Table of Contents
Page created on August 9, 2020. Not updated since.
Learning objectives
- What are the pharyngeal pouches?
- What are the pharyngeal clefts?
- What are the derivatives of the first pharyngeal pouch?
- What are the derivatives of the second pharyngeal pouch?
- What are the derivatives of the third pharyngeal pouch?
- What are the derivatives of the fourth pharyngeal pouch?
- What are the derivatives of the pharyngeal clefts?
- What is a branchial fistula?
- What is a lateral cervical cyst?
Pharyngeal pouches and clefts
The pharyngeal apparatus consists of not only pharyngeal arches, but pharyngeal pouches and clefts as well. The inside of the pharyngeal apparatus is lined by endoderm and forms the pharyngeal pouches, while the outside is lined by ectoderm and forms the pharyngeal clefts. The pharyngeal pouches and clefts, like the arches, give rise to several important adult structures.
There are five pairs of pharyngeal pouches, of which the fifth is rudimentary and doesn’t form anything. There are four pharyngeal clefts.
Derivatives of pharyngeal pouches
First pharyngeal pouch
The pharyngeal pouch forms the tympanic cavity and eustachian tube. The lining of the tympanic cavity later contributes to the formation of the internal aspect of the tympanic membrane.
Second pharyngeal pouch
The epithelium of the second pharyngeal pouch proliferates and forms buds. These buds are later invaded by mesoderm, eventually forming the palatine tonsils. The pouch itself forms the tonsillar fossa.
Third pharyngeal pouch
The third pharyngeal pouch has two “wings”, one pointing dorsally and one pointing ventrally. The dorsal wing of the third pharyngeal pouch gives rise to the inferior parathyroid glands. The ventral wing gives rise to the thymus.
The thymus eventually migrates in the caudal direction to its target position, pulling the inferior parathyroid glands with it. This explains how the inferior parathyroid glands, which originate more cranially than the superior parathyroid glands, eventually end up in a more inferior position.
Fourth pharyngeal pouch
Like the third, the fourth pharyngeal pouch has dorsal and ventral “wings”. The dorsal wing forms the superior parathyroid glands. The ventral wing forms the ultimobranchial body, which will become incorporated in the thyroid gland. This body eventually gives rise to the parafollicular cells or C cells of the thyroid gland, which secrete the hormone calcitonin.
Derivatives of pharyngeal clefts
First pharyngeal cleft
It was previously thought that the first pharyngeal cleft forms the external auditory meatus, but this has later been disproved. Instead, the external auditory meatus forms from an invagination of surface ectoderm from the first pharyngeal arch.
The first pharyngeal cleft itself disappears as the second pharyngeal arch grows.
Second to fourth pharyngeal clefts and the cervical sinus
As the second pharyngeal arch grows it eventually overlaps the third and fourth arches, causing them to lose contact with the outside. The clefts then form a cavity called the cervical sinus, which is lined by ectoderm. The cervical sinus normally disappears with further development.
Malformations
Branchial fistulas
If the second pharyngeal arch doesn’t grow to cover the third and fourth arches, the second, third, and fourth pharyngeal clefts will remain in contact with the outside by a small canal called a branchial fistula.
This fistula often occurs together with a lateral cervical cyst, which is the remnant of a cervical sinus which doesn’t obliterate. The branchial fistula often drains the lateral cervical cyst.
Rarely, the fistula may not be connected to the outside but to the inside, forming an internal branchial fistula. The lateral cervical cyst is then drained into the lumen of the pharynx.
Summary
- What are the pharyngeal pouches?
- They are invaginations on the inside of the pharyngeal apparatus, lined by endoderm
- What are the pharyngeal clefts?
- They are invaginations on the outside of the pharyngeal apparatus, lined by ectoderm
- What are the derivatives of the first pharyngeal pouch?
- Tympanic cavity, eustachian tube, internal aspect of tympanic membrane
- What are the derivatives of the second pharyngeal pouch?
- Palatine tonsils, tonsillar fossa
- What are the derivatives of the third pharyngeal pouch?
- Dorsal wing -> inferior parathyroid glands
- Ventral wing -> thymus
- What are the derivatives of the fourth pharyngeal pouch?
- Dorsal wing -> superior parathyroid glands
- Ventral wing -> ultimobranchial body -> parafollicular cells of thyroid gland
- What are the derivatives of the pharyngeal clefts?
- The external auditory meatus comes not from the first pharyngeal cleft but from an invagination in the first pharyngeal arch
- The first pharyngeal cleft disappears as the second pharyngeal arch grows
- The other pharyngeal clefts form the cervical sinus when the second pharyngeal arch grows
- The cervical sinus later obliterates
- What is a branchial fistula?
- If the third and fourth pharyngeal arches are never covered by the second, the second to fourth pharyngeal clefts will remain in contact with the outside through a small canal called the branchial fistula
- What is a lateral cervical cyst?
- If the cervical sinus doesn’t obliterate it remains as a lateral cervical cyst