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
Cleft IV
The
fourth pharyngeal cleft is incorporated into the cervical sinus with the third
cleft and eventually obliterated (see Figure 41.4).
Pouch IV
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.
Clinical relevance
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.
