Development of the digestive system

Starting from the 20th day of intrauterine development, the intestinal endoderm in the fetus's body contracts into the tube, forming the primary gut. The primary intestine is closed in its anterior and posterior parts and is located anterior to the chord. The primary gut gives rise to the epithelium and glands of the digestive tube (except the oral cavity and anal region). The remaining layers of the digestive tube are formed from a splinopleura - a medial plate of the unsegmented part of the mesoderm adjacent to the primary gut.

At the third week of embryogenesis, the ectodermal depression is formed at the head end of the embryo - the oral bay, on the caudal end - the anal (anal) bay. The oral cavity deepens toward the head end of the primary gut. The membrane between the oral cavity and the primary intestine (pharyngeal membrane) breaks through the 4th week of embryogenesis. As a result, the oral bay receives a message with the primary gut. Anal bay initially separated from the cavity of the primary intestine by the anal membrane, which breaks later.

At the 4th week of intrauterine development, the ventral wall of the primary intestine forms an outward protrusion (future trachea, bronchi, lungs). This protrusion serves as a boundary between the head (pharyngeal) gut and the posterior intestine. The anterior, middle and posterior intestine are distinguished in the trunk. From the ectodermal lining of the oral bay, the epithelium of the oral cavity and the salivary glands are formed. The pharynx gives rise to the epithelium and glands of the pharynx; the anterior colon - the epithelium and glands of the esophagus and stomach, the middle intestine - the epithelial cover of the blind, ascending and transverse colon of the intestines, as well as the epithelium of the liver and pancreas. The hindgut is the source of the development of the epithelium and glands of the descending, sigmoid colon and rectum. Of visceropleura formed the remaining structure of the walls of the digestive tube, including the visceral peritoneum. From the somatopleura, a parietal peritoneum and subperitoneal tissue are formed.

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 gill apparatus of the embryo. On each side wall of the pharynx, five pairs of protrusions (gill pockets) are formed, and between them - seals - gill arches. The first (jaw) and the second (hyoid) arc are called visceral, the three lower pairs are the gill arches. From the material of the first visceral arch, the upper and lower walls of the oral cavity, upper and lower jaws, lips, as well as small bones of the hearing organ (malleus, anvil) and chewing muscles are formed as a result of various transformations. From the tissues of the second visceral arch, small horns and the body of the hyoid bone are formed, the styloid process of the temporal bone, stapes, mimic muscles. The first branchial arch serves to form large horns of the hyoid bone, and the cartilages of the larynx form from the remaining gill arches. The epithelium of the first gill pocket forms the epithelial cover of the tympanum, auditory tube, etc., from the second pocket - the epithelium of the amygdala fossa, the epithelium of the third and fourth epithelial components of the thymus and parathyroid glands.

Language is formed, beginning with the 5th week of embryogenesis, from the unpaired ectodermal bookmark (the end and middle part of the body of the tongue) and paired ectodermal bookmarks (the back of the body, the root of the tongue). Bookmarks gradually grow together. Papillae of the tongue form on the 6-7th 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) sinks gradually into the mesenchyme of the alveolar processes. The pulp has a mesenchymal origin.

At the 2nd month of embryogenesis, the primary intestine undergoes complex transformations. A primary intestinal loop is formed, directed by a bend to the umbilical orifice. The intestine leaves partially from the abdominal cavity through the umbilical opening (physiological umbilical hernia). Only on the 4th month of intrauterine life the umbilical ring narrows, the intestinal loops return to the abdominal cavity. At the second month of embryogenesis, the anterior colon begins to form (the future stomach). Under the formed primary intestinal loop appears a small protrusion - the rudiment of the cecum. Their descending knee of the intestinal loop forms a small intestine, from the ascending one it is thick. The initial section of the descending knee of the intestine is later transformed into the duodenum, and the rest of the department is transformed into the mesenteric part of the small intestine. Dorsal to the rudiment of the cecum forms a left flexure of the colon, forming the transverse and descending colon. By the 6th month of embryogenesis, the ascending part of the colon and its right bend are formed. The terminal compartment of the colon is transformed into a sigmoid colon. The rectum is excreted from the large intestine due to the appearance in the lower parts of the trunk of the embryo of the transverse septum near the cloaca. A growing septum divides the cloaca into the genitourinary (anterior) and perineal (posterior) parts. After the breakthrough of the cloacal (anal) membrane and the formation of the anal opening, the rectum opens outward. Simultaneously with the differentiation of the intestine parts, it changes its position as it grows. In the second and third month of intrauterine life, the hindgut is displaced from the median plane in front of the intestinal loop to the left and upwards. The intestinal loop makes a right turn 180 ° (clockwise). The rudiment of the cecum goes to the right upper position; The upper knee of the intestinal loop descends behind the cecum. Against the backdrop of the growth of the intestinal loop, the insertion of the cecum in the southern half of the intrauterine development drops to the right and down into the right iliac fossa. The intestinal loop forms a bend to the right 90 °. Elongation of the descending knee of the intestine, the formation of numerous loops of the small intestine essentially shift the colon to the top, which changes its position. As a result, the ascending colon takes the right side in the abdominal cavity, the transverse colon is located in the transverse direction.

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