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Pharyngeal arches are one of 4 primary characteristics that all chordates share (along with a notochord, a dorsal nerve chord, and a post-anal tail). Although they are more obvious in fish where they include the gills, they are still a major developmental characteristic of early human embryos. They were formerly called branchial arches (branchia is the Greek word for gill) because even vertebrates which lack gills develop these arches in a pattern equivalent to those vertebrates that possess gills. In human embryos, these arches include cartilages (most of which degenerate), their own aortic arches for a blood supply (most of which degenerate), and grooves or slits between them (which later fuse). Human embryos have 4 visible pharyngeal arches (the first arch has two important subdivisions) and 2 more which are not visible externally.
While a number of adult structures develop from these pharyngeal arches (given below), it is significant that these arches begin their development as structures which are equivalent to those of the early embryos of fish.
--The first arch will eventually form structures such as the upper and lower jaws, the zygomatic bone, the squamous portion of the temporal bone, 2 middle ear bones, a number of muscles (including chewing muscles), and the eardrum.
--The second arch forms a middle ear bone, part of the temporal bone (styloid process), 2 parts of the hyoid bone, tonsils, the thyroid gland (which must descend into neck), and a number of muscles.
--The third arch forms 2 parts of the hyoid bone, the stylopharyngeus muscle, the upper parathyroid gland, and the thymus (which later descends).
--The fourth arch produces larynx cartilages, the lower parathyroid gland, and a few muscles.
--The fifth arch does not develop significantly; it contributes to a pouch (along with the 4th arch) to which the thyroid will fuse.
--The 6th arch produces larynx cartilages and a few muscles.
At 24 days the pharyngeal arches are first visible in the human embryo, beginning with the 1st (mandibular) and the 2nd (hyoid) arches. At 26 days there are 3 pairs of pharyngeal arches. (Moore, p. 91)
The following images are of the pharyngeal arches in a chick embryo.
|Each pharyngeal arch possesses a bar of cartilage, an aortic arch, and a cranial nerve (or branch of a cranial nerve).|
The cartilage of the first arch forms the maxillary and mandibular processes. Embryonic and genetic evidence suggests that the jaws of gnathostomes evolved from the first pair of pharyngeal arches in ancestral vertebrates. The mesenchyme of the maxillary process forms the premaxillary, maxillary, zygomatic, and squamosal (part of temporal) bones. Meckel’s cartilage which forms the mandibular arch forms the entire lower jaw in cartilaginous fish but degenerates in bony vertebrates except for the malleus and incus (Sadler, p.302). The cranial neural crest cells which migrate to the first embryonic arch form two condensations of cartilage, which have previously been called “maxillary” and “mandibular”. Evidence indicates that both the upper and lower jaws form from the cartilage of the “mandibular” condensation while that of the “maxillary” condensation (which was formerly thought to form the upper jaw) forms trabecular cartilage of the neurocranium (Cerny, 2004; Lee, 2004).
The cartilage of the second arch (Reichert’s cartilage) forms the stapes, styloid process, lesser horn of the hyoid, and part of the body of the hyoid.
The cartilage of the third arch forms the lower part of the body of the hyoid and the greater horn of the hyoid.
The fourth and sixth arch cartilages form the thyroid, cricoid, arytenoid, corniculate, and cuneiform cartilages of the larynx (Sadler, p. 304-5).
|The cleft of the first pouch forms the external auditory meatus, the middle ear cavity, and the Eustachian (pharyngotympanic) tube. The tissue which separates the outer and inner parts of the cleft forms the tympanic membrane.|
The second pouch forms the palatine tonsil. The tonsillar fossa in adults is a remnant of the pouch.
The third pouch forms the inferior parathyroid gland (the dorsal portion) and the thymus (the ventral portion). When the thymus descends, the inferior parathyroid gland descends with it.
The fourth pouch forms the superior parathyroid glands. The ventral portion of this pouch may form thymic tissue which later disappears.
The fifth pouch forms the ultimobranchial body which will contribute to the thyroid gland (it contributes the parafollicular cells or C cells which secrete calcitonin). Remnants of the ultimobranchial body can form fistulas and cysts (Sadler, p. 305-8).
The innervation of the pharyngeal pouches is simple, with cranial nerves V (two branches), VII, IX, and X innervating the first four arches (and parts of X innervate the 6th arch). Subsequent modifications of this pattern establishes the innervation pattern of many structures of the adult head and neck. The anterior portion of the tongue is formed by the first arch and is innervated by the mandibular branch of the trigeminal nerve. The posterior portion of the tongue is formed by the third arch and is innervated by the glossopharnyngeal nerve (Sadler, p. 311).
Unfortunately, the reorganization of the early pharyngeal arches does not always occur properly. Some individuals have cysts which develop at the sites of the first pharyngeal grooves. These may form branchial sinuses which open to the outside of the throat or rarely, they may open into the pharynx. (Moore, p. 227). Abnormalities of the second pharyngeal cleft can produce fistulas, sinuses (which may open to the exterior), cysts, and pharyngeal masses. Some require surgery (Gamble, 1998). Other abnormalities in development can result in remnants of branchial cartilages that do not degenerate, masses of thymus tissue which remain in the neck or are connected to the parathyroid glands, parathyroid glands that do not descend, and accessory thyroid glands.