Development of the Gastrointestinal System
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Introduction
Formation from Yolk Sack
The GI system begins to develop from the yolk sack during the fourth week, as the primary role of both is to provide nourishment. The gut epithelium develops from embryonic endoderm, the ventral layer of the trilaminar embryo, while connective tissue, muscle, and mesothelium develop from splanchnic mesoderm (splanchnopleure).
The gut tube becomes suspended by the dorsal mesentery into the primitive peritoneal cavity, formed from the intraembryonic coelom.
Longitudinal folding resulting in the formation of the foregut and hidgut as blind pouches covered by the oropharyngeal and cloacal membranes, respectively.
The spetum transversum, a thick plate of mesoderm that appears with folding of the embryo head, becomes the central tendon of the diaphragm and the ventral mesogastrium. After it is invaded by the developing liver, the ventral mesogastrium produces the falciform ligament, the capsule of the liver, and the lesser omentum. The dorsal mesogastrium forms the greater omentum.
Development of the Foregut
The foregut develops into the following structures:
Primordial pharynx: oral cavity, respiratory system, esophagus, stomach, duodenum, liver, biliary apparatus, and pancreas. With the exception of the pharynx, esophagus, and most of the esophagus, these structures are supplied by the celiac trunk.
Esophagus
The esophagus lengthens both prenatally and postnatally
Stomach
The stomach dilates and rotates during the 4th and 5th weeks
Duodenum
Development of the Midgut
The midgut gives rise to the distal duodenum, the rest of the small intestine,the ascending colon and proximal 2/3 of the descending colon.
The midgut, which is continuous with the yolk sack, grows faster than the rest of the embryo and as such enters the extraembryonic coelom of the umbilical cord during the 6th week, rotating a total of 270 degrees as it returns by the 10th week.
After the midgut returns, fixation of the duodenum, pancreas, and ascending and descending colon occurs through movement retroperitoneally. This reduces movement and the risk of volvulus and intestinal infarction.
Development of the Hindgut
The hindgut gives rise to the distal transverse colon and everything that comes after it up to the pectinate line, the urinary bladder, and the proximal urethra.
The urorectal septum divides the cloaca into the urogenital sinus and anorectal canal
The pectinate line represents the boundary between endoderm and ectoderm, which produces the epithelia of some of the anal canal. This line marks changes in musculature, innervation, arterial supply, and venous drainage.
Congenital GI Problems
esophageal atresia and trancheoesophageal fistula
Results from deviation of the tracheoesophageal septum or failure of esophageal recanalization. THis results in polyhydramnios, as the baby cannot swallow amniotic fluid and therefore pass it back to the mother through the placenta.
Pyloric stenosis
Hypertrophic pyloric stenosis affects 1:150 males and 1:750 females. It occurs in the circular muscle, results in projectile vomiting, and must be surgically corrected.
Extrahepatic biliary atresia
Obstruction or failure of canalization of the biliary ducts results in jaundice and must be corrected surgically.
Annular pancreas
Rare, but can cause duodenal obstruction
Duodenal atresia and stenoses
Occlusion by epithelial cells can accompany Down syndrome or prematurity. Most vomit contains bile, as the block usually occurs distal to the bile duct. Polyhdramnios will occur.
Cysts and intestinal duplication
Almost all duplications are caused by failure of normal recanalization, resulting in two lumina
Meckel's Diverticulim
An ileal diverticulum is one of the most common problems in the digestive tract, occurring in 2-4% of infants. The can contain gastric tissue, producing inflammation, ulceration, and bleeding, mimicking appendicitis.
Midgut rotation defects and defects of fixation
Failure in rotation or fixation can result in volvulvus (twisting), leading to infarction and gangrene.
Hirschprung disease (congenital megacolon)
Absence of proper neural crest migration and formation of autonomic ganglion cells (Auerbach and Meissner plexuses) results in dilation and absence of peristalsis, causing buildup of intestinal contents. The RET gene appears important in causing Hirschprung disease.
Anorectal malformations
Abnormal development of the urorectal septum results in imperforate anus, atresia, stenosis, or fistulas.
Gastroschisis
A split in the anterior body wall results in the protrusion of viscera into the amniotic cavity, and results from incomplete closure of the lateral folds during the 4th week of development.
Omphaolocele
Persistence of the intestine in the umbilical cord results in omphaolocele, with umbilical epithelium, a derivitive of the amnion, covering protruding structures.
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