Normal Embryology, Anatomy, and Physiology of the GI Tract Flashcards
What are the three germ layers in embryological development?
The gastrointestinal (GI) system involves three germ layers: mesoderm, endoderm, ectoderm.
MESODERM
The mesoderm gives rise to the connective tissue, including the wall of the gut and the smooth muscle.
ENDODERM
The endoderm is the source of the epithelial lining of the gastrointestinal tract, liver, gallbladder, and pancreas.
ECTODERM
The ectoderm divides into the surface ectoderm, neural tube, and neural crest.
How can the gastrointestinal tract be divided embryologically?
The gastrointestinal system has three divisions: the foregut, midgut, and hindgut.
FOREGUT
The foregut starts with the oral cavity and ends at the first part of the duodenum, blood supply is from the celiac artery.
MIDGUT
The midgut covers the mid-duodenum to the proximal two-thirds of the trans- verse colon, blood supply is from the superior mesenteric artery.
HINDGUT
The hindgut extends from the latter one-third of the transverse colon to the upper portion of the anus, blood supply is from the inferior mesenteric artery.
When does midgut herniation occur?
At week 6–10 of gestation the midgut herniates through the umbilical ring.
The midgut then develops almost entirely outside of the peritoneal cavity, then rotates and returns back into the abdominal cavity around week 10 of gestation.
What is the omphalomesenteric duct?
The omphalomesenteric duct or vitelline duct forms a connecting tract between the embryonic yolk sac and primitive midgut. It usually obliterates during week 8 of gestation.
Failure of complete obliteration of the omphalomesenteric duct produces several malformations including a Meckel’s diverticulum, patent omphalomesenteric duct and omphalomesenteric sinus.
What are the different cells of the intestinal mucosa?
Cell Types
1) Absorptive
- Epithelium/intestinal glands
- Digestion and absorption of nutrients and chyme
2) Goblet
- Epithelium/intestinal glands
- Secretion of mucus
3) G cells
- Intestinal glands
- Secretion of hormone intestinal gastrin
4) I cells
- Intestinal glands
- Secretion of the hormone cholecystokinin which stimulates release of pancreatic juice and bile
5) K cells
- Intestinal glands
- Secretion of the hormone glucose–dependent insulinotropic peptide which stimulates the release of insulin
6) M cells
- Intestinal glands
- Secretion of the hormone motilin, which accelerates gastric emptying, stimulates intestinal peristalsis and stimulates the production of pepsin
7) S cells
- Intestinal glands
- Secretion of the hormone secretin
What are the Auerbach and Meissner plexi?
The Auerbach plexus is located between the circular muscle layer and the longitudinal muscle layer in the lower esophagus, stomach, and intestines.
The Meissner plexus is located in the submucosal tissue, which connects the surface mucous membrane lining to the deeper muscle layers in the stomach and intestines.
In which part of the alimentary tract does absorption occur?
Absorption of most of the nutrients occurs in the jejunum.
Where do the following nutrients absorb?
• Iron—The absorption of iron nutrients occurs in the duodenum.
• Lipids and water—The absorption of lipids and water occurs through passive
diffusion within the small intestine.
• Bile salts—These are absorbed within the terminal ileum.
• Vitamin B12—In the terminal ileum after binding to the intrinsic factor which is secreted from the stomach.
• The Fat-Soluble Vitamins: A, D, E, and K—Need lipids and bile salts to be absorbed in the ileum [5].
What is the blood supply of the large colon?
The superior mesenteric artery supplies the cecum, ascending, and proximal two-thirds of the transverse colon from blood.
The inferior mesenteric artery supplies the splenic flexure, descending colon and sigmoid.
The venous drainage of the colon is similar to the arterial supply, the superior mesenteric and inferior mesenteric veins ultimately empty into the hepatic portal vein [6].
What is the blood supply of the rectum and anal canal?
- Superior rectal artery originates from the inferior mesenteric artery
- Middle rectal arteries, which are branches of the internal iliac arteries
- Inferior rectal arteries from the internal pudendal arteries
- The submucosal venous plexus above the pectinate line drains into the superior rectal veins (portal circulation)
- The submucosal plexus below the pectinate line drains into the inferior rectal veins (systemic circulation).
What are the porto-systemic anastomoses?
• It is the connection between the veins of the systemic venous system and the
portal vein system.
• The major anastomoses sites are:
Lower esophagus: Left gastric veins (portal system) and the oesophageal tributaries to the azygous system
Rectal: Superior rectal veins (portal) and inferior and middle rectal veins (systemic)
Paraumbilical: Paraumbilical veins (portal) and epigastric veins (systemic)
Retroperitoneal: The portal tributaries of the mesenteric veins and the retroperitoneal veins
What is the role of the ileocecal sphincter?
The ileocecal sphincter creates a mechanical barrier to prevent the entry of bacteria into the small intestine. It assists in controlling the movement of nutrients and fluids into the colon.
The loss of an ileocecal sphincter can also cause a condition known as small bowel bacterial overgrowth.
The illness causes poor absorption of vitamin B12 and fat, as well as diarrhea.
Other effects of the condition include bile salts’ de-conjugation, liver damage, abdominal pains, and loss of fluid.
Which characteristic features differentiate the large intestine from the small intestine?
- The large intestine has fat-filled, peritoneal bags attached to its surface called epiploic appendices.
- The large intestine contains the teniae coli that move vertically along the large intestine’s surface, which diverges at the recto-sigmoid junction to form an extensive coating in the rectum.
- The teniae coli’s contraction reduces the wall of the large intestine, thereby leading to haustration.
- The ascending and descending part of the colon are fixed to the retroperito- neum.
What is a watershed area?
A watershed area refers to any region within the body that receives a dual supply of blood from the two large arteries’ most distal branches and has the least vascular collaterals.
The watershed areas in the body are highly susceptible to ischemia. It includes the splenic flexure of the colon and the rectosigmoid area.
Name adult remnants of embryonic structures.
Foramen ovale: Fossa ovalis of the heart
Ductus arteriosus: Ligamentum arteriosum
Left umbilical vein:
- Extrahepatic: Ligamentum teres hepatis (round ligament)
- Intrahepatic: Ligamentum venosum (Ductus venosus)
Left and right umbilical arteries:
- Proximal: Umbilical branches of internal iliac
- Distal: Medial umbilical ligaments
*Lateral umbilical ligaments: folds of peritoneum
that run over the inferior epigastric arteries
Embryologic ventral mesentery: Falciform ligament
Fetal urachus: Median umbilical ligament
Omphalomesenteric duct: Meckel’s diverticulum
