Wk 3 - Anatomy: Jejunum, Ileum and Peritoneum

Question 1

What structures are derived from the midgut?

Answer 1

Midgut derived structures include the duodenum distal to the ampulla of Vater, the jejunum, ileum, cecum, ascending colon, and foremost two-thirds of the transverse colon.

Question 2

Unlike the foregut and hindgut, the midgut remains connected to the yolk sac via the…

Answer 2

Vitelline duct (connects midgut to yolk sac)

Question 3

What is Meckel diverticulum?

Answer 3

• Meckel diverticulum is a congenital intestinal diverticulum due to fibrous degeneration of the umbilical end of the omphalomesenteric (vitelline) duct that occurs around the distal ileum.
• ie Results as a failure of the vitelline duct to obliterate during the fifth week of fetal development.
• It is considered the most common structural congenital anomaly of the gastrointestinal tract.

Question 4

Describe physiological umbilical herniation of the embryo during gut development.

Answer 4

In the 6th week of embryological development, the rapidly growing liver and gut outgrow the abdominal cavity and the intestinal loop is forced into the umbilical cord (herniates through the umbilical ring).

Question 5

What portion of the intestinal loop (in the embryo) forms the distal duodenum, jejunum and proximal ileum?

Answer 5

Cephalic limb of the loop

Question 6

What portion of the intestinal loop (in the embryo) forms the distal ileum, caecum, appendix, ascending colon and proximal 2/3rds of the transverse colon?

Answer 6

Caudal limb of the loop

Question 7

The development of the midgut continues outside of the abdominal cavity until roughly week ____ of development where the ____ return to the abdominal cavity via ____.

Answer 7

The development of the midgut continues outside of the abdominal cavity until roughly week TEN of development where the intestinal loops return to the abdominal cavity via rotation.

Question 8

During retraction of the herniated loops, the ____ (cephalic limb/caecal bud) re-enters first, to the left while the ____ (cephalic limb/caecal bud) re-enters last, initially to right upper quadrant.

Answer 8

During retraction of the herniated loops, the cephalic limb re-enters first, to the left while the caecal bud re-enters last, initially to right upper quadrant.

Question 9

The ventral mesentery of the embryo becomes the…

Answer 9

• Falciform ligament
• Lesser omentum

Question 10

The dorsal mesentery of the embryo becomes the…

Answer 10

• Splenorenal ligament
• Gastrosplenic ligament
• Greater omentum

Question 11

What are mesenteries? and what is their function?

Answer 11

Mesenteries are double layers of peritoneum in the abdominal cavity and are continuations of the visceral and parietal peritoneum with the serous membranes adhered back to back so that the outer mesothelium secretes serous fluid into the peritoneal cavity. This decreases the friction between the adjacent visceral surfaces and allows some movement of the organs that occur during digestion.

Question 12

How many mesenteries are there in a fully formed abdominal cavity? Name them.

Answer 12

In the fully formed abdominal cavity, mesenteries are found dorsally and adhere the viscera to the posterior wall. There are three mesenteries, all named after their organ attachments in the abdominal cavity, as follows:

• the mesentery of the small intestine or mesentery proper,
• transverse mesocolon
• sigmoid mesocolon

Question 13

What is the mesentery proper?

Answer 13

• The mesentery of the small intestine is a large and broad fan-shaped mesentery.
• It is attached to the jejunum and ileum of the small intestine, connecting them to the posterior abdominal wall.
• The root extends from duodenojejunal flexure to ileocaecal junction.

Question 14

The peritoneal cavity can be divided into…

Answer 14

• Greater and lesser sacs.
• The greater sac comprises the majority of the peritoneal cavity.
• The lesser sac (also known as the omental bursa) is smaller and lies posterior to the stomach and lesser omentum.

Question 15

The lesser sac communicates with the greater sac via…

Answer 15

The epiploic foramen

Question 16

What are the two peritoneal recesses?

Answer 16

1. Subphrenic recess
2. Hepatorenal recess – fluid collects here in supine position (flows into pelvic area via paracolic gutter when seated – into rectovesical pouch or rectouterine pouch)

Question 17

Where is the subphrenic recess located?

Answer 17

Peritoneal cavity between diaphragm and diaphragmatic surface of liver

Question 18

Where is the hepatorenal recess located?

Answer 18

Peritoneal cavity between [right] visceral surface of liver and right kidney – HEPATORENAL - the LIVER IS ON THE RIGHT

Question 19

Describe sensory innervation and referred pain of the GI tract/organs.

Answer 19

• Visceral pain (distension, ischaemia, inflammation) is carried by afferent fibres and radiates to skin supplied by nerves of the same spinal cord segments – REFERRED PAIN.
  
  • Rectum (S2 and S4) referred to peritoneum not abdominal wall
• If parietal peritoneum is also inflamed, pain is much more localised and usually more severe - WHY? because nerve supply to viscera is autonomic vs peritoneum supplied by somatic nerves that supply body wall itself.

Question 20

If parietal peritoneum is also inflamed, pain is much more localised and usually more severe - WHY?

Answer 20

because nerve supply to viscera is autonomic vs peritoneum supplied by somatic nerves that supply body wall itself.

Question 21

The jejunum is mostly in the ____ quadrant while the ileum is in the ____ quadrant.

Answer 21

The jejunum is mostly in the left upper quadrant while the ileum is in the right lower quadrant.

Question 22

Describe the epithelia of the small intestine.

Answer 22

• Specialised for absorption - villi + microvilli + plicae circulares
• Epithelial cell types – The mucosa of the small intestine is lined by a simple columnar epithelium which consists primarily of absorptive cells (enterocytes), with scattered goblet cells and occasional enteroendocrine cells. In crypts, the epithelium also includes Paneth cells and stem cells.
  
  • Enterocytes
  • Goblet/mucous cells
  • Paneth cells
  • Neuroendocrine cells
  • Stem cells

Question 23

What are Paneth cells (found in small intestine)?

Answer 23

• Paneth cells are highly specialised secretory epithelial cells located in the small intestinal crypts of Lieberkühn.
• The dense granules produced by Paneth cells contain an abundance of antimicrobial peptides and immunomodulating proteins that function to regulate the composition of the intestinal flora.

Question 24

What are Peyers patches (found in small intestine)?

Answer 24

Peyer’s patches are small masses of lymphatic tissue found throughout the ileum region of the small intestine. Also known as aggregated lymphoid nodules, they form an important part of the immune system by monitoring intestinal bacteria populations and preventing the growth of pathogenic bacteria in the intestines.

Question 25

Describe the blood supply of the jejunum and ileum.

Answer 25

• Supplied by SMA
• 12-15 branches
• Between layers of mesentery proper
• Linked distally to form arterial arcades - These branches anastomose to form loops, called arcades.
• Marginal artery = last arcade
• Vasa recta directly supply intestine - From the arcades, long and straight arteries arise, called vasa recta.

Question 26

What is the innervation of the small intestine?

Answer 26

• Enteric NS composed of myenteric and submucosal plexuses
• ENS innervates gut independently
• ANS modulates the ENS