Pharyngeal pouch (embryology)

Overview
In the development of vertebrate animals, Pharyngeal or branchial pouches form on the endodermal side between the branchial arches, and pharyngeal grooves (or clefts) form the lateral ectodermal surface of the neck region to separate the arches. The pouches line up with the clefts, and these thin segments become gills in fish.

First pouch
This is the only pouch in which the endoderm and ectoderm remain close together, as the tympanic membrane. There is minimal mesoderm in the tympanic membrane.

Other mesoderm derivatives include: Meckel's cartilage (mandible, malleus, incus, sphenomandibular ligament), muscles (temporalis, masseter, later and medial pterygoids, mylohyoid, anterior belly digastric, tensor tympani, tensor veli palatini, anterior 2/3 tongue), and nerves (CN V2, CN V3).
 * The endoderm lines the future auditory tube, middle ear, mastoid antrum, and inner layer of the tympanic membrane.
 * The ectoderm lines the future external acoustic meatus and outer layer of the tympanic membrane.
 * Mesoderm remains as the middle portion of the tympanic membrane, where the endoderm of the first pharyngeal pouch and the ectoderm of the first pharyngeal cleft have met.

Second pouch

 * Contributes to the middle ear, tonsils, supplied by the facial nerve.

Third pouch

 * The third pouch possesses Dorsal and Ventral wings. Derivatives of Dorsal include the inferior parathyroid glands, while the ventral wings fuse to form the cytoreticular cells of the thymus. The main nerve supply to the derivatives of this pouch is Cranial Nerve IX, glossopharyngeal nerve.

Fourth pouch

 * Derivatives include the superior parathyroid gland and parafollicular C-Cells of the thyroid gland.

Fifth pouch

 * Rudimentary structure, becomes part of the fourth pouch contributing to thyroid C-cells.