Human Anatomy - Kotsan I. Y. 2009

Splanchnology
Large glands of the digestive system
Development of the human digestive system

The Digestive System begins to develop even before The formation of the embryo's body through The Emergence of the endodermal layer inside the spherical blastodermal vesicle. During the Formation of the embryonic body (after the 20th day of intrauterine development) and its Separation from extraembryonic elements, the embryonic (intestinal) endoderm rolls up into a tube (the primitive gut), which begins and ends blindly and communicates with the yolk sac. By the 4th week of intrauterine life, the primitive gut, which develops from the endoderm, is located ventral to the notochord. Subsequently, this endoderm gives rise to the epithelium of the digestive tract (except for a part of the Oral Cavity and the anal region), as well as small and large digestive glands (gastric, intestinal, Liver, Pancreas). The remaining layers of the digestive tract (the mucosa excluding the epithelium, submucosa, muscular, and Connective Tissue layers) originate from the splanchnopleure—the inner plate of the unsegmented mesoderm adjacent to the primitive gut. The ectoderm participates in the formation of the oral cavity and the terminal part of the rectum.

At the end of the 4th week of embryonic development, an ectodermal depression—the stomodeum (oral pit)—appears at the cranial end of the embryo, and the proctodeum (anal pit) appears at the caudal end. The stomodeum deepens until it reaches the closed anterior end of the primitive gut. Between the cavity of the primitive gut and the stomodeum, a bilayered buccopharyngeal membrane is formed, consisting of an outer ectodermal and an inner endodermal layer. Soon (at the 4th–5th week of development), the buccopharyngeal membrane ruptures, and the cavity of the stomodeum communicates with the cavity of the primitive gut. The proctodeum is separated from the cavity of the primitive gut by the anal membrane, which is formed, similarly to the buccopharyngeal membrane, by the ectodermal layer of the proctodeum and the endodermal layer of the primitive gut. The rupture of the anal membrane in the 5th week of development leads to the formation of the caudal opening of the primitive gut. Thus, As a result of the rupture of the buccopharyngeal and anal membranes, the embryonic primitive gut gains communication with the external environment at both ends.

In the primitive gut, the HEAD gut and trunk gut are distinguished, the boundary between which is an endodermal outgrowth of the primitive gut—the future Trachea and Lungs. The head gut, in turn, is divided into the oral part and the pharyngeal gut, and the trunk gut into the foregut, midgut, and hindgut. From the oral part, lined with epithelium of ectodermal origin, a part of the oral cavity is formed; from the pharyngeal gut, lined with endodermal epithelium, the deep sections of the oral cavity and the Pharynx are formed. The Esophagus, Stomach, and the initial part of the duodenum develop from the foregut; the Small Intestine and the initial section of the Large Intestine (cecum, ascending and transverse colons) develop from the midgut; and the descending and sigmoid colons and the rectum develop from the hindgut. The somatopleure and splanchnopleure form the Peritoneum.

The Development of the oral cavity is associated with the formation of the facial part of the embryo and the transformation of the pharyngeal arches and clefts. Up to five pairs of outgrowths—pharyngeal pouches—appear on the inner lateral walls of the pharyngeal gut. Pharyngeal clefts do not form in humans. Between the pharyngeal pouches are areas of tissue known as visceral (pharyngeal) arches: the first is the mandibular arch, the second is the hyoid visceral arch, and the third, fourth, and fifth are the pharyngeal arches proper.

From the mandibular (first visceral) arch and adjacent Tissues, the paired maxillary and mandibular processes are formed, which bound the stomodeum inferiorly and laterally. Its upper boundary is the frontonasal process, which extends from the developing Skull base. Subsequently, the frontonasal process divides into medial and lateral nasal processes, from which the External Nose, nasal septum, and the walls of the Nasal cavity are formed. At the same time, the maxillary processes approach each other and fuse with the lateral nasal processes, forming the upper lip. Ridges arise on the inner surface of the maxillary processes and grow toward each other, forming the palate. The maxillae are formed from the maxillary processes, while the lower lip, Mandible, and the floor of the oral cavity are formed from the mandibular processes. The ectoderm covering the edges of the maxillary and mandibular processes provides the primordia for the Teeth. The Tongue is formed from paired and unpaired primordia on the ventral wall of the pharynx in the region of the First and Second pharyngeal arches. The malleus and incus (auditory ossicles) also form from the first visceral arch.

The second through fifth visceral (pharyngeal) arches and adjacent tissues, fusing with each other along the midline, form the anterior part of the neck. The lesser horns of the Hyoid bone, the styloid process, and the stapes (auditory ossicle) are formed from the second visceral arch. The greater horns of the hyoid bone are formed from the third visceral (first pharyngeal) arch.

The epithelial lining of the mucosa of the auditory tube and the tympanic cavity of the Middle ear develops from the epithelium of the first pharyngeal pouch; the epithelium of the tonsillar fossa develops from the second; and the Thymus and Parathyroid glands develop from the third and fourth. The Thyroid Gland is formed from the epithelium of the anterior wall of the pharynx at the boundary between the first and second arches.

Already at the end of the first month of development, the primitive oral cavity, pharynx, esophagus, stomach, and intestine can be distinguished. In the second month, a part of the intestinal tube grows intensively, forming a spindle-shaped dilation—the future stomach, which subsequently bends and rotates 90° to the right—and the intestinal (umbilical) loop, in which descending and ascending limbs are distinguished. The vitelline duct extends from the apex of the intestinal tube, connecting the intestine to the yolk sac. The small intestine is formed from the descending limb, while the terminal part of the small intestine, cecum, ascending and transverse colons are formed from the ascending limb.

The endoderm of the duodenal wall forms cranial and caudal outgrowths from which The Liver and Gallbladder develop, and the pancreas is formed from its ventral and dorsal outgrowths in the same region.

During the 2nd–3rd months of development, the hindgut, located in the sagittal plane, shifts to the left and upward, and the intestinal loop rotates by 180°, as a result of which the cecal bud ends up in the upper right (ascending limb), and the descending limb of the intestinal loop is directed downward. In the second half of intrauterine development, the cecum descends into the right iliac fossa, and the intestinal loop rotates clockwise by another 90⁰ (a total rotation of 270°). The descending limb of the intestine lengthens significantly. As a result, the intestine bends many times, forming loops. The latter displace the developing transverse colon upward. The ascending colon is located on the right, adjacent to the posterior wall of the Abdominal cavity, and the transverse colon lies transversely. This leads to the formation of the right colic (hepatic) flexure. Simultaneously, the upper section of the hindgut shifts to the left and attaches to the posterior wall of the abdominal cavity, forming the descending colon. The left colic (splenic) flexure is formed between it and the transverse colon.

The fold of the dorsal mesogastrium, continuing to grow, descends from the greater curvature of The Stomach downward and lies anterior to the transverse colon and the loops of the small intestine. This is the greater omentum. Its posterior wall in the upper section fuses with the transverse colon and its mesentery. The cavity posterior to the stomach transforms into the omental bursa (lesser sac). The gap between the two leaves of the greater omentum obliterates after birth.