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Head And Neck: Arch II


Head And Neck: Arch II
Time period: day 21 onwards
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.

Head And Neck: Arch II, Pouch II

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.

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