HINDGUT LÀ GÌ

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Primitive Gut Tube

The primitive sầu guttube is derived from the dorsal part of the yolk sac, which is incorporated inkhổng lồ the body toàn thân of the embryo duringfolding of the embryo during the fourth week.The primitive gut tube is divided into lớn three sections.

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Table 11- Sections of the Gut Tube

Section

Blood supply

Adult derivatives

Foregut

Celiac artery

Pharynx, lower respiratory system, esophagus, stomach, proximal half of duodenum, liver và pancreas, biliary apparatus

Midgut

Superior mesenteric artery

Small intestine, distal half of duodenum, cecum & vermisize appendix, ascending colon, most of the transverse colon

Hindgut

Inferior mesenteric artery

Left part of transverse colon, descending colon, sigmoid colon, rectum, superior part of anal canal, epithelium of urinary bladder, most of the urethra

The epitheliumof và the parenchyma of glandsassociated with the digestive sầu tract (e.g., liver and pancreas) are derived from endoderm. The muscular wallsof the digestive sầu tract (lamina propria, muscularis mucosae, submucosa,muscularis externa, adventitia and/or serosa) are derived from splanchnic mesoderm.

During the solidstage of development the endoderm of the gut tube proliferates until the gutis a solid tube. A process of recanalizationrestores the lumen.

Proctodeum and Stomodeum

The proctodeum (anal pit) is the primordial anus, and the stomodeum is the primordial mouth. Inboth of these areas ectoderm is in direct liên hệ with endoderm withoutintervening mesoderm, eventually leading to lớn degeneration of both tissue layers.

Foregut

Esophagus

The tracheoesophagealseptum divides the foregut into lớn the esophagus and trachea.See the chapter on Respiratory system for more information.

Stomach

The primordium of the primitivestomach is visible about the kết thúc of the fourth week.It is initially oriented in the donghotantheky.vnian plane and suspended from thedorsal wall of the abdominal cavity by the dorsalmesentery or mesogastrium.During development the stomach rotates 90�in a clockwise direction along its longitudinal axis, placing the leftvagus nerve along its anterior side and the rightvagus nerve along its posterior side. Rotationof the stomach creates the omental bursaor lesser peritoneal sac.

Duodenum

The duodenum acquires its C-shaped loop as the stomachrotates. Because of its location atthe junction of the foregut & the midgut, branches of both theceliac trunk và the superiormesenteric artery supply the duodenum.

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Figure 9 - Primitive Digestive sầu Tract

Pancreas

The pancreas develops from two outgrowths of theendodermal epithelium, the dorsalpancreatic bud và the ventralpancreatic bud. During rotationof the gut these primordial come together to size a single pancreas.The ventral pancreatic bud forms the uncinate process và part of thehead, while the dorsal pancreatic bud forms the remainder of the head, body toàn thân, andtail of the pancreas. The ducts ofthe pancreatic buds join together to form the mainpancreatic duct, but the proximal part of the duct of the dorsal pancreaticbud may persist as an accessorypancreatic duct.

Liver & Biliary Apparatus

The liver develops from endodermal cells that size the hepaticdiverticulum. The liver growsin cthua trận association with the septumtransversum, which later forms part of the diaphragm.As it grows the hepatic diverticulum divides into lớn a cranial part, which forms the parenchymaof the liver, & the caudal part,which gives rise to lớn the gallbladdercystic duct.The hemopoietic cells,Kupffer cells, and connectivetissue of the liver are derived from mesenchymein the septum transversum. Theembryonic liver is large & fills much of the abdominal cavity during theseventh through ninth weeks of development.

Blood formation (hemopoiesis) begins in the liver during the sixth week of development, & bileformation begins in the twelfth week.

Spleen

The spleen develops from mesenchymal cells locatedbetween layers of the dorsal mesogastrium.

Midgut

The midgut communicates with the yolk sac via the yolkstalk. As the midgut forms, it elongates into a U-shaped loop (midgutloop) that temporarily projects inlớn the umbilical cord (physiologicalumbilical herniation). Thecranial limb of the midgut elongates rapidly during development & forms the jejunumcranial portion of the ileum. Thecaudal limb forms the cecum, appendix,caudal portion of the ileum, ascendingcolon, & proximal two-thirds of thetransverse colon. The caudal limb iseasily recognized during development because of the presence of the cecaldiverticulum.

The midgut loop rotates 270�counterclockwise around the superiormesenteric artery as it retracts inkhổng lồ the abdominal cavity during the tenth week of development.

Hindgut

The hindgut is defined khổng lồ begin where the blood supplychanges from the superior mesenteric artery khổng lồ the inferior mesenteric artery, i.e. at the distal third of thetransverse colon.

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Partitioning of the Cloaca

The cloaca is the endodermally lined cavity at the end ofthe gut tube. It has a diverticuluminto lớn the body toàn thân stalk called the allantois.The cloacal membrane separatesthe cloaca from the proctodeum (anal pit). Duringdevelopment a sheet of mesenchyme (urorectalseptum) develops lớn divide the cloaca into lớn a ventral (urogenital sinus) và a dorsal portion (anorectal canal). Byweek seven the urorectal septum reaches the cloacal membrane, dividing it intoventral (urogenital membrane) anddorsal (anal membrane) portions.

Anal Canal

The epithelium of the superior two-thirds of the analcanal is derived from the endodermal hindgut; the inferior one-third developsfrom the ectodermal proctodeum. Thejunction of these two epithelia is indicated by the pectinate line, which also indicates the approximate former site ofthe anal membrane that normallyruptures during the eighth week ofdevelopment.

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Figure10 - Partitioning of thecomtháng cloaca

Clinical Correlations

Esophageal Atresia

Esophageal atresiausually results from abnormal division of the tracheoesophageal septum. Thefetus is unable lớn swallow and this results in polyhydramnios (excessive sầu amount of amniotic fluid) because amnioticfluid cannot pass into lớn the intestines for return lớn the maternal circulation.

Congenital Hypertrophic Pyloric Stenosis

Overgrowth of the longitudinal muscle fibers of the pyloruscreates a marked thickening of the pyloric region of the stomach.The resulting stenosis (Gk. severe narrowing) of the pyloric canalobstructs passage of food inlớn the duodenum, và as a result after feeding theinfant expels the contents of the stomach with considerable force (projectilevomiting). This condition affectsapproximately 1/150 male infants, but only 1/750 female infants.

Annular Pancreas

The ventral & dorsal pancreatic buds form a ring aroundthe duodenum, thereby obstructing it.

Ileal Diverticulum (Meckel�s Diverticulum)

A remnant of the proximal part of the yolk stalk thatfails to lớn degenerate during the early fetal period results in a finger-lượt thích blindpouch that projects from the ileum. Whilethis condition occurs in about 1/50 people, it is usually asymptomic và onlyoccasionally leads to lớn abdominal pain and/or rectal bleeding.

Omphalocele

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

Malrotations of the Midgut

The midgut does not rotate normally as it retracts intothe abdominal cavity. This usuallypresents as symptoms of intestinal obstruction shortly after birth.Malrotation also predisposes the infant to lớn a volvulusof the midgut, wherein the intestines bind và twist around a shortmesentery. Volvulus usually interferes with the blood supply khổng lồ asection of the intestines, & can lead khổng lồ necrosis and gangrene.

Sub-hepatic Cecum and Appendix

The cecum & appendix adhere to lớn the inferior surface ofthe liver during the fetal period, and are carried upwards with it, resulting inan abnormal anatomical position that may create difficulties in diagnosingappendicitis.

Stenosis and Atresia of the Small Intestine

Failure of recanalization of ileum during the solid stageof development leads khổng lồ stenosis (narrowing) or atresia (complete obstruction)of the intestinal lumen. Somestenoses and atresias may be caused by an infarction of the fetal bowel owing toimpairment of its blood supply (cf. volvulus).

Congenital Aganglionic Megacolon (Hirschsprung�s disease)

This results from the failure of neural crest cells tomigrate và khung the myenteric plexus in the sigmoid colon and rectum.The resulting laông chồng of innervation results in loss of peristalsis, fecalretention, và abdominal distention.

Anorectal Agenesis

Abnormal formation of the urorectal septum causes therectum lớn end as a blind sac above sầu the puborectalis muscle.

Anal Agenesis

Abnormal formation of the urorectal septum causes therectum to over as a blind sac below the puborectalis muscle.

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Imperforate Anus

The anal membrane fails to break down before birth.The anus must be reconstructed surgically, with severity depending on thethickness of the intervening tissue.

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