Embryology · Head and neck: pharyngeal arches
Pharyngeal pouches: tonsil, thymus, parathyroids and C cells !
- Embryonic structure
Pouches are inner pockets of the embryo's pharynx. They form the middle ear, tonsil, thymus, parathyroids and part of the thyroid.
Updated 2026-11-01
See it in 3D: Auditory tube · Palatine tonsils · Thymus · Parathyroid glands · Thyroid gland
What it is
The pharyngeal pouches are outpocketings of pharyngeal endoderm that push between the arches. There are four well-formed pairs, and their lining gives the tympanic cavity, palatine tonsil, thymus, the four parathyroids and the thyroid C cells. The third-pouch crossover (inferior parathyroids that start higher than the superior ones) and the pouches' role in 22q11.2 deletion are classic questions.
Key facts
- The inferior parathyroid comes from the 3rd pouch and the superior from the 4th: the names 'cross'.
- Because of its long journey, the inferior parathyroid is the most variable: it may stay in the thymus, the mediastinum or near the carotid bifurcation.
- The thymus grows until puberty and is then replaced by fat.
Structure
- Endodermal lining; each gland's connective tissue comes from neural crest mesenchyme.
- The 3rd and 4th pouches have a dorsal and a ventral wing.
- The 5th pouch is rudimentary and is usually absorbed into the 4th.
What it becomes
- 1st: tubotympanic recess → tympanic cavity and auditory tube; its end helps form the tympanic membrane.
- 2nd: epithelial buds invaded by lymphocytes → palatine tonsil; the rest of the pouch remains as the tonsillar fossa.
- 3rd: inferior parathyroids and thymus.
- 4th: superior parathyroids and the ultimobranchial body, which joins the thyroid and gives the parafollicular (C) cells that make calcitonin.
Step by step
- Weeks 4–5: pouches form front to back, guided by FGF and retinoic acid.
- Week 5: the dorsal wing of the 3rd pouch gives the inferior parathyroid, the ventral wing the thymus.
- Weeks 6–7: the thymus descends to the anterior mediastinum and drags the inferior parathyroid along.
- The 3rd-pouch parathyroid ends up below the 4th-pouch one, which has a shorter descent.
- Week 7: the parathyroids settle on the back of the thyroid.
- Between the third and fifth months the tonsil fills with lymphoid tissue.
What each pharyngeal pouch forms
| Pouch | Derivatives |
|---|---|
| 1st | Tympanic cavity (middle ear), auditory tube, mastoid air cells |
| 2nd | Epithelium of the palatine tonsil and tonsillar fossa |
| 3rd | Dorsal wing: inferior parathyroids; ventral wing: thymus |
| 4th | Dorsal wing: superior parathyroids; ventral wing: ultimobranchial body (C cells) |
Markers
- TBX1
- HOXA3
- PAX9
- PAX1
- GCM2
- FOXN1
- EYA1
Clinical notes
- 22q11.2 deletion (DiGeorge): pouches 3 and 4 fail; small or absent thymus (T-cell deficiency) and hypocalcaemia from hypoparathyroidism.
- Ectopic parathyroids: a common cause of persistent hyperparathyroidism after surgery.
- Thymic or parathyroid rests in the neck from incomplete migration.
- C cells give medullary thyroid carcinoma (calcitonin as marker), part of multiple endocrine neoplasia type 2.
- Auditory tube dysfunction (1st pouch): otitis media with effusion, especially with cleft palate.
Test yourself
Think of the answer, then tap the question to see it.
Which pharyngeal pouch gives the inferior parathyroid glands?
The third pouch (dorsal wing), together with the thymus (ventral wing).
Why do third-pouch parathyroids end up below fourth-pouch parathyroids?
The thymus drags them down as it migrates to the mediastinum, a longer journey.
Where do the thyroid C cells come from?
From the ultimobranchial body, which arises from the ventral wing of the fourth pouch.
What comes from the first pharyngeal pouch?
The tympanic cavity (middle ear), the auditory tube and the mastoid air cells.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- The pharyngeal apparatus: arches, pouches, clefts and membranes
- Pharyngeal arches: nerve, muscles, skeleton and artery (the table)
- Pharyngeal clefts and membranes: external ear and cervical sinus
- 22q11.2 deletion (DiGeorge): the third and fourth pouches
- Thyroid gland: from the foramen cecum to the neck
- Ear: inner, middle and external ear
- Branchial cleft cysts, sinuses and fistulas
Related conditions
- DiGeorge syndrome (22q11.2 deletion syndrome)
- Hypoparathyroidism (incl. post-thyroidectomy hypocalcaemia)
- Hypocalcaemia
- Acute otitis media
- Thyroid cancer (papillary)
- Multiple endocrine neoplasia (MEN 1, 2A, 2B)
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Embryology, Pharyngeal Pouch - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023); Embryology, Parathyroid - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023); Langman's Medical Embryology, 13th ed., ch. 17: Head and Neck (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.