Pharyngeal Arches

Sumamry This lesson will go through the constituents of each arch and the head and neck structures they develop into.

Recap:

In the previous lesson we looked at how two haploid cells (sperm and ova) develop into three germ layers; trilaminar disc. These layers consist of ectoderm, mesoderm and endoderm, each destined for specific structures and functions. These layers further differentiated and migrated to produce the neural tube, neural crest cells and somites (among others). As the embryo grows and cells proliferate, it folds and is moulded into the foetus. In this lesson we will look at how the pharyngeal arches form, and relate them to the anatomy of the adult head and neck.

Key germ cells:

Neural Crest Cells:

Cranial nerves and sensory ganglia Structures:Adrenal medulla and many connective tissue types of the head and neck e.g. bone, dentine, PDL. Migration of the neural crest cells is an essential process for normal development of the face (and teeth). If this fails to occur we see facial deformities occurring for example: Treacher Collins Syndrome. If the neural crest cells are affected during development, there is a high probabilityReviseDental.com dental abnormalities will be present as well. Mesenchyme:

Derived from mesoderm (head and neck - mainly paraxial mesoderm) Structure: connective tissue of the body

Ectomesenchyme Derived from the neural crest cells When in the head and neck region mesenchyme is known as ectomesenchyme. This is because of the neuroectoderm origin.

Pharyngeal Arches:

Molecular guidance and instructions are vast and if you wish to explore this further in more detail, please see the references below and third party reading. Intra cellular signalling controls, proliferation, migration, differentiation, maintenance and apoptosis. Some important genes and transcription factors include Homeobox genes: Otx, Mxs, Dlx and Barx. Common local signally pathways include sonic hedgehog (Shh), Fibroblast growth factor (Fgf), Bone mophorgenetic proteins (BMP) and vascular endothelial growth factor (VEGF).

If we were to look down on the embryo from above around week 4, small bulges called rhombomeres would be developed laterally from the developing hindbrain. With various molecular guidance and gene expression they migrate forward from the "back of the head" to where the "face and front of neck" would be, to provide the majority of the ectomesenchyme to the development of the pharyngeal structures. The arches proliferate and migrate forward till they meet their counterpart and eventually separate the head from the cardiac bulge.

Arch Anatomy: ReviseDental.com 5 Pharyngeal arches Numbered 1-6 (the 5th arch is transient and doesn't exist)

Each arch consists of:

Germ layers: Ectoderm (outer layer) Mesoderm (middle layer) - head neck: infiltrated by ectomesenchyme Endoderm (inner layer)

Note: Arch one develops anterior to the buccopharyngeal membrane therefore doesn't have an endoderm component and its inner lining is formed of ectoderm (dental example: enamel formation). The imaginary line is considered to be just posterior to the third molar teeth where ectoderm becomes endoderm.

The arches also contain:

Artery Cranial Nerve Skeletal cartilage (support network) Skeletal muscle Separated by a pouch (inner) and a cleft (outer) - note: no mesoderm in these areas.

Note: Arches 1,2 and 3 are the main arches to contribute to the face, oral cavity and tongue. (see next lesson)

The arches ectomesenchyme proliferate and migrate to contribute to the 5 facial processes/ prominences:

Frontal Maxillary (x2) Mandibular (x2)

Therefore, whenReviseDental.com we go on to describe the development of the face and various structures in the head, (see next lesson) we will be referring to the above processes (swelling/ prominences), which have derived from particular arches. Further migration, proliferation, elimination and fusion allows the facial features take form and function. Arches, Clefts and Pouches - What does each develop into?

Tip: Remember the cranial nerve

Arch bone/ Arch Cranial No. Cleft Pouch Adult Vessel cartilage Musculature Nerve Mandible Maxilla ReviseDental.com Muscle of Zygoma mastication Squamous portion Tympanic Mylohyoid of the temporal membrane and Anterior belly of Trigeminal (Vb 1 bone EAM cavity Maxillary artery digastric Vc) Meckel's cartilage: Eustachian Tensor tympani Incus Tube Tensor veli Malleus palatani Sphenomandibular ligament Stapedial: this Reichert's embryonic artery cartilage: Stapaedius connects the Stapes Muscles of facial precursors of the Styloid process expression Contributes to internal carotid, Facial 2 Styloid ligament Posterior belly - the palatine maxillary and (VII) Lesser Horn of of digastric tonsils middle hyoid bone Stylohyoid meningeal Upper body of the Platysma arteries. It is not hyoid bone present at birth. Caroticotympanic Lower body of Inferior hyoid bone Stylopharyngeus parathyroid Glossopharyngeal 3 - Common carotids Greater horns of muscle glands (IX) the hyoid bone Thymus Superior parathyroid glands Pharyngeal Ultimobranchial Larynx cartilages: Right side: constrictors body Superior Thyroid c. subclavian 4 Levator veli - (Parafollicular laryngeal - Vagus Corniculate Left side: aortic palatini cells of the (X) Cuneiform arch Cricothyroid Thyroid-C cells these cells secrete calcitonin) Intrinsic Recurrent Right: pulmonary Cricoid cartilage muscles of the Laryngeal - arteries 6 Arytenoid larynx (note: - - Vagus Left: Ductus cartilages NOT the (X) arteriosus cricothyroid) - obliterated with development or transient Note: if the clefts aren't fully obliterated, a brachial fistula can remain.

Note: Dependent of the text you are reading the cartilage components of arches four and six seem to interchange. ReviseDental.com ReviseDental.com Somites:

Recap: The mesoderm divides into the paraxial, intermediate and lateral plates. The paraxial mesoderm further divides into somites consisting of the myotome, dermatome and sclerotome.

In the area of the hind brain occipital somites begin to develop. The myotome constituents migrate to the area of the future lounge, taking with them what will become the hypoglossal nerve. The sclerotome constituents migrate and contribute to the posterior skull, including: occipital bone, parietal bones and the petrous part of the temporal bone.

In the area of the forebrain and midbrain, paraxial mesoderm differentiates into somatomeres which migrate to contribute to the extra-ocular muscles. They take with them what will become the trochlear and oculomotor nerve. The dermatome segments of the eye is supplied by the ophthalmic branch of the trigeminal nerve (Va).

Conclusion If you remember the nerve that goes with the each arch, you will be able to figure out the majority of the other structures within it. The majority of the facial skeleton is derived from ectomesenchyme (neural crest) with the posterior cranial vault and base derived from mesodermal somites. In the next lesson we will look at how the face develops and the main structures it contains.

Third Party Links ReviseDental.com References Berkovitz BK, Holland GR, Moxham BJ. Oral Anatomy, Histology and Embryology E-Book. Elsevier Health Sciences; 2017 Jul 11. Nanci A. Ten Cate's Oral Histology-e-book: development, structure, and function. Elsevier Health Sciences; 2017 Aug 15. Tortora GJ, Derrickson BH. Principles of anatomy and physiology. John Wiley & Sons; 2018 May 15. Marieb EN, Keller SM. Essentials of Human Anatomy and Physiology: Books a la Carte Edition. Benjamin-Cummings; 2011. Norton NS. Netter's head and neck anatomy for dentistry e-book. Elsevier Health Sciences; 2016 Sep 13. Logan BM, Reynolds P, Rice S, Hutchings RT. McMinn's Color Atlas of Head and Neck Anatomy E-Book. Elsevier Health Sciences; 2016 Oct 21. Atkinson ME. Anatomy for dental students. Oxford University Press; 2013 Mar 14. This content has been written by and uploaded to ReviseDental.com. It is the work of the author and should not be reproduced without express prior permission from the author through ReviseDental.com. © Revise Dental. All rights reserved.

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