Pharyngeal pouches

	L.Moss-Salentijn

Outline

	Pharyngeal grooves
	Development of external ear
	Development of tongue
	Development of thyroid gland
	Pharyngeal pouches

Pharyngeal pouch evolution

Slide 4

 Fate of pharyngeal grooves 2-4

Fate of 1^st pharyngeal groove and pouch

First groove     external auditory meatus

Slide 8

First pouch      pharyngotympanic tube

External ear receives 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.

Endodermal swellings on arches  1-4 contribute to the tongue

Merging of lingual swellings

Thyroid gland development. Thyroglossal duct

Descent of developing thyroid. Thyroglossal tract is no longer intact, allowing gland to move.

Adult thyroid gland

Thyroid gland

Slide 19

Pharyngeal pouch at 4 weeks

Slide 21

Derivatives of dorsal and ventral parts of pharyngeal pouches

Second pharyngeal pouch,ventral: development of palatine tonsil

	Third month: subepithelial infiltration of lymphoid tissue: tonsillar stroma
	Extension of solid epithelial strands into stroma
	Epithelial strands open up into tonsillar crypts
	Third trimester: lymphatic follicles develop around crypts.

Palatine tonsil

Palatine tonsil: epithelium of  tonsillar crypt

Third pharyngeal pouch, ventral:
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

Slide 28

Thymus histogenesis

	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
	12th week: development of well-defined medulla and cortex
	Hassall’s corpuscles in medulla: ectodermally derived (third pharyngeal groove?)

Thymus

Third pharyngeal pouch, dorsal: inferior parathyroid gland

	Closely associated with thymus primordium.
	Detach together from pharyngeal wall and travel together; parathyroid bud encased in thymic tissue.
	When primordia pass thyroid by 7th week, primordia separate: inf. parathyroid becomes located on infero-posterior surface of thyroid.

Fourth pharyngeal pouch, dorsal: superior parathyroid gland

	Primordia of superior parathyroid glands develop very near the supero-posterior surface of the thyroid gland.
	They become readily attached or even embedded in the thyroid parenchyma.

Parathyroid gland

Fourth pharyngeal pouch, ventral: ultimobranchial bodies