Embryology20 Dr.Ban

The midgut Organs in the adult mid gut: Duodenum Jejunum Ileum Cecum Appendix Ascending colon Hepatic flexure of colon Transverse colon (proximal 2/3rd )

The mid gut is the portion of the embryo from which most of the intestine develop. During development, the human mid gut undergoes a rapid phase of growth in which the loop of mid gut ( U shaped loop )herniates outside of the abdominal cavity of the fetus and protrudes into the umbilical cord. This herniation is physiological (occurs normally). • The upper limb of the U is destined to be form the future small intestine • The lower limb forms the ascending and transverse colon. • At the tip of the U, the mid gut is attached to the umbilicus by a thin duct called the vitellointestinal duct which disappears during the later stages of development. • The space between the 2 limbs of the U has the mesentry – a fan shaped structure that holds all the loops of intestine together.

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The midgut loops slips back out of the umbilical cord and the physiological hernia ceases to exist. This change coincides with :  the termination of the yolk sac  and the counter clockwise rotation of the two limbs of the midgut loop around their combined central axis.

The U loop undergoes 3 rotations in a step wise manner:

First it rotates by 90° in the anticlockwise direction (as seen from the front) along the axis of the superior mesentric artery. At the end of this first rotation the upper limb of the U, or the future ileum comes to lie on the fetus’s right and the lower limb of U or the future colon lies on the left.At the end of 10th week, the midgut retracts back into the abdominal cavity. Though the reason for this retraction is not exactly known, the midgut undergoes an additional 180 degrees anticlockwise rotation.The net rotation of the entire midgut is 270° anticlockwise (90° + 90° +90° ). This brings the cecum (developed from the lower limb of the U) to the right side.

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Rotation and herniation of the midgut through the umbilicus (A and B) At the end of the sixth week, the primary intestinal loop herniates into the umbilicus, rotating through 90 degrees counterclockwise. (C) The small intestine elongates to form jejunalileal loops, the cecum and appendix grow, and at the end of the tenth week, the primary intestinal loops retracts into the abdominal cavity, rotating an additional 180 degrees counterclockwise. (D and E) During the eleventh week, the retracting midgut completes this rotation as the cecum is positioned just inferior to the liver. The cecum is then displaced inferiorly, pulling down the proximal hindgut to form the ascending colon. The descending colon is simulataneously fixed on the lest side of the posterior abdominal wall. The jejunum, ileum, transverse colon, and gigmoid colon remain suspended by mesentery.

The cranial limb of the midgut elongates rapidly during development and forms the jejunum and cranial portion of the ileum. The caudal limb forms the cecum, appendix, caudal portion of the ileum, ascending colon, and proximal two-thirds of the transverse colon. The caudal limb is easily recognized during development because of the presence of the cecal diverticulum.

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Diagram showing the process by which the intestine rotates and herniates during normal development. From panel A to B (left-sided views), the midgut loop rotates 90° in a counterclockwise direction, so that its position changes from midsagittal (A) to transverse (B1). The small intestine forms loops (B2) and slides back into the abdomen (B3) during resolution of the hernia. Meanwhile, the cecum moves from the left to the right side, which represents the additional 180° counterclockwise rotation of the intestine (C, central view). Hindgut

The hindgut is the posterior (caudal) part of the alimentary canal extends from the left 1/3rd of the transverse colon to the cloaca (rectum), is supplied by the inferior mesenteric artery . In contrast to the midgut, no intestinal rotation occurs here but rather this part gets pushed to the left side by the midgut returning from the umbilical coelom. It consists of: Transverse colon -left 1/3rd Descending colon Sigmoid colon Rectum Anal canal

Partitioning of the cloaca

The cloaca is the endodermally lined cavity at the end of the gut tube. It has a diverticulum into the body stalk called the allantois.The cloacal membrane separates the cloaca from the proctodeum (anal pit). During development a sheet of mesenchyme (urorectal septum) develops to divide the cloaca into:

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-ventral (urogenital sinus) -dorsal portion (anorectal canal).

Proctodeum and Stomodeum The proctodeum (anal pit) is the primordial anus, and the stomodeum is the primordial mouth. In both of these areas ectoderm is in direct contact with

endoderm without intervening mesoderm.

Mesentery

Portions of the gut tube and its derives are suspended from the dorsal and ventral body wall by mesenteries, double layers of peritoneum that enclose an

organ and connect it to the body wall. Such organs are called intraperitoneal. Mesenteries provide pathways for vessels, nerves, and lymphatic structures to and from abdominal viscera.

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Clinical Correlations Esophageal atresia

Esophageal atresia usually results from abnormal division of the tracheoesophageal septum. The fetus is unable to swallow and this results in polyhydramnios (excessive amount of amniotic fluid) because amniotic fluid cannot pass into the intestines for rturn to the maternal circulation.

Annular pancreas The ventral and dorsal pancreatic buds form a ring around the duodenum, thereby obstructing it.

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Omphalocele

The midgut fails to retract into the abdominal cavity. At birth, coils of intestine covered with only a transparent sac of amnion protrude from the umbilicus.

Malrotations of the midgut

The midgut does not rotate normally as it retracts into the abdominal cavity. This usually presents as symptoms of intestinal obstruction shortly after birth. Malrotation also predisposes the infant to a volvulus of the midgut, wherein the intestines bind and twist around a short mesentery. Volvulus usually interferes with the blood supply to a section of the intestines, and can lead to necrosis and gangrene. Midgut volvulus is a condition in which the intestine has become twisted as a result of malrotation of the intestine during fetal development. Malrotation of the intestine occurs when the normal embryologic sequence of bowel development and fixation is interrupted

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Stenosis and atresia of the small intestine

Failure of recanalization of ileum during the solid stage of development leads to stenosis (narrowing) or atresia (complete obstruction) of the intestinal lumen. Some stenoses and atresias may be caused by an infarction of the fetal bowel owing to impairment of its blood supply.

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