Development of the digestive system

Beginning on the 20th day of intrauterine development, the intestinal endoderm in the body of the embryo folds into a tube, forming the primitive gut. The primitive gut is closed in its anterior and posterior sections and is located in front of the chord. The primitive gut gives rise to the epithelium and glands of the digestive tract (except for the oral cavity and anal region). The remaining layers of the digestive tract are formed from the splanchopleura - the medial plate of the unsegmented part of the mesoderm adjacent to the primitive gut.

In the 3rd week of embryogenesis, an ectodermal depression forms at the head end of the embryo - the oral bay, and at the caudal end - the anal (anal) bay. The oral bay deepens towards the head end of the primary intestine. The membrane between the oral bay and the primary intestine (pharyngeal membrane) breaks through in the 4th week of embryogenesis. As a result, the oral bay communicates with the primary intestine. The anal bay is initially separated from the cavity of the primary intestine by the anal membrane, which breaks through later.

In the 4th week of intrauterine development, the ventral wall of the primary intestine forms a protrusion forward (the future trachea, bronchi, and lungs). This protrusion serves as a boundary between the head (pharyngeal) intestine and the posterior trunk intestine. The trunk intestine is divided into the foregut, midgut, and hindgut. The ectodermal lining of the buccal bay forms the epithelium of the oral cavity and the salivary glands. The pharyngeal intestine gives rise to the epithelium and glands of the pharynx; the foregut gives rise to the epithelium and glands of the esophagus and stomach, the midgut gives rise to the epithelial lining of the cecum, ascending and transverse colons, as well as the epithelium of the liver and pancreas. The hindgut is the source of development of the epithelium and glands of the descending, sigmoid colon, and rectum. The remaining structures of the walls of the digestive tract, including the visceral peritoneum, are formed from the visceropleura. The parietal peritoneum and subperitoneal tissue are formed from the somatopleura.

The development of the walls of the oral cavity, the bones of the facial skull, and some internal organs is associated with the transformation of the branchial apparatus of the embryo. Five pairs of protrusions (branchial pockets) are formed on both lateral walls of the pharyngeal intestine, and between them there are seals - branchial arches. The first (maxillary) and second (hyoid) arches are called visceral, the three lower pairs are branchial arches. From the material of the first visceral arch, as a result of various transformations, the upper and lower walls of the oral cavity, the upper and lower jaws, lips, as well as small bones of the organ of hearing (malleus, incus) and chewing muscles are formed. From the tissues of the second visceral arch, the lesser horns and body of the hyoid bone, the styloid process of the temporal bone, the stapes, and the facial muscles are formed. The first branchial arch serves to form the large horns of the hyoid bone, the remaining branchial arches form the cartilages of the larynx. The epithelial lining of the tympanic cavity, auditory tube, etc. is formed from the epithelium of the first branchial pocket, the epithelium of the tonsillar fossa is formed from the second pocket, and the epithelial components of the thymus and parathyroid glands are formed from the epithelium of the third and fourth.

The tongue is formed, starting from the 5th week of embryogenesis, from an unpaired ectodermal rudiment (the end and middle part of the tongue body) and paired ectodermal rudiments (the back of the body, the root of the tongue). The rudiments gradually grow together. The papillae of the tongue are formed in the 6th-7th month of intrauterine life.

Teeth are formed from the ectoderm covering the edges of the maxillary and mandibular processes. The resulting ectodermal dental plate (thickening) gradually sinks into the mesenchyme of the alveolar processes. The pulp is of mesenchymal origin.

In the 2nd month of embryogenesis, the primary intestine undergoes complex transformations. A primary intestinal loop is formed, directed by a bend towards the umbilical opening. The intestine partially exits the abdominal cavity through the umbilical opening (physiological umbilical hernia). Only in the 4th month of intrauterine life does the umbilical ring narrow, and the intestinal loops return to the abdominal cavity. In the 2nd month of embryogenesis, the expansion of the foregut (the future stomach) begins to form. Under the formed primary intestinal loop, a small protrusion appears - the rudiment of the cecum. The small intestine is formed from the descending knee of the intestinal loop, and the large intestine from the ascending knee. The initial section of the descending knee of the intestine is subsequently transformed into the duodenum, and the remaining section - into the mesenteric part of the small intestine. Dorsal to the rudiment of the cecum, the left flexure of the colon is formed, and the transverse and descending colons are formed. By the 6th month of embryogenesis, the ascending part of the colon and its right flexure are formed. The terminal section of the colon is transformed into the sigmoid colon. The rectum is separated from the large intestine due to the formation of a transverse septum at the cloaca in the lower sections of the embryo's body. The growing septum divides the cloaca into the urogenital (anterior) and perineal (posterior) parts. After the rupture of the cloacal (anal) membrane and the formation of the anus, the rectum opens outward. Simultaneously with the differentiation of the intestinal sections, it changes its position as it grows. In the 2nd-3rd month of intrauterine life, the hindgut shifts from the median plane in front of the intestinal loop to the left and upward. The intestinal loop makes a 180° turn to the right (clockwise). The rudiment of the cecum moves to the right upper position; the upper knee of the intestinal loop descends behind the cecum. Against the background of the growth of the intestinal loop, the rudiment of the cecum in the first half of intrauterine development descends to the right and down into the right iliac fossa. The intestinal loop forms a 90° bend to the right. The lengthening of the descending knee of the intestine, the formation of numerous loops of the small intestine significantly displace the colon upward, which changes its position. As a result, the ascending colon takes place on the right in the abdominal cavity, the transverse colon is located in the transverse direction.

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