The Floor of the Pharyngeal Intestine

The floor of the pharyngeal intestine The floor of the pharyngeal intestine is also the source of proliferations which will allow the formation of the tongue and the thyroid gland: 1) The tongue derives from buds that develop from the branchial arches and raise the floor of the pharyngeal intestine: ✪the first arc is at the origin of a bud located on the line median, the tuberculum impar, and a lateral bud on each side. These three buds develop very quickly and together constitute the anterior part of the tongue, in front of the lingual “V”, which projects into the buco-nasal cavity. ✪the second arc is the origin of a bud on the line median which develops little, the copula, ✪the third arc is the origin of a bud on each side which come together to form the hypobranchial eminence. These three buds from the second and third arches together constitute the root of the tongue. The mesenchyme of the branchial arches is at the origin of the connective and vascular elements but the striated muscular fibers derive from the occipital somites. The endoderm which borders the lumen of the pharyngeal intestine gives rise to the lingual epithelium (non-keratinized squamous cell) where the papillae and the first taste buds appear very early next to the nerve endings as well as the intussusceptions (see glossary) which are at the origin of the numerous intralingual salivary glands. 2)The thyroid gland is born from budding of the endodermis on the midline at the junction between the 1st and 2nd arch (between tuberculum impar and copula). This budding invaginates into the underlying mesenchyme in the form of a diverticulum, the end of which develops to give rise to the main thyroid anlage. This anlage migrates in a caudal direction to be placed at the 7th week on the anterior surface of the neck, in front of the thyroid cartilage. During this migration the anlage remains connected to the floor of the pharyngeal intestine by the thyroglossal canal. Normally, this canal disappears, only the distal end persists which gives the pyramidal lobe of the thyroid and its proximal orifice in the form of a blind dimple at the tip of the lingual V, the foramen caecum. The anlage develops on each side to form the thyroid lobes, this development corresponds to a proliferation of cells of endodermal origin in the form of cell cords which fragment, around the 10th week of development, into clusters giving rise to thyroid vesicles. These become functional around the 11th week (presence of colloid and first hormonal secretions). C cells develop at the periphery of the vesicles from neural crest cells embedded in the wall of the pharyngeal intestine. 2) 1-The respiratory diverticulum The respiratory diverticulum appears at the 4th week in the form of a groove from the ventral side of the pharyngeal intestine to its caudal part. This anlage will remain in communication with the cavity of the pharyngeal intestine through the laryngeal orifice, it is separated from the base of the tongue by the epiglottis resulting from budding of the mesenchymal tissue of the 4th arch which raises the endodermal covering on the midline just posterior to the copula. This diverticulum will be at the origin of the development of the respiratory system 2) 2-Development of the esophagus The segment of the primitive anterior intestine located immediately downstream of the respiratory diverticulum changes little during development. It maintains a cylindrical shape around a longitudinal axis and lengthens due to the descent of the gastric anlage into the abdominal cavity. It becomes the thoracic segment of the primitive anterior intestine, after the formation of the diaphragm which separates the pleuroperitoneal cavity into two regions. The internal tunic, derived from the endodermis, transforms into a stratified squamous epithelium, not keratinized in the human species. It is also at the origin of the glandular elements which develop in the surrounding mesenchyme, the esophageal glands and the esophagogastric or cardiac glands in the area of junction with the stomach. The rest of the mesenchyme differentiates during development to produce the connective and muscular constituents of the wall.