The development of the pharyngeal pouches and clefts explained in a very simple way.

If you are completely new to embryology and you want to understand it quickly, this should be the first video you watch:
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Post any questions you have about the video below, I read all the comments:


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In summary, the pouches become:
Pouch 1: Part of the tympanic membrane (eardrum); middle ear cavity; and internal auditory canal (eustachian tube).
Pouch 2: Palatine tonsils.
Pouch 3: Dorsal wing becomes the inferior parathyroid glands. The ventral wing becomes the thymus.
Pouch 4: Dorsal part becomes the superior parathyroid glands. The ventral part becomes the ultimobranchial body.
Pouch 5: There is no pouch five, it is rudimentary.

The clefts become:
Cleft 1: External auditory meatus and part of the tympanic membrane (eardrum).
Clefts 2, 3, and 4 should disappear. Otherwise branchial fistula or cysts may persist.



Summary of video for your notes:
There are four pairs of pouches, the fifth is rudimentary.
The first pharyngeal pouch forms a stalklike diverticulum called the tubotympanic recess. It comes into contact with the epithelial lining of the first pharyngeal cleft.
The second pharyngeal pouch proliferates and forms buds that penetrate into the surrounding mesenchyme. By month 5 the tonsil is infiltrated by lymphatic tissue.
The third pharyngeal pouch is characterised by a dorsal and ventral wing. In the 5th week, the dorsal wing differentiates into the inferior parathyroid gland; while the ventral into the thymus. Both glands lose their connection with the pharyngeal wall, and the thymus migrates caudally and medially pulling the parathyroid with it. The parathyroids will hopefully find the dorsal surface of the thyroid as it descends in the thyroglossal duct.
The fourth pouch also has dorsal and ventral components. The dorsal is the superior parathyroid glands and they detach and find the thyroid as it descends. The ultimobranchial body is later incorporated into the thyroid as the C cells (parafollicular cells). These cells secrete calcitonin, which "calcitonescalcium down" (if high serum calcium is detected, it is released and reduces serum calcium).