Pharyngeal Arches PDF

# General Anatomy & Embryology ## SLO: Pharyngeal (branchial or visceral) Arches By the end of this course, each student should be able to describe the early embryogenesis and the branchial arches and their derivatives. The mesoderm of the lateral wall of the pharynx on both sides proliferates le...

# General Anatomy & Embryology ## SLO: Pharyngeal (branchial or visceral) Arches By the end of this course, each student should be able to describe the early embryogenesis and the branchial arches and their derivatives. The mesoderm of the lateral wall of the pharynx on both sides proliferates leading to the formation of six cylindrical elevations known as pharyngeal arches. Each pharyngeal arch consists of: an outer ectodermal covering, an inner endodermal lining and a mesodermal core between the ectoderm and the endoderm. Four pairs of ectodermal grooves called pharyngeal clefts appear on the external surface of the lateral wall of the pharynx. Five pairs of endodermal grooves called pharyngeal pouches appear on the internal surface of the lateral wall of the pharynx. The level of the ectodermal clefts corresponds to the level of the endodermal pouches and both come in close contact with each other. ### Branchial arches: **Lateral view:** - Cardiac bulge **Frontal view:** - 1st arch - 2nd arch - 3rd arch - 4th arch ## Derivatives of the pharyngeal clefts (ectoderm): **(1) First cleft:** - Its dorsal part forms the external auditory meatus and the outer layer of tympanic membrane. **(2) 2nd, 3rd & 4th clefts:** - The mesoderm of the 2nd arch grows in a downward direction pushing the overlying ectoderm till it fuses with the lower part of the neck forming the skin of the neck and burying the 2nd, 3rd and 4th clefts. The buried clefts form a cavity lined by ectoderm called cervical sinus which later disappears completely and does not give rise to any structure. ### Congenital anomalies: 1. Branchial cyst: due to failure of obliteration of the cervical sinus. 2. Branchial sinus: the cyst may rupture on the surface forming a sinus. 3. Branchial fistula: the cyst may open into the skin and the lumen of the pharynx through a narrow canal. ## Derivatives of the pharyngeal pouches (endoderm): ### 1st Pouch: - Eustachian tube. - Middle ear cavity. - Mastoid antrum. - Inner layer of the ear drum. ### 2nd Pouch: - It forms the primordium of palatine tonsils. ### 3rd Pouch: - It forms the thymus gland and the inferior parathyroid glands. ### 4th Pouch: - It forms the superior parathyroid glands. ### 5th Pouch: - It forms the ultimo-branchial body which is incorporated into the thyroid gland later. | Ectoderm (Clefts) | 4 | |--------------|---| | Endoderm (Pouches) | 5 | | Mesoderm (arches) | 6 | ## Derivatives of the mesoderm of the pharyngeal arches: ### 1st arch: - Meckel's cartilage: the mandible develops as a membrane around it. - Malleus and incus. 2 bones of middle ear - Sphenomandibular ligament. - Muscles: muscles of mastication (temporalis, masseter, medial and lateral pterygoids), mylohyoid m. and anterior belly of digastric m. - Nerve supply: mandibular nerve. ### 2nd arch: - Stapes, Styloid process. - Lesser horn and upper part of the body of the hyoid bone. - Stylohyoid ligament. - Muscles: muscles of scalp and face and posterior belly of digastric muscle. - Nerve supply: facial nerve. ### 3rd arch: - Greater horn and the lower part of the body of the hyoid bone. - One muscle: stylopharyngeus muscle - Nerve supply: glossopharyngeal nerve. ### 4th arch: - Thyroid cartilage of the larynx. - One muscle of the larynx: cricothyroid muscle - Nerve supply: superior laryngeal nerve. - ### 5th arch: - DEGENERATES. - ### 6th arch: - All laryngeal cartilages except the thyroid cartilage. - All muscles of the larynx except the cricothyroid m. - Nerve supply: recurrent laryngeal nerve.