The pharyngeal arches, also known as visceral arches , are transient structures seen in the embryonic development of and other , that are recognisable precursors for many structures.
In the human embryo, the arches are first seen during the fourth week of development. They appear as a series of outpouchings of mesoderm on both sides of the developing pharynx. The vasculature of the pharyngeal arches are the aortic arches that arise from the aortic sac.
Structure
In
and other
vertebrates, the pharyngeal arches are derived from all three
germ layers (the primary layers of cells that form during embryonic development).
Neural crest cells enter these arches where they contribute to features of the
skull and
facial skeleton such as bone and cartilage.
However, the existence of pharyngeal structures before neural crest cells evolved is indicated by the existence of neural crest-independent mechanisms of pharyngeal arch development.
The first, most anterior pharyngeal arch gives rise to the
mandible. The second arch becomes the
hyoid and jaw support.
In fish, the other posterior arches contribute to the branchial skeleton, which support the gills; in tetrapods the anterior arches develop into components of the ear, tonsils, and thymus.
The genetic and developmental basis of pharyngeal arch development is well characterized. It has been shown that
and other developmental genes such as
DLX are important for patterning the anterior/posterior and dorsal/ventral axes of the
.
Some fish species have a second set of jaws in their throat, known as
, which develop using the same genetic pathways involved in oral jaw formation.
During embryonic development, a series of pharyngeal arch pairs form. These project forward from the back of the embryo toward the front of the face and neck. Each arch develops its own artery, nerve that controls a distinct muscle group, and skeletal tissue. The arches are numbered from 1 to 6, with 1 being the arch closest to the head of the embryo, and arch 5 existing only transiently.
These grow and join in the ventral midline. The first arch, as the first to form, separates the mouth pit or stomodeum from the pericardium. By differential growth the neck elongates and new arches form, so the pharynx has six arches ultimately.
Each pharyngeal arch has a cartilage stick, a muscle component that differentiates from the cartilaginous tissue, an artery, and a cranial nerve. Each of these is surrounded by mesenchyme. Arches do not develop simultaneously but instead possess a "staggered" development.
Pharyngeal pouches form on the endodermal side between the arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches.
The development of the pharyngeal arches provides a useful landmark with which to establish the precise stage of embryonic development. Their formation and development corresponds to Carnegie stages 10 to 16 in mammals, and Hamburger–Hamilton stages 14 to 28 in the chicken. Although there are six pharyngeal arches, in humans the fifth arch exists only transiently during embryogenesis.
First arch
The
first pharyngeal arch, also
mandibular arch (corresponding to the first branchial arch of fish), is the first of six pharyngeal arches that develops during the fourth week of development.
[William J. Larsen (2001). Human embryology. Edinburgh: Churchill Livingstone. ] It is located between the
stomodeum and the first pharyngeal groove.
Processes
This arch divides into a maxillary process and a mandibular process, giving rise to structures including the
of the lower two-thirds of the face and the jaw. The maxillary process becomes the
maxilla (or
upper jaw, although there are large differences among animals
), and
palate while the mandibular process becomes the
human mandible or
lower jaw. This arch also gives rise to the muscles of mastication.
Meckel's cartilage
Meckel's cartilage forms in the mesoderm of the mandibular process and eventually regresses to form the
incus and
malleus of the
middle ear, the anterior ligament of the malleus and the sphenomandibular ligament. The
Human mandible or lower jaw forms by perichondral
ossification using Meckel's cartilage as a 'template', but the maxillary does
not arise from direct ossification of Meckel's cartilage.
Derivatives
The skeletal elements and muscles are derived from mesoderm of the pharyngeal arches.
Skeletal
Muscles
Other
Mucous membrane and glands of the anterior two thirds of the tongue are derived from ectoderm and endoderm of the arch.
Nerve supply
The mandibular and maxillary branches of the
trigeminal nerve (
CN V) innervate the structures derived from the corresponding processes of the first arch. In some lower animals, each arch is supplied by two cranial nerves. The nerve of the arch itself runs along the cranial side of the arch and is called post-trematic nerve of the arch. Each arch also receives a branch from the nerve of the succeeding arch called the pre-trematic nerve which runs along the caudal border of the arch. In human embryo, a double innervation is seen only in the first pharyngeal arch. The mandibular nerve is the post-trematic nerve of the first arch and
chorda tympani (branch of facial nerve) is the pre-trematic nerve. This double innervation is reflected in the nerve supply of anterior two-thirds of tongue which is derived from the first arch.
[Inderbir Sing, G.P Pal-Human Embryology]
Blood supply
The artery of the first arch is the first
aortic arches,
[McMinn, R., 1994. Last's anatomy: Regional and applied (9th ed).] which partially persists as the
maxillary artery.
Second arch
The
second pharyngeal arch or
hyoid arch, is the second of fifth pharyngeal arches that develops in fetal life during the fourth week of development
and assists in forming the side and front of the
neck.
Reichert's cartilage
Cartilage in the second pharyngeal arch is referred to as Reichert's cartilage and contributes to many structures in the fully developed adult.
[Sudhir, Sant, 2008. Embryology for Medical Students 2nd edition] In contrast to the Meckel's cartilage of the first pharyngeal arch it does not constitute a continuous element, and instead is composed of two distinct cartilaginous segments joined by a faint layer of
mesenchyme.
Dorsal ends of Reichert's cartilage
Ossification during development to form the
stapes of the
middle ear before being incorporated into the middle ear cavity, while the ventral portion ossifies to form the lesser cornu and upper part of the body of the
hyoid bone. Caudal to what will eventually become the
stapes, Reichert's cartilage also forms the styloid process of the
temporal bone. The cartilage between the
hyoid bone and styloid process will not remain as development continues, but its
perichondrium will eventually form the stylohyoid ligament.
Derivatives
Skeletal
From the cartilage of the second arch arises
Muscles
Nerve supply
Facial nerve (CN VII)
Blood supply
The artery of the second arch is the second
aortic arches,
which gives origin to the
stapedial artery in some mammals but atrophies in most humans.
Muscles derived from the pharyngeal arches
Pharyngeal muscles or Branchial muscles are
of the head and neck. Unlike
skeletal muscles that developmentally come from
somites, pharyngeal muscles are developmentally formed from the pharyngeal arches.
Most of the skeletal musculature supplied by the cranial nerves (special visceral efferent) is pharyngeal. Exceptions include, but are not limited to, the extraocular muscles and some of the muscles of the tongue. These exceptions receive general somatic efferent innervation.
First arch
All of the pharyngeal muscles that come from the first pharyngeal arch are innervated by the mandibular divisions of the
trigeminal nerve.
These muscles include all the muscles of mastication, the anterior belly of the
digastric, the
Mylohyoid muscle,
tensor tympani, and tensor veli palatini.
Second arch
All of the pharyngeal muscles of the second pharyngeal arch are innervated by the
facial nerve. These muscles include the
facial muscles, the posterior belly of the
digastric, the
stylohyoid muscle, the auricular muscle
and the
stapedius muscle of the middle ear.
Third arch
There is only one muscle of the third pharyngeal arch, the
stylopharyngeus. The stylopharyngeus and other structures from the third pharyngeal arch are all innervated by the glossopharyngeal nerve.
Fourth and sixth arches
All the pharyngeal muscles of the fourth and sixth arches are innervated by the superior laryngeal and the recurrent laryngeal branches of the
vagus nerve.
These muscles include all the muscles of the palate (exception of the tensor veli palatini which is innervated by the
trigeminal nerve), all the muscles of the pharynx (except
stylopharyngeus which is innervated by the glossopharyngeal nerve), and all the muscles of the larynx.
In humans
It has been proposed that the five arches in amniotes numbered 1–4 and 6, be re-named as simply 1–5.
The fifth arch is seen to be a transient structure and becomes the sixth arch, (the fifth being absent). More is known about the fate of the first arch than the remaining four. The first three contribute to structures above the larynx, whereas the last two contribute to the
larynx and trachea.
The recurrent laryngeal nerves are produced from the nerve of arch 5, and the laryngeal cartilages from arches 4 and 5. The superior laryngeal branch of the vagus nerve arises from arch 4. Its arteries, which project between the nerves of the fourth and fifth arches, become the left-side arch of the aorta and the right subclavian artery. On the right side, the artery of arch 5 is obliterated while, on the left side, the artery persists as the ductus arteriosus; circulatory changes immediately following birth cause the vessel to close down, leaving a remnant, the ligamentum arteriosum. During growth, these arteries descend into their ultimate positions in the chest, creating the elongated recurrent paths.
|
| | [[Maxillary artery]], external carotid artery, [[Vidian artery]] |
| | Ascending pharyngeal artery, Inferior tympanic artery, primitive hyoid artery, [[Stapedial artery]] |
| | common carotid, [[internal carotid]] |
| | Right 4th aortic arch: subclavian artery
Left 4th aortic arch: aortic arch
|
| | Right 6th aortic arch: [[pulmonary artery]]
Left 5th aortic arch: pulmonary artery and ductus arteriosus |
Terminology
It has been proposed that the arches be re-named simply as 1–5. The argument is the existence of the fifth arch (and pouch), held to be a transient structure in the embryo.
==Additional images==
with first (mandibular), second (hyoid), pharyngeal arches and third arches labelled]]
See also
-
Branchial cleft cyst
-
Congenital cartilaginous rest of the neck
-
First arch syndrome
-
Splanchnocranium
External links
