Head And Neck: Arch III
The third and fourth pharyngeal arches form in the fifth week (Figure 42.1). Both are considerably smaller than the first two arches and have fewer derivatives.
The connective tissue element of the third arch will become the greater horn (cornu) and caudal parts of the hyoid bone (Figures 42.2 and 42.3). The second arch formed the other parts of the hyoid (see Chapter 41).
The muscle element becomes the stylopharyngeus muscle, passing from the styloid process to the pharyngeal constrictor muscles. The third arch is involved in the development of pharyngeal structures, and its nerve is the glossopharyngeal nerve (CN IX). Motor fibres to the stylopharyngeus muscle are supplied by CN IX, and this nerve also supplies motor fibres and sensory fibres to the pharyngeal plexus for innervation of the muscles and mucosa of the pharynx.
The pharyngeal plexus forms from glossopharyngeal and vagus nerve (CN X) fibres. The vagus nerve is the nerve of the fourth and sixth arches, and the overlap between structures of the head and neck derived from different pharyngeal arches is apparent in adult anatomy.
The third aortic arches, the arteries of the third pharyngeal arches (one on either side of the neck), form major parts of the adult common and internal carotid arteries (see Figure 43.3). The internal carotid arteries are formed from the third aortic arch on either side in combination with the dorsal aortae (see box).
Structures formed from the third pharyngeal arch (Figure 42.3):
Bones Greater horn and the inferior part of the body of the hyoid bone
Muscles Stylopharyngeus muscle
Nerve Glossopharyngeal nerve (CN IX)
Blood supply Common carotid artery and the first part of the internal carotid artery
The third orated into the cervical sinus and eventually obliterated (see Chapter 41).
The endoderm lining the third pharyngeal pouch is involved in the development of endocrine glands in the neck. The third pouch can be split into a ventral wing and a dorsal wing.
Cells of the ventral parts of the left and right third pharyngeal arches come together and migrate caudally to form the majority of the thymus gland (Figure 42.4). Neural crest cells are also involved in development of the thymus gland, which continues to mature after birth.
The cells of the dorsal part of the third pouch differentiate into parathyroid gland cells in the sixth week, and migrate with the cells of the thymus gland to descend into the neck (Figure 42.4). These cells will form the inferior parathyroid gland and later separate from the thymus gland, moving dorsally to it as found in the adult.
Defects in pharyngeal arch III related development affect the formation of the thymus and parathyroid glands. Neural crest cell migration or proliferation problems may affect thymus development.
Congenital hypoparathyroidism is a condition in which parathyroid hormone (PTH) is secreted in low levels, potentially caused by a failure of one or more parathyroid glands to develop normally. Symptoms are wide ranging but often not diagnosed until 2 years of age, and include seizures and poor growth. Treatment options include administering vitamin D and calcium supplements.
Ectopic parathyroid tissue left behind during migration of the cells of the third pouch is relatively common but asymptoma c.
This may be more common for the inferior parathyroid gland.