Evolution of the Anterior Primitive Gut

EVOLUTION OF THE ANTERIOR PRIMITIVE GUT 1. Development of the pharyngeal gut Initial portion of the primitive anterior intestine, the pharyngeal intestine follows the oral cavity and corresponds to the covering of the internal surface of the branchial arches. These mesoblastic masses are covered on the outside by the ectoderm which forms folds between the arches, the ecto branchial pockets. The first of these pockets will be at the origin of the external acoustic meatus, the following will be covered by the significant expansion of the second arch and will give a tubular cavity which disappears during development, the cervical sinus. The endodermis covers the internal surface of the branchial arches and also forms folds between the arches, the endobronchial pockets, located opposite the ecto branchial pockets. 1.1. The oral-pharyngeal mucosa The internal covering of the gill region is the origin of the mucous membrane which lines the pharynx and the posterior part of the cavity oral. It is made of a non-keratinized squamous epithelium, deriving from the endodermis, resting on a very vascularized connective chorion, deriving from the intra-embryonic mesenchyme, which contains numerous small salivary glands of endodermal origin. 1.2. Endobronchial pockets The endobronchial pockets, which appeared on the lateral surfaces from the 4th week, give important derivatives during the 6th and 7th weeks of development: ✪ the first deepens and lengthens at the same time as it widens the oral region, it gives the tubo-tympanic recess at the origin of the auditory tube and the tympanic cavity. Its end joins the bottom of the first ecto branchial pocket which becomes the external acoustic meatus (the tympanum will remain a membrane resulting from the joining of the endodermal and ectodermal layers separated by a thin layer of connective tissue of mesenchymal origin). ✪ the second, shallow, gives budding which sinks into the underlying chorion and will subsequently be the site of colonization by lymphoid cells; thus the palatine tonsil is formed on each side (the endodermal trabeculae are at the origin of the crypts). ✪ The third gives two distinct buddings: the ventral part gives on each side, a budding voluminous, the main anlage of the thymus, which plunges towards the midline and migrates in a caudal direction to join its contralateral counterpart the dorsal part gives a small distinct bud which is transforms into parathyroid gland outline. ✪The fourth also gives two buddings: the dorsal part, an outline of the parathyroid gland the ventral part an accessory thymic anlage . ✪The fifth pocket is rudimentary in the human species which does not have an ultimo branchial body. (The C cells of the thyroid which secrete the hypocalcemic hormone calcitonin, derive from neural crest cells. From the 8th week, the thymic anlages meet at the midline and the thymus migrates in a caudal direction to reach its final location in the upper part of the thorax. The primordia are colonized from the 11th week of development by dendritic cells and lymphoid cells, the thymus becomes a site of differentiation of lymphoid cells. The migration of the thymic primordia leads to that of the parathyroid primordia: the parathyroid primordia derived from the 4th endobronchial pouches are placed at the lower pole of the thyroid lobes, these are the lower parathyroids, those derived from the 3rd endobronchial pouches stop behind the upper pole of the thyroid lobes, these are the superior parathyroids.