The floor of the pharyngeal intestine The floor of the pharyngeal intestine is also the source of proliferations which will allow theformation of the tongue and the thyroid gland: 1) The tongue derives from buds that develop from the branchial arches and raise the floorof 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 constitutethe anterior part of the tongue, in front of the lingual “V”, which projects into the buco-nasalcavity. ✪ 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 togetherconstitute the root of the tongue. The mesenchyme of the branchial arches is at the origin ofthe connective and vascular elements but the striated muscular fibers derive from theoccipital somites. The endoderm which borders the lumen of the pharyngeal intestine givesrise to the lingual epithelium (non-keratinized squamous cell) where the papillae and the firsttaste buds appear very early next to the nerve endings as well as the intussusceptions (seeglossary) 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 junctionbetween the 1st and 2nd arch (between tuberculum impar and copula). This buddinginvaginates into the underlying mesenchyme in the form of a diverticulum, the end of whichdevelops to give rise to the main thyroid anlage. This anlage migrates in a caudal direction tobe 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 intestineby the thyroglossal canal. Normally, this canal disappears, only the distal end persists whichgives the pyramidal lobe of the thyroid and its proximal orifice in the form of a blind dimple atthe tip of the lingual V, the foramen caecum. The anlage develops on each side to form thethyroid lobes, this development corresponds to a proliferation of cells of endodermal origin inthe form of cell cords which fragment, around the 10th week of development, into clustersgiving rise to thyroid vesicles. These become functional around the 11th week (presence ofcolloid and first hormonal secretions). C cells develop at the periphery of the vesicles fromneural 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 ventralside of the pharyngeal intestine to its caudal part. This anlage will remain in communicationwith the cavity of the pharyngeal intestine through the laryngeal orifice, it is separated fromthe base of the tongue by the epiglottis resulting from budding of the mesenchymal tissue ofthe 4th arch which raises the endodermal covering on the midline just posterior to thecopula. 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 therespiratory diverticulum changes little during development. It maintains a cylindrical shapearound a longitudinal axis and lengthens due to the descent of the gastric anlage into theabdominal cavity. It becomes the thoracic segment of the primitive anterior intestine, afterthe formation of the diaphragm which separates the pleuroperitoneal cavity into two regions.The internal tunic, derived from the endodermis, transforms into a stratified squamousepithelium, not keratinized in the human species. It is also at the origin of the glandularelements 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 themesenchyme differentiates during development to produce the connective and muscularconstituents of the wall.