L7 Development of Head & Neck PDF

L7 Development of Head & Neck ILOs By the end of this lecture, students will be able to 1. Interpret the development & structure of branchial apparatus. 2. Differentiate between the derivatives of the branchial arches. 3. Correlate the nerve supply of different head& neck structures to...

L7 Development of Head & Neck ILOs By the end of this lecture, students will be able to 1. Interpret the development & structure of branchial apparatus. 2. Differentiate between the derivatives of the branchial arches. 3. Correlate the nerve supply of different head& neck structures to their development. 4. Appraise the congenital anomalies affecting the head & neck relative to their development. BRANCHIAL (PHARYNGEAL) APPARATUS: The branchial arches are six arches, which develop in successive manner from above downward around the pharynx. Each arch contains: a cartilaginous component, a cranial nerve, an artery, muscular component. The artery supplying the fifth arch degenerates early, so the 5th arch itself degenerates too. 1st arch divides into small maxillary and large mandibular parts. Fig. 1: Embryo showing the pharyngeal arches from outside. A. Coronal section in the pharynx of an embryo. Page 1 of 6 What are the derivatives of each pharyngeal arch& its innervation? Ectodermal; Endodermal, and Mesodermal derivatives. ggggggggggg Some of the components ossify; others disappear or become ligamentous. The maxillary process and Meckel’s cartilage of the first arch are replaced by the maxilla and mandible, respectively, which develop by membranous ossification. The cartilage of the 2nd arch is called Reichert's cartilage. Page 2 of 6  Skeletal derivatives of pharyngeal arches Fig. 2: Skeletal derivatives of pharyngeal arches Endodermal derivatives of the pharyngeal pouches: 1st Pharyngeal Pouch: Primitive tympanic cavity& auditory tube 2 nd Pharyngeal Pouch: Palatine tonsil 3rd Pharyngeal Pouch: Forms 2 diverticula: dorsal and ventral. dorsal: becomes the inf. Parathyroid gland. ventral: becomes the thymus. 4th Pharyngeal Pouch: Develops into dorsal and ventral diverticula. The dorsal bud becomes the Sup. Parathyroid. The ventral bud becomes the Ultimobranchial body which fuses with the thyroid gland to give the parafollicular C cells which produce calcitonin. Fig. 3: Endodermal derivatives of the pharyngeal pouches Page 3 of 6 Ectodermal derivatives of pharyngeal clefts The first cleft penetrates the underlying mesoderm to form the external auditory meatus. The mesoderm of the second arch proliferates to form the operculum. This operculum grows caudally to overlap the third and fourth clefts. The operculum forms a space between it and the clefts called cervical sinus. This sinus will disappear to give smooth contour of the neck. Fig. 4: Ectodermal derivatives Abnormalities of Branchial Arches: 1. Ectopic thymus and parathyroid. 2. Branchial fistulas: occurs when pharyngeal pouch 2 and pharyngeal groove 2 persist, forming a patent opening from the internal tonsillar area to the external neck. It is found along the anterior border of the sternocleidomastoid muscle. 3. Branchial (cervical) cyst: occurs when parts of the pharyngeal grooves 2, 3, and 4 persist, thereby forming a cyst. Fig. 5: Abnormalities Page 4 of 6 DEVELOPMENT OF THE FACE: The face is developed from five processes: 1. The maxillary processes (2 in number, originated from the first branchial arch). 2. The mandibular processes (2 in number, originated from the first branchial arch). 3. The frontonasal process formed by proliferation of the mesoderm ventral to the brain. The ectoderm on both sides of the frontonasal process will form nasal placode. The nasal placode invaginates the underlying structures& will transform into nasal pit. The nasal pits divide the frontonasal process into lateral & medial nasal processes. ❖ The frontonasal process will form the bridge of the nose; ❖ The medial nasal processes will form the tip of nose and the filtrum of upper lip, ❖ The lateral nasal processes will form the alae of the nose. The maxillary processes increase in size and grow medially; they meet the medial nasal processes, and fuse with them. The maxillary process will form the upper lip. The lateral nasal process and the maxillary process are separated by deep groove called (nasolacrimal groove). Canalization of this groove leads to formation of nasolacrimal duct. The ectoderm of the maxillary processes and the lateral nasal processes will fuse together burring the nasolacrimal duct. This fusion leads to the formation of the upper part of the cheek and the maxilla. Fig. 6: Development of the face Page 5 of 6 Nerve supply of the processes: 1. The frontonasal process: ophthalmic nerve. 2. The maxillary processes: maxillary nerve. 3. The mandibular processes: mandibular nerve. Abnormalities of the face: 1. Cleft upper lip (hare lip): Most common congenital anomaly, it is due to failure of fusion of the maxillary process and the medial nasal process. 2. Oblique facial cleft: Defect in the face extends from the medial angle of eye to upper lip. It is due to failure of fusion of lateral nasal process with the maxillary process. The nasolacrimal duct is exposed to the surface. 3. Median cleft lower lip It is a rare anomaly. It is due to failure of fusion of the 2 mandibular processes. Fig. 7: facial abnormalities Page 6 of 6