Head And Neck: Arch I
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).
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.
