Editing Embryology (section)

== Head and neck ==

* Pharyngeal (previously called branchial) arches consist of bars of mesenchymal tissue separated by pharyngeal clefts
** Form in about the fourth week
** A core of mesenchymal tissue covered on the outside by surface ectoderm and on the inside by endodermal epithelium
** Each arch contains its own artery (aortic arch), cranial nerve, muscle element, and cartilage bar or skeletal element
** These arches play important roles in facial development
** The fifth arch disappears during development and does not lead to any structures

*[[File:Table - derivatives of arches.png|frameless|663x663px]]

* [[File:Derivatives of pouches.png|right|frameless]]Endodermal pharyngeal pouches form simultaneously along the lateral walls of the pharynx
** Embryo has four pairs of pouches - the fifth is rudimentary
** These pouches penetrate the surrounding mesenchyme but do not establish an open communication with the external clefts; so the whole structure is not really analogous to gills
** Endoderm of the pharyngeal pouches gives rise to a number of endocrine glands and parts of the middle ear

* Pharyngeal clefts are transient structures that all disappear, but in some cases can persist as cervical cysts/fistulas