Expansion
Ear, tonsil, thymus below, thymus above
Mnemonic
The endodermal pouches, numbered with their derivatives:
- Pouch 1 - middle ear cavity and Eustachian tube
- Pouch 2 - palatine tonsil
- Pouch 3 - inferior parathyroids and the thymus
- Pouch 4 - superior parathyroids and the ultimobranchial body (C cells)
“3 goes down below 4” is the counterintuitive fact worth carrying: the third pouch gives the inferior parathyroids, because they migrate down with the thymus and are dragged past the fourth pouch derivatives.
That migration explains two clinical points: ectopic parathyroid tissue in the mediastinum is almost always inferior gland, and the inferior glands are more variable in position at surgery.
DiGeorge syndrome is failure of the third and fourth pouches, giving thymic aplasia (T cell deficiency) and absent parathyroids (hypocalcaemia), with cardiac outflow defects, from a 22q11 deletion.
Expansion
- Pouch 1 - middle ear cavity and eustachian tube
- Pouch 2 - palatine tonsil
- Pouch 3 - thymus and inferior parathyroid glands
- Pouch 4 - superior parathyroid glands