Head And Neck: Arch III
Introduction
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
Cleft III
The third orated into the cervical sinus and eventually
obliterated (see Chapter 41).
Pouch III
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.
Clinical relevance
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.
