The primitive gut tube is derived from the dorsal part of the yolk sac, which is incorporated into the body of the embryo during folding of the embryo during the fourth week. The primitive gut tube is divided into three sections.
Table 11 - Sections of the Gut Tube
The epithelium of and the parenchyma of glands associated with the digestive tract (e.g., liver and pancreas) are derived from endoderm. The muscular walls of the digestive tract (lamina propria, muscularis mucosae, submucosa, muscularis externa, adventitia and/or serosa) are derived from splanchnic mesoderm.
During the solid stage of development the endoderm of the gut tube proliferates until the gut is a solid tube. A process of recanalization restores the lumen.
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, eventually leading to degeneration of both tissue layers.
The tracheoesophageal septum divides the foregut into the esophagus and trachea. See the chapter on Respiratory system for more information.
The primordium of the primitive stomach is visible about the end of the fourth week. It is initially oriented in the median plane and suspended from the dorsal wall of the abdominal cavity by the dorsal mesentery or mesogastrium. During development the stomach rotates 90° in a clockwise direction along its longitudinal axis, placing the left vagus nerve along its anterior side and the right vagus nerve along its posterior side. Rotation of the stomach creates the omental bursa or lesser peritoneal sac.
The duodenum acquires its C-shaped loop as the stomach rotates. Because of its location at the junction of the foregut and the midgut, branches of both the celiac trunk and the superior mesenteric artery supply the duodenum.
Figure 9 - Primitive Digestive Tract
The pancreas develops from two outgrowths of the endodermal epithelium, the dorsal pancreatic bud and the ventral pancreatic bud. During rotation of the gut these primordial come together to form a single pancreas. The ventral pancreatic bud forms the uncinate process and part of the head, while the dorsal pancreatic bud forms the remainder of the head, body, and tail of the pancreas. The ducts of the pancreatic buds join together to form the main pancreatic duct, but the proximal part of the duct of the dorsal pancreatic bud may persist as an accessory pancreatic duct.
Liver and Biliary Apparatus
The liver develops from endodermal cells that form the hepatic diverticulum. The liver grows in close association with the septum transversum, which later forms part of the diaphragm. As it grows the hepatic diverticulum divides into a cranial part, which forms the parenchyma of the liver, and the caudal part, which gives rise to the gallbladder and cystic duct. The hemopoietic cells, Kupffer cells, and connective tissue of the liver are derived from mesenchyme in the septum transversum. The embryonic liver is large and fills much of the abdominal cavity during the seventh through ninth weeks of development.
Blood formation (hemopoiesis) begins in the liver during the sixth week of development, and bile formation begins in the twelfth week.
The spleen develops from mesenchymal cells located between layers of the dorsal mesogastrium.
The midgut communicates with the yolk sac via the yolk stalk. As the midgut forms, it elongates into a U-shaped loop (midgut loop) that temporarily projects into the umbilical cord (physiological umbilical herniation). 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.
The midgut loop rotates 270° counterclockwise around the superior mesenteric artery as it retracts into the abdominal cavity during the tenth week of development.
The hindgut is defined to begin where the blood supply changes from the superior mesenteric artery to the inferior mesenteric artery, i.e. at the distal third of the transverse colon.
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 a ventral (urogenital sinus) and a dorsal portion (anorectal canal). By week seven the urorectal septum reaches the cloacal membrane, dividing it into ventral (urogenital membrane) and dorsal (anal membrane) portions.
The epithelium of the superior two-thirds of the anal canal is derived from the endodermal hindgut; the inferior one-third develops from the ectodermal proctodeum. The junction of these two epithelia is indicated by the pectinate line, which also indicates the approximate former site of the anal membrane that normally ruptures during the eighth week of development.
Figure 10 - Partitioning of the common cloaca
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 return to the maternal circulation.
Congenital Hypertrophic Pyloric Stenosis
Overgrowth of the longitudinal muscle fibers of the pylorus creates a marked thickening of the pyloric region of the stomach. The resulting stenosis (Gk. severe narrowing) of the pyloric canal obstructs passage of food into the duodenum, and as a result after feeding the infant expels the contents of the stomach with considerable force (projectile vomiting). This condition affects approximately 1/150 male infants, but only 1/750 female infants.
The ventral and dorsal pancreatic buds form a ring around the duodenum, thereby obstructing it.
Ileal Diverticulum (Meckel’s Diverticulum)
A remnant of the proximal part of the yolk stalk that fails to degenerate during the early fetal period results in a finger-like blind pouch that projects from the ileum. While this condition occurs in about 1/50 people, it is usually asymptomic and only occasionally leads to abdominal pain and/or rectal bleeding.
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. Ugh.
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.
Sub-hepatic Cecum and Appendix
The cecum and appendix adhere to the inferior surface of the liver during the fetal period, and are carried upwards with it, resulting in an abnormal anatomical position that may create difficulties in diagnosing appendicitis.
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 (cf. volvulus).
Congenital Aganglionic Megacolon (Hirschsprung’s disease)
This results from the failure of neural crest cells to migrate and form the myenteric plexus in the sigmoid colon and rectum. The resulting lack of innervation results in loss of peristalsis, fecal retention, and abdominal distention.
Abnormal formation of the urorectal septum causes the rectum to end as a blind sac above the puborectalis muscle.
Abnormal formation of the urorectal septum causes the rectum to end as a blind sac below the puborectalis muscle.
The anal membrane fails to break down before birth. The anus must be reconstructed surgically, with severity depending on the thickness of the intervening tissue.