Outline • Pharyngeal grooves Pharyngeal pouches • Development of external ear • Development of tongue • Development of thyroid gland L.Moss-Salentijn • Pharyngeal pouches Pharyngeal pouch evolution Pouches lined with foregut endoderm. Grooves lined with ectoderm. Fate of pharyngeal grooves 2-4 Fate of 1st pharyngeal groove and pouch Covered by rapid outgrowth of 2nd arch “operculum.” 1 First groove external auditory meatus First pouch pharyngotympanic External ear receives tube contributions from arches 1 and 2 External ear development by merging of 6 auricular hillocks External ear and tongue development require merging: the elimination of a groove between facial processes by differential growth. 2 Endodermal swellings on arches 1-4 contribute to the tongue Merging 1. Paired lingual swellings and single median tuberculum impar of lingual 2. Single median copula swellings 3-4. Combined median hypobranchial eminence Thyroid gland development. Thyroglossal duct Descent of developing thyroid. Thyroglossal tract is no longer intact, allowing gland to move. Adult thyroid gland Thyroid gland Arrowheads: parafollicular cells. Follicular cells (a). Thyroglobulin in thyroid follicles. Arrowheads: capillaries. 3 Pharyngeal pouch at 4 weeks Epithelio-mesenchymal Derivatives of dorsal and ventral interactions. parts of pharyngeal pouches Specific transcription factors. Second pharyngeal pouch,ventral: Palatine tonsil development of palatine tonsil • Third month: subepithelial infiltration of lymphoid tissue: Crypt in transverse tonsillar stroma section. • Extension of solid epithelial strands into stroma Tonsillar crypt with • Epithelial strands open up into lymphatic follicles (a) tonsillar crypts and stratified • Third trimester: lymphatic follicles squamous epithelium develop around crypts. (b). 4 Palatine tonsil: epithelium of Third pharyngeal pouch, ventral: tonsillar crypt development of thymus Thymus development •4th week: initially bilateral hollow tubes •5th week: elongation of primordia - still attached to pharyngeal pouches •6th week tips of primordia meet and fuse in midline below sternum Thymus histogenesis Thymus • During downward (caudal) growth lumina are lost. Epithelium is transformed into solid branching cords: future thymic lobules • Densely packed epithelium develops into loose reticulum into which lymphocytes appear Hassall’s corpuscles (a) th •12 week: development of well-defined medulla Cortex (a); medulla (b) and cortex • Hassall’s corpuscles in medulla: ectodermally derived (third pharyngeal groove?) 5 Third pharyngeal pouch, dorsal: Fourth pharyngeal pouch, dorsal: inferior parathyroid gland superior parathyroid gland • Primordia of superior • Closely associated with parathyroid glands thymus primordium. develop very near the • Detach together from supero-posterior pharyngeal wall and travel together; parathyroid bud surface of the thyroid encased in thymic tissue. gland. • When primordia pass thyroid • They become readily by 7th week, primordia attached or even separate: inf. parathyroid embedded in the becomes located on infero- posterior surface of thyroid. thyroid parenchyma. Parathyroid Fourth pharyngeal pouch, ventral: Oxyphil cells (a) and gland ultimobranchial bodies Principal or chief cells Ultimobranchial bodies migrate inferiorly. They become embedded in posterior wall of thyroid and give rise to C- cells or parafollicular cells (arrowheads) 6.