branchial pouch

Additional portions of thymus tissue are sometimes developed from the fourth branchial pouches.

They are the remains of the epithelial tubes, which grow out from the third branchial pouches of the embryo to form the thymus.

The parathyroid glands originate from the interaction of neural crest mesenchyme and third and fourth branchial pouch endoderm.

The thymus appears in the form of two flask-shape diverticula, which arise from the third branchial pouch (pharyngeal pouch) of the endoderm.

A pharyngeal groove (or branchial groove, or pharyngeal cleft) is the counterpart of the branchial pouch on the ectodermal side.

Fish gill slits may be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches.

Embryologic origin is from a median endodermal mass in the region of the tongue (foramen cecum) in contrast to the parafollicular (C) cells that arise from the 4th branchial pouch.

Pharyngeal pouches (or branchial pouches) form on the endodermal side between the arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches.

The thymic epithelium is the first to develop, and appears in the form of two flask-shape endodermal diverticula, which arise, one on either side, from the third branchial pouch (pharyngeal pouch), and extend lateralward and backward into the surrounding mesoderm and neural crest-derived mesenchyme in front of the ventral aorta.