Development

Question 1

Describe the gut tube. (4)

Answer 1

An endoderm lines tube that runs the length of the body with two blind end pouches on either end which become the foregut and hindgut. The tube is connected to the yolk sac at the umbilicus, and this becomes the midgut.

Question 2

Describe the derivatives of the foregut. (6)

Answer 2

Foregut - oesophagus, stomach, pancreas, liver, gall bladder, proximal duodenum.

Question 3

Describe the derivatives of the midgut. (6)

Answer 3

Midgut - duodenum distal to bile duct, jejunum, ileum, caecum, ascending colon, proximal 2/3 of the transverse colon.

Question 4

Describe the derivatives of the hindgut. (8)

Answer 4

Distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum, cloaca [bladder lining, urethral lining, anal canal].

Question 5

Describe the intraembryonic coelom. (2)

Answer 5

The precursor to the abdominal and thoracic cavities that is divided by the diaphragm.

Question 6

Describe how mesenteries and peritoneum form. (3)

Answer 6

Begins forming as the mesoderm later of the trilaminar disk begins to separate so one edge can curl round and surround the ectoderm.
This is then smothered in peritoneum.

Question 7

Describe the mesenteries of the gut tube. (2)

Answer 7

Two mesenteries: dorsal which runs the length of the gut tube, and ventral which only runs on foregut.

Question 8

Describe the sacs of the gut tube. (7)

Answer 8

Foregut is divided into left and right sacs, but distal to foregut there is only the left sac.
With folding, the left sac becomes the greater sac and the right sac becomes the lesser sac. The greater sac is most of the peritoneal cavity, with the lesser sac lying just behind the stomach, connected by the Foramen of Winslow or epiploic foramen.

Question 9

Describe the omenta of the gut tube. (5)

Answer 9

They are layers of fused visceral peritoneum that form the boundaries of the greater and lesser sacs.
The greater omentum is 4 layers of visceral peritoneum flopping into the abdominal cavity but attached to the greater curvature of the stomach and the transverse colon.
The lesser omentum is 2 layers that is taught between the lesser curve of the stomach and the liver.

Question 10

Describe the free edge of the lesser omentum. (3)

Answer 10

Has a free edge because the ventral mesentery only exists in the foregut. This edge contains the bile duct, portal vein and hepatic artery.

Question 11

Describe the rotation of the stomach. (4)

Answer 11

Long tube begins to rotate longitudinally, flinging one edge out which will become the greater curvature
This edge grows faster than the other, causing the duodenal end to raise up in anteroposterior rotation.

Question 12

Describe the two classifications of organs without mesentery. (2)
Give 2 examples of each. (4)

Answer 12

Retroperitoneal - never covered by peritoneum - kidneys, oesophagus.
Secondarily retroperitoneal - did have a mesentery that fused with the posterior abdominal wall as the organ grew - ascending and descending colon.

Question 13

Describe the ligaments of the abdominal cavity. (8)

Answer 13

Starts off with a line anterior to posterior: dorsal mesentery > spleen > gut tube > liver > ventral mesentery.
With rotation it becomes anterior to posterior: falciform ligament > liver on right > lesser omentum > gut tube > gastrosplenic ligament > spleen on left > splenorenal ligament.

Question 14

Describe the development of the lung bud. (2)

Answer 14

The respiratory diverticulum forms from the anterior part of the foregut. At week 4, the tracheoesophageal septum grows, seperating the GI and resp tracts.

Question 15

Describe the livers peritoneal covering. (4)

Answer 15

Bare area posteriorly - no peritoneum because the diaphragm is here.
Lesser omentum - posterior to lesser curve of stomach.

Question 16

Describe the physiological herniation of the midgut. (6)

Answer 16

The midgut and the liver both grow much faster than the abdominal cavity, and so the midgut herniated out of the umbilicus. The loop has the SMA as it’s axis and is connected to the yolk sac by the Viteline duct.
The loop makes 3 90 degree turns anticlockwise so the cranial limb enters the abdominal cavity first, moving it to the left side.

Question 17

Describe the blood supply of the gut tube. (3)

Answer 17

Foregut - coeliac trunk
Midgut - superior mesenteric artery
Hindgut - inferior mesenteric artery.

Question 18

Describe two pathologies relating to malrotation of the midgut. (4)

Answer 18

Incomplete rotation - only one 90 gpdegree turn - all colon on left.
Reversed rotation - midgut makes one 90 degree turn clockwise - transverse colon passes posterior to duodenum.

Question 19

Describe 3 midgut defects. (8)

Answer 19

Volvulus - twisting of a loop of bowel on the mesentery.
Remnants of the yolk sac - the vitelline duct can persist - vitelline cysts, vitelline fistulae or Meckel’s diverticulum.
Meckel’s diverticulum - blind end pouch that can contain ectopic foregut tissue (eg pancreatic) causing imflammation.

Question 20

Describe the need for recanalisation and problems if it doesn’t happen. (3)

Answer 20

In some structures, cellular growth is so rapid the lumen becomes obscured so recanalisation needs to occur.
If it doesn’t, atresia or stenosis occur.

Question 21

Describe the development of the anal canal and explain the different pain sensors here. (6)

Answer 21

Above the pectinate line of the anal canal: IMA, pelvic parasympathetics (visceral), columnar epithelium.
Below the pectinate line: pudendal artery, pudendal nerve (somatic pain), stratified squamous epithelium.