Embryology

Question 1

How does the midgut remain connected to the yolk sac

Answer 1

Via the vitelline duct

Question 2

What does the foregut form

Answer 2

Oesophagus

Stomach

Pancreas

Liver

Gall bladder

Duodenum - proximal to bile duct

Question 3

What does the midgut form

Answer 3

Duodenum - distal to bile duct

Jejunum

Ileum

Caecum

Ascending colon

Proximal 2/3 of transverse colon

Question 4

What does the hindgut form

Answer 4

Distal 1/3 of transverse colon

Descending and sigmoid colon

Rectum

Upper anal canal

Internal lining of bladder and urethra

Question 5

What forms the stroma, muscle, CT and peritoneal components of the gut wall

Answer 5

The visceral mesoderm

Question 6

What is the blood supply of the primitive gut tube

Answer 6

Foregut - celiac trunk

Midgut - superior mesenteric artery

Hindgut - inferior mesenteric artery

Question 7

What are mesenteries

Answer 7

Double layers of peritoneum that enclose an organ and connect it to the body wall - neurovascular supply runs in mesentery

Question 8

What are peritoneal ligaments

Answer 8

Double layers of peritoneum that pass from one organ to another or from an organ to the body wall

Question 9

What does the dorsal mesentery form in the following regions:

Stomach

Duodenum

Colon

Answer 9

Stomach - forms greater omentum/dorsal mesogastrium

Duodenum - forms dorsal mesoduodenum

Colon - forms dorsal mesocolon

Question 10

What does the ventral mesentery divide into and what causes this divide

Answer 10

Liver growth cause the ventral mesentery to divide into:

Lesser omentum

Falciform ligament - liver to ventral body wall

Question 11

Describe what happens during the oesophagus development

Answer 11

In 4th week, respiratory diverticulum/lung bud appears in ventral wall of foregut

Tracheoesophageal septum then separates diverticulum from dorsal part of foregut to divide the foregut into respiratory primordium and oesophagus

Oesophagus develops rapidly as heart and lungs descend

Question 12

Describe rotation of the stomach and how this affects nerve innervation

Answer 12

Stomach rotates 90^o clockwise around its longitudinal axis -> left side faces anteriorly and right side faces posteriorly

Original posterior wall grows faster during rotation so have formation of greater and lesser curvatures

Causes left vagus nerve to innervate anterior wall and right vagus nerve to innervate posterior wall

Question 13

How is the lesser sac formed

Answer 13

Rotation of stomach around longitudinal axis pulls dorsal mesogastrium left to create lesser sac behind the stomach

Rotation also pulls ventral mesogastrium right

Question 14

Describe the formation of the spleen - where it develops and where it ends up

Answer 14

Spleen primordium appears as mesodermal proliferation between two leaves of dorsal mesogastrium

As rotation continues, spleen is pushed left and the dorsal mesogastrium between the spleen and dorsal midline fuses with peritoneum of posterior abdominal wall

Spleen is conencted to body wall by lienorenal ligament and to the stomach by gastrolienal ligament

Question 15

How is the greater omentum formed

Answer 15

Stomach rotation around anteroposterior axis causes dorsal mesogatrium to bulge down and continue to grow downwards to form double-layered sac extending over the transverse colon and small intestinal loops

Question 16

Why is there a bare area of the liver on its superior surface

Answer 16

Mesoderm on liver surface differentiates into visceral peritoneum expect on cranial surface where liver remains in contact with septum transversum - forms central tendon of the diaphram

Question 17

What is the accessory pancreatic duct

Answer 17

A duct which remains if the proximal part of the dorsal pancreatic duct does not obliterate

Question 18

Where do the main and accessory pancreatic ducts enter the duodenum

Answer 18

Main duct - major papilla

Accessory duct - minor papilla

Question 19

What does the cephalic limb of the primary intestinal loop form

Answer 19

Distal duodenum

Jejunum

Part of the ileum

Question 20

What does the caudal limb of the primary intesrinal loop form

Answer 20

Distal ileum

Caecum

Appendix

Ascending colon

Proximal 2/3 of transverse colon

Question 21

Describe physiological herniation

Answer 21

During 6th week, primary intestinal loop enters the extraembyronic cavity in the unbilical cord due to the abdominal cavity becoming too small to contain the intestinal loops

Question 22

Describe rotation of the midgut

Answer 22

Primary intestinal loop 90^o counterclockwise around the axis formed by superior mesenteric artery - occurs three times

During each rotation, elongation continues

Question 23

Describe how the herniated loops return into the abdominal cavity

Answer 23

During 10th week, herniated loops return to abdominal cavity

Proximal jejunum re-enters first -> lies on left side

Question 24

Describe the development and movement of the cecal bud

Answer 24

Cecal bud forms in 6th week as a small dilation of caudal limb

It is the last part of the gut to re-enter the cavity

Initally lies below the right lobe of the liver before descending into the right iliac fossa to place the ascending colon and hepatic flexure on the right side of the abdominal cavity

Distal end of cecal bud forms appendix

Question 25

What happens to the mesenteries of the ascending and descending colons after they find their final positions

Answer 25

The mesenteries press against the peritoneum of the posterior wall and then the layers fuse

This results in the ascending and descending colons becoming retroperitoneal

Question 26

What happens to the transverse mesocolon during development

Answer 26

Transverse mesocolon fuses with posterior wall of greater omentum but retains its motility

Question 27

Describe how the mesentery of the jejunoileal loops changes its attachment site

Answer 27

At the start, jejunoileal loop mesentery is continuous with mesentery of ascending colon

Once ascending colon mesentery fuses with posterior wall, mesentery of jejunoileal loop forms a new line of attachment to keep it mobile

Question 28

What is the cloaca

Answer 28

Endoderm-lined cavity covered at its ventral boundary by surface ectoderm

Boundary between endoderm and ectoderm forms cloacal membrane

Question 29

What is the anal canal derived from and what divides the anal canal

Answer 29

Upper 2/3 of anal canal is from endoderm of hindgut

Lower 1/3 of canal is from ectoderm

Anal canal is divided by the pectinate line

Question 30

Describe the cranial part of the anal canal - neurovasculature, lymph and epithelium

Answer 30

Cranial part of anal canal:

Superior rectal artery from inferior mesenteric artery

Innervated by visceral nerves - S2,3,4 pelvic parasympathetic

Columnar Epithelium

Internal iliac nodes

Question 31

Describe the caudal part of the anal canal - neurovasculature, lymph and epithelium

Answer 31

Caudal part of the anal canal:

Inferior rectal arteries from internal pudendal

Somatic nerves - pudendal nerve (S2,3,4)

Stratified squamous epithelium

Superficial inguinal nodes