Head and Neck Anatomy PDF

Embryology Gabin: [email protected] Time Frame of Head anc Neck Development Carnegie Stage 15 = Week 4 ○ Pharyngeal apparatus present Tongue, thyroid and face also begin developing Carnegie Stage 16 = Week 6 ○ Ears appear Carnegie Stage 23 = Week 8 ○ Eye...

Embryology Gabin: [email protected] Time Frame of Head anc Neck Development Carnegie Stage 15 = Week 4 ○ Pharyngeal apparatus present Tongue, thyroid and face also begin developing Carnegie Stage 16 = Week 6 ○ Ears appear Carnegie Stage 23 = Week 8 ○ Eyes moved medially ○ Ears have moved superiorly Pharyngeal Apparatus Development The pharyngeal apparatus develops between the 4th through to 8th week after fertilisation ○ Timing = 4th- 8th week ○ Location = future head and neck ○ Key outcomes = craniofacial development The pharyngeal apparatus is associated with gut tube and hence is lined with endoderm ○ At the oropharyngeal membrane (primordial septum between mouth and pharynx) , this endoderm transitions to ectoderm ○ This is very similar to anus where the transition from ectoderm to endoderm Pharyngeal Apparatus Pharyngeal arch = entire ‘bead’ structure including ectoderm, mesoderm and endoderm Pharyngeal clefts (external) = invaginations of ectoderm Pharyngeal pouches (internal) = evaginations of endoderm Cranial nerves grow and elongate from the brain to innervate each arch and its structures Pharyngeal Arches Each pharyngeal arch is separated from one another by a pharyngeal cleft and pharyngeal pouch Each arch contains 4 structural components ○ Skeletal/ cartilage tissue → mesoderm ○ Muscular tissue → mesoderm ○ Sensory and motor nerves → neural tissue from brain ○ Arch artery Pharyngeal Arches Skeletal tissue and cartilage Muscular derivative Innervation Vasculature Arch 1 Zygomatic bone, Muscles of Trigeminal nerve (CNV2 and Maxillary artery maxilla and mandible mastication V 3) Incus and malleus (upper inner ear bones) Arch 2 Lower part of inner ear bone Muscles of facial Facial nerve (CNVII) Stapedial artery → upper part of hyoid bone expression (embryo)/ Stapes caroticotympanic Styloid process These muscles are artery Stylohyoid ligament superior to muscles of Upper hyoid mastication as develop later and migrate after Arch 3 Lower part of hyoid bone Stylopharyngeus Glossopharyngeal nerve Common carotid muscle (CNIX) artery and 1st part of internal carotid) Pharyngeal Arches Skeletal tissue and Muscular derivative Innervation Vasculature cartilage Arch 4 & 6 Pharyngeal and Laryngeal muscles Branches of vagus nerve Arch 4 = right subclavian laryngeal Arch 4 = superior artery and aortic arch Merge to form cartilages laryngeal nerve thyroid and (cricothyroid Arch 6 = ductus arteriosus pharynx muscle) and pulmonary artery Arch 6 = recurrent laryngeal nerve (all other intrinsic laryngeal muscles) Arch 5 Disappears Forms gills but not in humans Pharyngeal Clefts and Pouches 1st Cleft and Pouch These merge to form the ear canal Intermediate mesoderm in between ectoderm and endoderm forms the tympanic membrane 2nd Cleft and Pouch Pouch 2 invades the mesenchyme to form the palatine tonsils (lymphatic tissue later invades the tonsils) Cleft 2 migrates downwards to fuse with Cleft 3 & 4 to form a smooth neck (persistence of clefts → fluid accumulation e.g. cysts 3rd Cleft and Pouch Inferior parathyroid gland and thymus develops from Pouch 3 Thymus moves inferiorly to Arch 4 pulling the inferior parathyroid gland with it ○ Hence inferior parathyroid gland originates superiorly to the superior parathyroid gland 4th Cleft and Pouch Superior parathyroid gland - Pouch 4 Attaches to the thyroid and becomes superior to Pouch 3 derivatives Tongue Development Summary of tongue development Timing = Week 4 Location = swelling of the pharyngeal apparatus Key outcomes = structures developing near the pharyngeal apparatus Relating the tongue, pharyngeal apparatus and sensory innervation Sensory innervation of the tongue is related to its development from the pharyngeal apparatus The anterior ⅔ of the tongue develops from bilateral lingual swellings of Arch 1 and Arch 2 ○ Over time, Arch 1 overtakes Arch 2 → innervation of the tongue is hence by the lingual nerve (branch of V3) ○ The lingual nerve also carries additional special sensory fibres from CN7 (facial nerve) ○ These fibres travel from CN7 via the chorda tympani The posterior ⅓ of the tongue develops from the copula of Arch 3 ○ Hence it is innervated by the glossopharyngeal nerve ○ Provides both general and special sensation The extreme posterior of the tongue is termed the epiglottis and develops from Arch 6 ○ Innervated by the vagus nerve, providing general and special sensation Musculature of the Tongue Originates from post-otic somites All are intrinsic muscles of the tongue are innervated by the hypoglossal nerve except for palatoglossus m. Thyroid Development Summary of thyroid development Timing = Week 3.5 Location = floor of the primordial pharynx Key outcomes = adult anatomical development of thyroid gland Embryological origin and development The thyroid gland begins development from the thyroid diverticulum (between anterior ⅔ and posterior ⅓ of the tongue) ○ Between lingual swellings and copula During development, migrates inferiorly, anterior to the hyoid bone to adult location in the neck ○ Migration leaves a depression in the tongue (foramen cecum) ○ Migration also creates a duct called the thyroglossal duct Obliterated in most people but not all → can lead to infection Can also develop into the pyramidal/ third lobe of the thyroid Facial and Palatine Development Summary of Facial Development Timing = Week 4 to 8 ○ However, there is still continual development through foetal and post-partum life Location = Cephalic region Key outcomes = bilateral fusion, division of oral and nasal cavities (via hard palate) Facial and palatine development involves the formation of 5 prominences 1 frontonasal prominence 2 maxillary prominences 2 mandibular prominences Frontonasal Prominence Formed from mesenchyme near the brain During Week 5, the frontonasal prominence will develop 2 thickenings near the lateral edges (2 nasal placodes) By Week 6, each develops a central depression called the nasal pit The nasal pit will eventually open to form the nostril and connect to the gut tube Nasal placodes Divided into the: Medial nasal prominence ○ Forms the intermaxillary process, philtrum, columella and primary palate ○ 2 medial nasal prominences fuse during Week 7 to form the intermaxillary process which becomes the nasal septum, philtrum and primary palate - failure of fusion creates a medial cleft lip Lateral nasal prominence ○ Forms the ala of the nose ○ Separated from the maxillary prominence by the nasolacrimal groove - failure of fusion creates an oblique facial cleft Maxillary Prominence Formed by the upper part of Arch 1 Gives rise to the 2 palatine shelves which form the secondary palate after midline fusion ○ Also forms the uvula ○ Failure of fusion creates a bifid uvula and cleft palate Secondary palate also fuses with the primary palate, leaving an incisive foramen which conducts: ○ Nasopalatine nerve ○ Sphenopalatine arteries Also functions to form the cheeks Mandibular Prominence Formed by the lower part of Arch 1 Forms the mandible and base of the oral cavity If the mandible doesn’t expand enough, then the tongue can’t come down. This means the palatine shelves can’t meet because the tongue is in the way. This can lead to a cleft palate.