Knowing the development of the pharyngeal (brachial) arches, provides us with a fundamental understanding of the relationships the head and neck structures have with one another. Therefore, when things stray from the norm, we have a greater understanding which structures are likely to have been affected. Thank you to @mydentalnotebook for the wonderful images.
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.
Neural Crest Cells:
Mesenchyme:
Ectomesenchyme
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.
5 Pharyngeal arches Numbered 1-6 (the 5th arch is transient and doesn't exist)
Each arch consists of:
Germ layers:
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:
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:
Therefore, when 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.
Tip: Remember the cranial nerve
| No. | Arch bone/ cartilage | Arch Musculature | Cleft | Pouch | Cranial Nerve | Adult Vessel |
| 1 | Mandible Maxilla Zygoma Squamous portion of the temporal bone Meckel's cartilage: Incus Malleus Sphenomandibular ligament | Muscle of mastication Mylohyoid Anterior belly of digastric Tensor tympani Tensor veli palatani | EAM | Tympanic membrane and cavity Eustachian Tube | Trigeminal (Vb Vc) | Maxillary artery |
| 2 | Reichert's cartilage: Stapes Styloid process Styloid ligament Lesser Horn of hyoid bone Upper body of the hyoid bone | Stapaedius Muscles of facial expression Posterior belly of digastric Stylohyoid Platysma | - | Contributes to the palatine tonsils | Facial (VII) | Stapedial: this embryonic artery connects the precursors of the internal carotid, maxillary and middle meningeal arteries. It is not present at birth. Caroticotympanic |
| 3 | Lower body of hyoid bone Greater horns of the hyoid bone | Stylopharyngeus muscle | - | Inferior parathyroid glands Thymus | Glossopharyngeal (IX) | Common carotids |
| 4 | Larynx cartilages: Thyroid c. Corniculate Cuneiform | Pharyngeal constrictors Levator veli palatini Cricothyroid | - | Superior parathyroid glands Ultimobranchial body (Parafollicular cells of the Thyroid-C cells these cells secrete calcitonin) | Superior laryngeal - Vagus (X) | Right side: subclavian Left side: aortic arch |
| 6 | Cricoid cartilage Arytenoid cartilages | Intrinsic muscles of the larynx (note: NOT the cricothyroid) | - | - | Recurrent Laryngeal - Vagus (X) | Right: pulmonary arteries Left: Ductus arteriosus |
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).
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.
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Nanci A. Ten Cate's Oral Histology-e-book: development, structure, and function. Elsevier Health Sciences; 2017 Aug 15.
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Marieb EN, Keller SM. Essentials of Human Anatomy and Physiology: Books a la Carte Edition. Benjamin-Cummings; 2011.
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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.
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