Development Of The Foregut

Question 1

Bilaminar disc

Answer 1

Develops from the inner cell mass

Question 2

Gastrulation

Answer 2

Primitive streak forms on the epiblast

Epiblast cells migrate to the primitive streak and invaginate through it

Some cells displace the hypoblast and form the endoderm

Some cells create a new layer between the epiblast and endoderm = mesoderm

Epiblast = ectoderm

Question 3

Gut is derived from the endoderm and visceral mesoderm

Answer 3

Endoderm
-Epithelium of bowel
Hepatocytes of liver
-endo/exocrine cells of pancreas

Visceral mesoderm
-Muscle wall
-Connective tissue
-Wall
-Pancreas + liver
-Visceral peritoneum

Question 4

Boundaries of foregut/midgut/hindgut

Answer 4

Foregut = lower hypopharynx → SoO
-Coeliac trunk
-Greater splanchnic nerve (T6-T9)

Midgut = SoO → ⅔ along transverse colon
-Superior mesenteric artery
-Lesser splanchnic nerve ( T10-T11)

Hindgut = ⅔ along transverse colon → anal canal
-Inferior mesenteric artery
-Least splanchnic nerve (T12//L1)

Question 5

The gut tube differentiates

Answer 5

The gut tube starts to differentiate whilst lateral folding is bringing the ventral body wall together.

Concentration gradient of retinoic acid starts to specify the different parts
Lowest levels cranially
Highest levels distally

Differential expression of transcription factors and genes along the tube specify how regions will develop.

Question 6

The foregut has 2 mesenteries

Answer 6

The dorsal mesentery- extends from the lower esophagus to rectum and throughout its length serves as a pathway to the gut for blood vessels, nerves, and lymphatics

The ventral mesentery:
arises from the septum transversum
the liver grows into it, and splits it into 2:

-the lesser omentum – connects the liver to the stomach and duodenum

-the falciform ligament – connects the liver to the anterior abdominal wall

Question 7

Oesophagus

Answer 7

Oesophagus epithelium, glands derived from foregut endoderm
Epithelium proliferates, initially fills lumen
By week 8: becomes hollow tube via recanalisation

Oesophageal muscles, adventitious derived from surrounding mesoderm

Upper 2/3: striated muscle innervated by the vagus nerves.

Lower 1/3: smooth muscle innervated by splanchnic nerves.

The lung bud appears at the ventral wall of the foregut in the 4th week – become separated from each other

Question 8

The stomach

Answer 8

This section of the gut tube starts to dilate in week 4

Changes shape due to different rates of growth of different parts

Changes position – rotates 900 clockwise around its long axis: brings the left side to lie anteriorly and the right side to lie posteriorly

Brings duodenum to the right

Question 9

Development of the liver

Answer 9

Liver bud is an outgrowth from the distal foregut (endoderm epithelium) - appears in week 3

Cells proliferate - grow into the septum transversum

Connection between the liver bud and foregut (duodenum) narrows > bile duct

Small outgrowth from the bile duct > gallbladder

As the liver grows, the remaining mesoderm either side becomes membranous > falciform ligament and lesser omentum

Question 10

Development of the pancreas

Answer 10

Dorsal and ventral buds arise from the duodenum

Dorsal bud develops in the dorsal mesentery

Rotation of the stomach swings the ventral bud posteriorly

Dorsal and ventral buds fuse

Question 11

Final positions of the foregut viscera

Answer 11

Rotation of the stomach brings its left side to lie anteriorly and swings the duodenum to the right

The dorsal mesentery along the greater curvature bulges down and grows – the greater omentum. This becomes fixed to the mesentery of the transverse colon (and posterior wall).

Rotation alters the position of the mesenteries, omenta and peritoneal ligaments

Some organs are brought into contact with the posterior abdominal wall (e.g., the pancreas and duodenum) and become retroperitoneal

A small space behind the stomach – the lesser sac – is formed

Question 12

Summary

Answer 12

The foregut has a ventral and a dorsal mesentery: the liver develops in the ventral, the spleen develops in the dorsal, and the pancreas develops from 2 buds – one ventral and one dorsal – which later fuse

Differential growth of the stomach walls change its shape and gives rise to the greater and lesser curvatures

Rotation of the stomach 900 clockwise changes its own position, as well as the position of the duodenum, foregut derivatives and the attached mesenteries and peritoneal folds

Rotation of the stomach and fixation of the liver and greater omentum to the posterior wall creates a space behind the stomach: the lesser sac

The remaining, much larger part of the peritoneal cavity is the greater sac