Digestive System 1

Question 1

What are the adult derivatives of the primordial gut tubes?

Answer 1

Foregut

• Pharynx and its derivatives
• Lower respiratory tract
• Esophagus, stomach
• Duodenum up to the major duodenal papilla
• Liver, Biliary apparatus and Pancreas

Midgut

• Duodenum distal to the major duodenal papilla
• Jejunum & Ileum; Cecum & Appendix
• Ascending colon & right 2/3 of transverse colon

Hindgut

• Left 1/3 of transverse colon
• Descending colon, sigmoid colon & rectum
• Anal canal up to the pectinate line
• Urinaary bladder & most of urethra

Question 2

Describe parts of the primitive gut

Answer 2

After the embryonic folding, intra-embryonic part of the yolk sac is divided into the foregut [within head fold]; Midgut & Hindgut [within tail fold]

-Yolk stalk/ vitellointestinal duct connects midgut with the yolk sac vesicle

Question 3

Give the overview of the primordial gut

Answer 3

Suspended in the peritoneal cavity by dorsal and ventral mesenteries

• Epithelial lining & glands- from yolk sac endoderm
• Smooth muscle & connective tissue- from the splachnic mesoderm

Question 4

Explain organogenesis of the foregut

Answer 4

Foregut
Arterial supply- celiac trunk

Structures- esophagus, stomach, duodenum (1st and 2nd), liver, biliary apparatus, and pancreas (also pharynx and lower respiratory system)

The spleen is not an outgrowth of foregut. It develops in the dorsal mesentary from mesenchyme

Rotation due to differential growth: the spleen migrates to the left, the stomach rotates so that the left side faces anteriorly and the right side posteriorly (e.g. left vagus innervates anterior wall) //lesser curve faces right, greater faces left

Question 5

Summarize foregut rotation

Answer 5

1. Longitudinal axis rotation
2. Ventraldorsal rotation axis rotation

Pyloric stenosis- the smooth muscle in the pyloric region of the stomach hypertrophied

-The passage of foods and liquids is obstructed, and can cause severe vomiting

Question 6

Summarize rotation of the biliary and pancreatic ducts

Answer 6

1. Abdominal accessory organs arise as foregut outgrowths
2. Proximal duodenum rotates clockwise
3. Ventral & dorsal pancreatic fuse-Bile and pancreatic ducts join to drain together at major papilla

Annular pancreas- left and right portions of ventral pancreatic bud wrap around the duodenum

-Can constrict/obstruct duodenal contents

Question 7

Summarize the development of pancreas

Answer 7

Dorsal Pancreatic bud forms upper part of the head, body, and tail

Ventral pancreatic bud- lower part of the head and uncinate process

Question 8

Summarize the steps in midgut development

Answer 8

1. Herniation & 90 degree rotation
2. Elongation & formation of jejunoileal loops
3. Retraction & 1 degree rotation
4. Final positions

Question 9

Summarize the development of hindgut

Answer 9

1. Cloaca is common end for hindgut and urogenital tract
2. Urorectal septum grows towards cloacal membrane
3. Perineal body separates urogenital and anal membranes

Question 10

Give the key points of midgut development

Answer 10

• Primary intestinal loop elongates, outgrows abdominal cavity
• Rotates 90 degrees within umbilical cord
• Retracts, grows, and rotates an additional 180 degrees
• Gives rise to distal duodenum, jejunum, ileum, ascending colon and proximal 2/3 transverse colon

Question 11

What are the key points of hindgut develop?

Answer 11

• Cloacal region differentiates to form separate urogenital and GI channels
• Gives rise to distal 1/3 transverse colon, descending, & sigmoidal colons, and proximal 2/3 anorectal canal

Question 12

Describe organogenesis of Midgut

Answer 12

Arterial supply- Superior Mesenteric artery

• Rapid elongation of the gut and its mesentery resulting in the formation of the primary intestinal loop
• The Cephalic limb of the primary loop —> distal duodenum, jejunum, and some ileum
• The Caudal limb of the primary loop—> lower ileum, cecum, appendix, ascending colon, and 2/3rds of the transverse colon

Question 13

Explain organogenesis of the hind gut

Answer 13

Arterial supply-Inferior mesentric artery

Structures- distal 1/3 rd of transverse colon, descending colon, sigmoid colon, rectum, and superior portion of the rectal canal

Develops from the caudal loop, it is the last part of the bowel to renter the body following rotation

Question 14

Explain the role of the ileal diverticulum in the development of the midgut

Answer 14

A small portion of the umbilical vesicle [vitelline duct] may persist-ileal (Meckel’s) diverticulum (2-4% frequency)

-It is located 40-60 cm from the ileocecal valve on the anti-mesenteric border of the ileum. May be asymptomatic, may contain gastric or pancreatic tissue. May become infected —> symptoms resemble appendicitis but close to umbilicus

Question 15

Explain the development of the anal canal

Answer 15

• Superior 2/3 derived from the hindgut
• Inferior 1/3 derived from proctodeum
• Pectinate line- junction of the epithelia derived from the ectoderm of the proctoderm and endoderm from the hind gut

Pectinate line is the former site of the anal membrane

Question 16

What is the blood supply of the anal canal?

Answer 16

Superior 2/3 supplied by the superior rectal artery- IMA

Inferior 1/3 supplied by the inferior rectal artery -Internal pudendal

Question 17

What is the innervation of the anal canal?

Answer 17

Above the pectinate line- autonomic

Below the pectinate line- inferior rectal nerves- pudendal nerve