Digestive System Embryology L2 PDF

The development of digestive system (Lecture 2) Dr.Aida Alobaidi 2024-9-23  PANCREAS 1.Formed by two buds, dorsal and ventral, originating from the endodermal lining of the duodenum. 2.Whereas the dorsal pancreatic bud is in...

The development of digestive system (Lecture 2) Dr.Aida Alobaidi 2024-9-23  PANCREAS 1.Formed by two buds, dorsal and ventral, originating from the endodermal lining of the duodenum. 2.Whereas the dorsal pancreatic bud is in the dorsal mesentery, the ventral pancreatic bud is close to the bile duct. 3.When the duodenum rotates to the right and becomes C-shaped, the ventral pancreatic bud moves dorsally in a manner similar to the shifting of the entrance of the bile duct. 4.Finally, the ventral bud comes to lie immediately below and behind the dorsal bud.Later, the parenchyma and the duct systems of the dorsal and ventral pancreatic buds fuse. 5.The ventral bud forms the uncinate process and inferior part of the head of the pancreas. 6.The remaining part of the gland is derived from the dorsal bud.. 1.The main pancreatic duct (of Wirsung) is formed by the distal part of the dorsal pancreatic duct and the entire ventral pancreatic duct. 2.The proximal part of the dorsal pancreatic duct either is obliterated or persists as a small channel, the accessory pancreatic duct (of Santorini). 3.The main pancreatic duct, together with the bile duct, enters the duodenum at the site of the major papilla; the entrance of the accessory duct (when present) is at the site of the minor papilla. 4.In about 10% of cases, the duct system fails to fuse, and the original double system persists  Pancreatic Abnormalities 1.The ventral pancreatic bud consists of two components that normally fuse and rotate around the duodenum so that they come to lie below the dorsal pancreatic bud. Occasionally, however, the right portion of the ventral bud migrates along its normal route, but the left migrates in the opposite direction. In this manner, the duodenum is surrounded by pancreatic tissue, and an annular pancreas is formed The malformation sometimes constricts the duodenum and causes complete obstruction. 2. Accessory pancreatic tissue may be anywhere from the distal end of the esophagus to the tip of the primary intestinal loop. Most frequently, it lies in the mucosa of the stomach and in Meckel's diverticulum, where it may show all of the histological characteristics of the pancreas itself. Development of the midgut characterized by: 1.Rapid elongation of the gut and its mesentery, resulting in formation of the primary intestinal loop 2.At its apex, the loop remains in open connection with the yolk sac by way of the narrow vitelline duct The cephalic limb of the loop develops into 1. The distal part of the duodenum 2. The jejunum 3. part of the ileum The caudal limb becomes 1. The lower portion of the ileum 2. The cecum 3. The appendix 4. The ascending colon 5. The proximal two thirds of the transverse colon.  Physiological Herniation As a result of the rapid growth of the primary intestinal loop and the expansion of the liver, the intestinal loops enter the extraembryonic cavity in the umbilical cord during the sixth week of development  Rotation of the Midgut 1.The primary intestinal loop rotates around an axis formed by the superior mesenteric artery. 2.Rotation 270° counterclockwise occurs during herniation (about 90°), as well as during return of the intestinal loops into the abdominal cavity (remaining 180°). 3.Elongation of the small intestinal loop continues, and the jejunum and ileum form a number of coiled loops The large intestine likewise lengthens considerably but does not participate in the coiling phenomenon. During the 10th week, herniated intestinal loops begin to return to the abdominal cavity A. Anterior view of the intestinal loops after 270° counterclockwise rotation. Note the coiling of the small intestinal loops and the position of the cecal bud in the right upper quadrant of the abdomen. B. Similar view as in A, with the intestinal loops in their final position. Displacement of the cecum and appendix caudally places them in the right lower quadrant of the abdomen. Anomalies 1.Omphalocele herniation of abdominal viscera through an enlarged umbilical ring. The viscera, which may include liver, small and large intestines, stomach, spleen, or gallbladder, are covered by amnion. The origin of the defect is a failure of the bowel to return to the body cavity from its physiological herniation during the 6th to 10th weeks.  Omphalocele occurs in 2.5/10,000 births and is associated with a high rate of mortality (25%) and severe malformations, such as cardiac anomalies (50%) and neural tube defects (40%). Approximately 15% of live-born infants with omphalocele have chromosomal abnormalities 2.Gastroschisis The term applied to a projection of abdominal contents through the body wall directly into the amniotic cavity. It occurs lateral to the umbilicus usually on the right, and the defect is most likely due to abnormal closure of the body wall around the connecting stalk. ❖Viscera are not covered by peritoneum or amnion, and the bowel may be damaged by exposure to amniotic fluid. ❖Gastroschisis occurs in 1/10,000 births but is increasing in frequency, especially among young women; this increase may be related to cocaine use. ❖Unlike omphalocele, gastroschisis is not associated with chromosome abnormalities or other severe defects, so the survival rate is excellent. 3.Volvulus (rotation of the bowel) resulting in a compromised blood supply may, however, kill large regions of the intestine and lead to fetal death Gut Rotation Defects A. Abnormal rotation of the intestinal loop, resulting in a left-sided colon only 90 degree rotation. may result in twisting of the intestine (volvulus) and a compromise of the blood supply. B. Reversed rotation of the intestinal loop, occurs when the primary loop rotates 90° clockwise, the transverse colon passes behind the duodenum (and lies behind the superior mesenteric artery. C. Duplications of intestinal loops and cysts Their origin is unknown, although they may result from abnormal proliferations of gut parenchyma Gut Atresias and Stenoses Atresias and stenoses may occur anywhere along the intestine. 1. Most occur in the duodenum 2. Fewest occur in the colon 3. And equal numbers occur in the jejunum and ileum (1/1,500 births). 4. Atresias in the upper duodenum are probably due to a lack of recanalization. 5. From the distal portion of the duodenum caudally are most likely caused by vascular “accidents.” These accidents may be caused by: 1. Malrotation 2. Volvulus 3. Gastroschisis 4. Omphalocele And other factors Development of Hindgut Gives rise to: ▪ Distal third of the transverse colon ▪ Descending colon ▪ Sigmoid ▪ Rectum, and the upper part(two-thirds) of the anal canal ▪ While the lower part (one-third) is derived from the ectoderm of the cloacal membrane. 1.The terminal portion of the hindgut enters into the posterior region of the cloaca, the primitive anorectal canal. 2.The allantois enters into the anterior portion, the primitive urogenital sinus. 3.The cloaca itself is an endoderm-lined cavity covered at its ventral boundary by surface ectoderm. This boundary between the endoderm and the ectoderm forms the cloacal membrane. 4.A layer of mesoderm, the urorectal septum, separates the region between the allantois and hindgut. 5.The tip of the urorectal septum comes to lie close to the cloacal membrane.At the end of the seventh week, the cloacal membrane ruptures, creating the anal opening for the hindgut and a ventral opening for the urogenital sinus. 6.Between the two, the tip of the urorectal septum forms the perineal body 7.At the 7th week the cloacal membrane raptures and make the anal opining of the hindgut and a ventral opening of the urogenital sinus 8.Since the caudal part of the anal canal originates from ectoderm, it is supplied by the inferior rectal arteries, branches of the internal pudendal arteries. 9.However, the cranial part of the anal canal originates from endoderm and is therefore supplied by the superior rectal artery, a continuation of the inferior mesenteric artery. 10.The junction between the endodermal and ectodermal regions of the anal canal is delineated by the pectinate line, just below the anal columns. At this line, the epithelium changes from columnar to stratified squamous epithelium  Hindgut Abnormalities A,B-Rectourethral and rectovaginal fistulas, which occur in 1/5,000 live births, may be caused by abnormalities in formation of the cloaca and/or the urorectal septum. For example, if the cloaca is too small, or if the urorectal septum does not extend far enough caudally, then opening of the hindgut shifts anteriorly leading to an opening of the hindgut into the urethra or vagina.  Hindgut Abnormalities C-Rectoanal atresias vary in severity and may leave a fibrous remnant or loss of a segment of the rectum and anus. These defects are probably due to vascular accidents in this region similar to those that cause atresias in other regions of the gut. D-Imperforate anus occurs when the anal membrane fails to breakdown Thanks for your attention