertebrates exhibit gill slits even though they never breathe through gills because they share a common ancestor i.e. fish in which these structure first evolved.