Development of the GI tract

Question 1

Give an overview of the development of the GI tract

Answer 1

-GI tract development occurs during 3rd to 12th weeks embryonic age
-primary germ layers
-gut tube formation
-regionalised changes cause by rotation, swelling and elongation
-organs and glands produced by budding from the gut tube
-molecular control of some key events

Question 2

What happens at gastrulation

Answer 2

At gastrulation, the human embryo is arranged as a flat disc that contains three primary germ layers

Question 3

What are the primary germ layers

Answer 3

the primary germ layers are:

1) endoderm- epithelium of the gut tube lining

2) mesoderm- surrounding muscle, connective tissue and mesenteries

3) ectoderm- innervation of the gut (inside the gut)

Question 4

Explain how initial gut folding and tube formation occurs

Answer 4

1) the gut tube is formed by folding sheets of cells in two directions

2) folding towards the midline along the cranial-caudal axis

3) folding towards the yolk sac at the cranial and caudal ends

Question 5

Giveaway summary of the formation of the gut tube

Answer 5

1) the embryo is initially a solid flat disk attached to hemispherical yolk sac

2) part of the yolk sac cavity is enclosed within the embryo by pinching off the yolk sac to form a yolk stalk and balloon like yolk sac.

3) within the embryo, the cranial and caudal intestinal portals extend the tube towards the mouth and anus (delimited by prochordal and cloacal plates.

Question 6

What is the primary gut made up of?

Answer 6

the primary gut is made up of:

-sheet of endoderm, which makes the epithelia.

-surrounding mesoderm, which makes the muscle and connective tissue

Question 7

Explain the sub divisions of the foregut

Answer 7

Foregut:
-pharynx
-oesophagus
-stomach
-cranial half of duodenum
-ampulla of Vater
(Joining of common bile duct and pancreatic duct)

Question 8

What are the subdivisions of the midgut

Answer 8

Midgut:
-caudal duodenum
-jejunum
-ileum
-cecum
-appendix
-ascending colon
-proximal 2/3 of transverse colon

Question 9

What are the subdivisions of the hindgut

Answer 9

Hindgut:
-distal 1/3 of transverse colon
-descending colon
-rectum

Question 10

What do Hox genes determine

Answer 10

Homeotic (hox) genes determine position along the cranial-caudal axis Which specifies the position of GI tract structures

Question 11

what are different arteries that supply blood to the foregut, midgut, hind gut?

Answer 11

Foregut:
-aorta
-celiac artery

Midgut:
-superior mesenteric artery

Hindgut:
-inferior mesenteric artery

Question 12

Explain arterial blood supply to the GI tract

Answer 12

-Gut is surrounded by a plexus of blood vessels, joining vitelline vessels to aorta
-plexus resolves to form the arteries that supply the GI tract from the aorta

Question 13

What defines the boundaries of the gut?

Answer 13

these define the boundaries of the gut:
-celiac artery to the foregut
-superior mesenteric artery to the midgut
-inferior mesenteric artery to the hindgut

Question 14

Explain where sympathetic ganglia develop

Answer 14

Sympathetic ganglia develop next to major branches of the aorta

Question 15

Explain the role of post ganglionic sympathetic axons

Answer 15

post ganglionic sympathetic axons:
-innervate the same tissues that the arteries supply with blood

Question 16

Explain the role of the celiac ganglion (Foregut)

Answer 16

-controls upper digestive organs like stomach, liver, pancreas etc.

Question 17

Explain the role of superior mesenteric ganglion (midgut)

Answer 17

-controls the small intestine and part of the large intestine

Question 18

Explains the role of inferior mesenteric ganglion (hindgut)

Answer 18

-controls the lower part of the large intestine and rectum

Question 19

Explain characteristics of the foregut development

Answer 19

-regionalised development of the gut tube occurs simultaneously.
-gut development is dimensional and 3D process
-driven throughout by growth, expansion and rotation

Question 20

How does the stomach expand and rotate during GI development

Answer 20

1) The tube:

the stomach starts as a simple straight tube.
The left side becomes the greater curvature (red line), and the right becomes the lesser curvature (green line).
Vagus nerves (from the brain) travel alongside:
Left vagus becomes anterior
Right vagus becomes posterior This happens because the stomach rotates!

🔁 2. Rotation (Image A → B → C)
The stomach rotates 90° clockwise around its longitudinal axis (from head to toe).
The left side moves forward (anterior)
The right side moves backward (posterior)
This rotation explains why:
The left vagus nerve ends up on the front (anterior) of the stomach
The right vagus nerve ends up on the back (posterior)

🌀 3. Expansion (Image C to E)
The stomach starts to swell and curve, especially more on the left side (which becomes the greater curvature, highlighted in blue).
The lesser curvature forms on the opposite side (right side originally).
Image D and E show how the stomach gains its familiar shape, curving and forming the pylorus (exit to the duodenum) and esophagus (entry).

Question 21

What is an abnormality of foregut development?

Answer 21

Pyloric Stenosis is an abnormality of foregut development

Question 22

Explain characteristics of pyloric stenosis

Answer 22

-gastric outlet obstruction caused by smooth muscle hypertrophy
-3 in 1000 incidence
-projectile vomiting shortly after feeding
-pyloric Chanel elongation; railroad track
-L > 16mm, wall >4mm, diameter >14mm

Question 23

What is the origin of the mesenteries

Answer 23

Mesoderm layers:
In embryonic development, the lateral plate mesoderm splits into two layers:
-somatic (parietal) mesoderm: lines the body wall
-splanchnic (visceral) mesoderm: wraps around the gut.

These two layers are separated by intraembryonic coelom- the future body cavity

Where mesenteries come from:
-splanchnic mesoderm gives rise to;
-mesenteries
-connective tissue of the gut
-blood vessels and smooth muscle of gut wall
-it also contributes to the splanchopleure (gut covering + tissue)

Question 24

Explain mesentery attachment

Answer 24

mesenteries hold together the stomach and liver of the embryo

Question 25

Explain the stomach development (foregut)

Answer 25

1) As the stomach rotates, the dorsal mesogastrium is drawn with it 2) This mesogastrium encloses a space, the omental bursa 3) The folded mesogastrium grows to form the greater omentum, the folds fusing to obliterate the bursa.

Question 26

What fixes the mature gut in place

Answer 26

Fusion of the mesenteries with the posterior abdominal wall fixes the mature gut in place

Question 27

Explain organ budding from the liver- foregut

Answer 27

1) Inducing signal; heart to ventral gut endoderm 2) hepatic diverticulum grows into mesenchyme of septum transversum 3) cords of hepatic endoderm, bile drainage ducts, and blood vessels proliferate, arranged as sinusoids. 4) liver exceeds size of septum transversum, expands into ventral mesentery 5) remaining ventral mesentery gives rise to: -falciform ligament between liver and body wall -lesser omentum between liver and stomach

Question 28

Explain organ budding from the pancreas- foregut

Answer 28

1) two pancreatic buds: -dorsal from duodenal endoderm (induced by notochord) -ventral from hepatic diverticulum (induced mesoderm) 2) as duodenum rotates, ventral and dorsal buds meet and fuse. 3) if ventral bud bifid (bi-lobed), and one rotates around duodenum annular pancreas forms, which can obstruct duodenum

Question 29

Explain how intestines are formed (midgut)

Answer 29

-attached throughout length by dorsal mesentery -the mesentery and gut grow at different rates -a ventral branch of aorta supplies the mid gut: superior mesenteric artery -with a very rapid increase in length, the intestines rotate around the SMA -abdomen is two small to accommodate, so herniates into umbilical stalk by 6/7 weeks -by 10 weeks, the abdomen is bigger, the intestines return

Question 30

Explain midgut development

Answer 30

1) The midgut is connected to the yolk sac via the vitelline duct, it is initially a straight tube at around week 4 2) The midgut grows rapidly, it becomes too large to fit inside abdominal cavity (so temporarily herniates into umbilical cord) 3) While herniated, the midgut rotates 90 degrees counterclockwise around superior mesenteric artery (SMA) -as it returns to the abdomen at week 10, it rotates 180 degrees counterclockwise around superior 4) the intestines retract back into the abdominal cavity, the jejunum and ileum settle on the left side. Whilst the cecum and ascending colon move to the right side 5) The mesentery of the intestines fuses with the posterior abdominal walls in certain areas, intestine becomes fixed in place.

Question 31

What are abnormalities of intestinal (midgut) development

Answer 31

-persistence of yolk duct is the most common intestinal abnormality -the yolk duct is attached to ileum, near ilea decal junction- apex of midgut loop -Meckal’s diverticulum (2-4% of the population) is usually asymptomatic

Question 32

What is an umbilical hernia

Answer 32

-intestines return normally, but rectus abdominis fails to fuse around umbilicus

Question 33

what is omphalocele

Answer 33

Omphalocele is a congenital abdominal wall defect where a baby is born with some of their intestines (and sometimes liver or other organs) outside the belly, but covered by a thin sac made of amniotic membrane and peritoneum.

Question 34

What is gastroschisis

Answer 34

-failure of ventral body wall to fuse -increasing incidence (1 in 3000) -marked association with young maternal age, low maternal BMI, recreational drugs

Question 35

Describe Hirschsprung’s disease

Answer 35

-ganglia present in dilated/hypertrophic region -aganglionic segment shows contraction

Question 36

Explain characteristics of Hirschsprung’s disease

Answer 36

-aganglionic megacolon -primarily affects the hindgut -dilation of sections of the colon, with lack of tone and peristalsis, leading to profound constipation -a sense of parasympathetic enteric ganglia -caused by lack of neural crest cells -innervation normally inhibits contraction

Question 37

What are neural crest cells

Answer 37

-neural crest cells originate from the dorsal region of the neural tube\ -contribute to a wide variety of tissues in the embryo -parasympathetic innervation of the gut

Question 38

Explain anal development- the cloaca (hindgut)

Answer 38

-the cloaca is the transient common end of digestive and urogenital systems (including the base of allantois) -covered by cloacal (proctodeal) membrane over ectoderm depression, the proctodeum -split by the urorecal septum

Question 39

Explain how the cloaca gives rise to the rectum and urogenital sinus

Answer 39

1) The hindgut opens into the cloaca, the allantois also connects to it. This occurs week 4-5 2) Urorectal septum (mesodermal wedge) grows downward between the hindgut and the allantois. -the septum divides the cloaca into two parts: -the dorsal part- rectum + anal canal -ventral part- urogenital sinus 3) The cloaca is sealed from outside by cloacal membrane This membrane also divides into: -anal membrane (posterior) -urogenital membrane (anterior) Both membranes eventually rupture creating the anus and the external urethral orifice

Question 40

What does the dorsal, ventral, pelvic and phallic part become

Answer 40

Dorsal: -forms rectum and upper anal canal Ventral: -vesical part- becomes most of urinary bladder -pelvic part- becomes prostatic and membranous urethra Pelvic: -becomes prostatic and membranous urethra in males -entire urethra in females Phallic : -forms penile urethra -contributes to vestibule of vagina (in females)