Head And Neck: Arches IV–VI
The fourth arch is associated with the superior laryngeal branch of the vagus, and the sixth arch is associated with the recurrent laryngeal nerve. The muscles of the sixth arch are the intrinsic muscles of the larynx, and these muscles receive motor innervation from the recurrent laryngeal nerve. The vagus nerve also contributes to the pharyngeal plexus with the glossopharyngeal nerve, innervating pharyngeal mucosa and musculature.
You may also recall the lengthy detour that the recurrent laryngeal nerve takes in the adult, descending from the neck to loop around the subclavian artery on the right and the arch of the aorta on the left, before ascending between the oesophagus and trachea to the larynx. The right subclavian artery forms from the right fourth aortic arch (this is the artery of the fourth pharyngeal arch) and the arch of the adult aorta forms from the left fourth aortic arch (Figure 43.3). The left sixth aortic arch forms the left pulmonary artery and its connection to the aorta: the ductus arteriosus. The right sixth aortic arch forms the right pulmonary artery but its link to the embryonic right dorsal aorta is lost.
The nerves form here too, at the same levels. Evidence that the recurrent laryngeal nerve is the nerve of the sixth arch can be found in the adult, as the left recurrent laryngeal nerve passes around the ligamentum arteriosum, the remnant of the ductus arteriosus, the sixth arch artery linking the left pulmonary artery with the aorta (Figure 43.3). The nerves are prevented from ascending fully into the neck as the embryo grows and lengthens by the ductus arteriosus on the right and the subclavian artery on the left (see box).
Structures formed from the fourth and sixth pharyngeal arches (Figure 43.2)
Cartilage Thyroid, cricoid, arytenoid, corniculate, cuneiform
Muscles Arch IV: cricothyroid, levator veli palatini and the pharyngeal constrictors Arch VI: intrinsic muscles of the larynx (except cricothyroid)
Nerve Arch IV: superior laryngeal, branch of the vagus nerve Arch VI: recurrent laryngeal, branch of the vagus nerve At this level in the embryo we see differences in the cardiovascular system develop between left and right sides.
Blood Arch IV: left, aortic arch; right, subclavian artery Arch VI: left, pulmonary artery and ductus arteriosus; right, pulmonary artery
The fourth pharyngeal cleft is incorporated into the cervical sinus with the third cleft and eventually obliterated (see Figure 41.4).
From the endoderm of the fourth pharyngeal pouch the superior parathyroid glands form. From the fifth pouch (often considered part of the fourth pouch) the ultimobranchial body (or ultimopharyngeal body) forms. Cells of the ultimo-branchial body invade the thyroid gland and differentiate into the C cells (or parafollicular cells) which will produce calcitonin (Figure 43.3). There is no pharyngeal pouch VI.
Congenital cricoid cartilage abnormalities tend to affect the size or shape of the cartilage. This can lead to congenital subglottic stenosis, causing difficulty in breathing, and requires surgery.
Laryngomalacia is a common congenital laryngeal abnormality and patients have a larynx that collapses during breathing causing significant breathing difficulties. Other symptoms include a noise that can be heard during inspiration and gastroesophageal reflux. In most cases as the larynx continues to develop the symptoms are eased and are insignificant by 2 years of age. In severe cases surgery may be required.
Abnormal development of pharyngeal arch IV can affect the parathyroid glands and subsequently the quantity of hormones that these cells produce. Low levels of parathyroid hormone (hypoparathyroidism) can result in hypocalcaemia (low serum calcium levels). DiGeorge syndrome is a known cause of hypoparathyroidism. Symptoms are wide ranging and include muscle cramps, pain in the face and abdomen, dry hair, nails and skin and weak tooth enamel. Treatment includes calcium, vitamin D and synthetic parathyroid hormone supplements.