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Head And Neck: Arch I


Head And Neck: Arch I
Time period: day 21 onwards
Introduction
Pharyngeal (or branchial) arches are paired structures that develop in the ventrolateral parts of the head of the embryo (Figures 40.1 and 40.2). Six arches will form and contribute to the development of head and neck structures, although arch V is ignored as it fails to appear in human embryos. In this chapter we concentrate on arch I and its derivatives.

Each pharyngeal arch is a bud, or bar of mesenchymal tissue, with clefts separating the arches externally, and pouches separating them internally (Figure 40.3). Pharyngeal pouches develop internally as blebs of the foregut at the level of the pharynx.
Each pharyngeal arch consists of mesenchyme from paraxial and lateral plate mesoderm and receives an influx of neural crest cells. Neural crest cells from rhombomeres 1 and 2 (see Chapter 45 and Figure 45.4) migrate into the first pharyngeal arch. Hox genes, important in the organisation of the segmentation of vertebrates and in setting up the anteroposterior axis, are also important in neural crest cell migration here.
Each arch has its own nerve, artery, connective tissue cells and muscle cells (Figure 40.4).
Head And Neck: Arch I, Ligaments , Muscles, Nerve

Arch I
In week 4 a depression in the surface ectoderm of the embryo forms in the future face, the stomodeum (Figure 40.2). It is continuous with the gut tube and will become the mouth. It forms the centre of the face early in development, and surrounding it are the first pair of pharyngeal arches.
The first arch can be divided into a dorsal maxillary process and a ventral mandibular process (Figure 40.2). The mandibular process contains Meckel’s cartilage, which provides the horseshoe‐ shaped model for the mandible to form around and later degenerates after contributing to the connective tissue structures of the first arch, such as the incus and malleus of the middle ear (see box). The mandible forms by intramembranous ossification, rather than endochondral ossification.
Structures formed from the first pharyngeal arch (Figure 40.5)
Bones Incus, malleus, maxilla, mandible (also squamous part of the temporal bone, zygomatic bone, palatine bones)
Ligaments           Sphenomandibular ligament, anterior ligament of malleus
Muscles               Muscles of mastication (temporalis, masseter, pterygoids), anterior belly of digastric, mylohyoid, tensor veli palatini and tensor tympani
Nerve                   Trigeminal nerve (CN V) (maxillary and mandibular branches – V2 and V3)
Blood supply     Maxillary artery (first branch from the first aortic arch)

Note how the mandibular branch of the trigeminal nerve (CN V) supplies motor fibres to the muscles of mastication in the adult. All these structures are derived from first arch cells. The trigeminal nerve is the cranial nerve of the first arch and is the major sensory nerve of the skin of the face. Only its maxillary and mandibular branches (V2 and V3) supply structures derived from the first arch, however.
Ectoderm and endoderm from this arch also form the mucous membrane and glands on the anterior two‐thirds of the tongue.

Cleft I
The only cleft of any embryological importance in humans is present in week 5 and will form the external auditory meatus of the ear (Figures 40.6 and 40.7). Considering that the overlap between arch I and II structures is at the ear (note the origins of the malleus, incus and stapes bones) this makes sense. The other clefts largely disappear.
The pharyngeal cleft has an ectodermal surface (Figure 40.3).

Pouch I
The first pharyngeal pouch becomes a lengthy ingrowth which becomes the tubotympanic recess (Figure 40.6). With other pharyngeal structures this will form the pharyngotympanic tube. The first pouch extends towards the first cleft. The tubotympanic recess can be divided into dorsal and ventral parts, forming the middle ear cavity and auditory tube, respectively.
The pharyngeal pouch has an endodermal surface (Figure 40.3).

Clinical relevance
Facial abnormalities can be caused by failures of neural crest cell migration and are associated with other abnormalities including cardiac defects. Failure of neural crest cells to migrate into the arches can affect any of the structures that arise from pharyngeal arch I.
Treacher Collins syndrome is characterised by craniofacial abnormalities, including a cleft palate, a small mandible and malformed or absent ears. Linked to a mutation on chromo- some 5 it can be inherited but can also arise from a random mutation. This mutation prevents neural crest cell migration into the first pharyngeal arch. Deformities can vary in severity and surgery is often required. Hearing problems are common, as you might imagine given the ear structures formed by the first arch.
Robin sequence is also characterised by facial abnormalities but is a sequence of events rather than a genetic abnormality, and its cause is not completely understood. Patients show a small mandible, cleft palate and upper airway obstruction. Nasopharyngeal s often necessary to aid breathing and feeding. Surgical treatment is also required.