Describe the development and derivatives of the pharyngeal pouches

Introduction:  the human embryo has five pairs of pharyngeal pouches. The last one is atypical and therefore considered as part of the fourth. The epithelial endodermal lining of the pouches gives rise to a number of important organs.
First pouch: this form a stalk-like diverticulum, the tubotympanic recess, which comes in contact with the epithelium of the first pharyngeal cleft, the future external auditory meatus. The distal portion of the outpoketing forms the primitive tympanic cavity while the proximal part forms the auditory (Eustachian) tube.
Second pouch: the epithelial lining of this pouch proliferates into the surrounding mesenchyme. The bud is then invaded by mesenchymal tissue forming primodium of the palatine tonsil. The tonsils are gradually infiltrated during the third and fifth month.
Third pouch: the third and fourth pouches have dorsal and ventral wings in their distal ends. In the fifth week the epithelium of the third pouch differentiates into the inferior parathyroid gland, while the ventral part forms the thymus.
Fourth pouch: the epithelium of the dorsal wing of this pouch forms the superior parathyroid gland.
Fifth pouch: it is always considered as part of the fourth. It gives rise to the ultimobranchial body, which is later incorporated into the thyroid gland. The cells of the ultimobrachial body, in the adult, give rise to the parafollicular or C-cells of the thyroid gland.

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