Head And Neck: Arch II
Introduction
The second arch forms caudally to
the first arch (Figure 41.1). Pharyngeal arches I and II are bigger than III
and IV. Arch II grows rapidly and inferiorly to cover the smaller arches
forming the s growth forms a ‘lid’ over the other arches and creates the smooth covering of the neck.
Arch II
Highlighting the overlap between
arches I and II at the ear, the stapes bone is formed from the connective
tissue element of the second arch, whereas the malleus and incus bones develop
from the first arch. Likewise, the tensor tympani muscle of the ear forms from
the first arch but the stapedius muscle is derived from the second arch.
The second arch also contributes
to the bony styloid process of the skull, cranial parts and lesser horn (cornu)
of the hyoid bone and the stylohyoid ligament that connects them. The remainder
of the hyoid bone develops from the third arch, highlighting another overlap
between arches.
The cranial nerve of the second
arch is the facial nerve (CN VII; Figure 41.2). Consider this when looking at
the anatomical route that CN VII neurons take in relation to the middle and
inner ear and the styloid process. The facial nerve will innervate the muscle
and mucosal derivatives of the second arch (e.g. the mucosa of the tongue).
The facial nerve is also the
nerve to the muscles of facial expression, and these muscles are derived from
the muscle block of the second arch.
The blood vessels of the
pharyngeal arches are described in the cardiovascular chapters, and in general
the second aortic arches are lost. The second aortic arch forms a stapedial
artery that links internal and external carotid arteries, but normally does not
persist beyond foetal life. The stapedial artery passes through stapes, forming
its foramen. Of note, the middle meningeal artery is associated with the
development of the first and second aortic arches (see box).
Structures formed from the second
pharyngeal arch (Figure 41.3)
Bones Stapes,
styloid process of temporal bone, lesser horn
and superior part of the body of the hyoid bone
Muscles Muscles of
facial expression, stapedius, mylohyoid, posterior
belly of digastric, auricular, buccinator, platysma
Ligaments Stylohyoid ligament
Nerve Facial
nerve (CN VII)
Blood supply Hyoid
artery (foetal), stapedial artery (foetal)
Cleft II
The second pharyngeal cleft
becomes isolated from the external environment by
growth of the second arch when it forms the operculum. Consequently, it forms a sinus with the third
and fourth clefts, the cervical sinus, lined
with ectodermal epithelia (Figure 41.4).
With further growth this sinus
normally disappears entirely. The
ectoderm of the sinus is eventually involved in the epithelial cells of Hassall’s corpuscles found in the
thymus gland.
Pouch II
The endoderm of the second
pharyngeal pouch proliferates and pushes
into the mesenchyme beneath to form lymphoid tissue. The palatine tonsils form as a result, with the
lymphoid tissue derived from the mesoderm, and
epithelial cell‐lined crypts (Figure 41.5).
Clinical relevance
A persistent stapedial artery
presents as a pulsing in the ear or accidentally
found during surgery, but can cause hearing loss.
Pharyngeal cleft (or branchial)
cysts can be found after birth, in which
cysts are located under the platysma muscle, laterally, and anterior to the sternocleidomastoid muscle. The cysts
commonly enlarge slowly and appear much
later in life. The cysts have formed from the remnants
of the pharyngeal clefts that normally combine to briefly create the cervical sinus. The cyst may open
externally as a sinus.
Congenital facial paralysis is
rare but results in a lack of facial expression
and can affect lateral eye movement. Goldenhar syndrome affects both first and second pharyngeal
arches and the affected individual can
show facial palsy, but also malformations of other
facial bones including the maxilla and zygomatic bones.
