Development of GI tract

Question 1

When does GI tract development occur

Answer 1

During 3rd to 12th week of embryonic age

Question 2

In chronological order what is formed

Answer 2

Primary germ layer first
Then the gut tube formation
Then regionalised changes caused by rotation, swelling and elongation

Question 3

How are organs and glands produced

Answer 3

By budding from the gut tube

Question 4

What is the blastocyst made up of

Answer 4

Outer layer of cells = TROPHECTODERM/TROPHOBLAST

Middle = INNER CELL MASS (ICM)

Question 5

What can the inner cell mass differentiate into

Answer 5

Epiblast

Hypoblast

Question 6

Embryo is a flat disc comprised of 2 cell layers

Answer 6

Epiblast

Hypoblast

Question 7

When are the primary layers formed

Answer 7

In grastulation

Question 8

Ingressing cells give rise to

Answer 8

Mesoderm

Question 9

Epiblast give rise to

Answer 9

Ectoderm

Question 10

Hypoblast give rise to

Answer 10

Endoderm

Question 11

How is the gut tube formed

Answer 11

Folding of sheets of cells in 2 directions

1) Folding towards the midline along the cranial caudal axis
2) Folding towards the yolk sac at the cranial caudal ends

Question 12

What mesoderms surround the gut

Answer 12

Somatic mesoderm

Splanchnic mesoderm

Question 13

Steps for the formation of gut tube

Answer 13

1) Embryo initially a solid flat disk attached to hemispherical yolk sac
2) Part of yolk sac cavity 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 the prochordal and cloacal plates
4) The primary gut tube made up of :
A sheet of endoderm which makes up the epithelia and glands. And surrounding mesoderm which makes muscle and connective tissue

Question 14

Regions in foregut

Answer 14

Pharynx to Ampulla of Vater

Question 15

Regions in midgut

Answer 15

Caudal duodenum to Proximal 2/3 of transverse colon

Question 16

Regions in hindgut

Answer 16

Distal 1/3 of transverse colon to rectum

Question 17

Where is the foregut arterial supply from

Answer 17

Celiac artery

Question 18

Where is the midgut arterial supply from

Answer 18

Superior mesenteric artery

Question 19

Where is the hindgut arterial supply from

Answer 19

Inferior mesenteric artery

Question 20

What innervates the foregut

Answer 20

Celiac ganglion

Question 21

What innervates the mid gut

Answer 21

Superior mesenteric ganglion

Question 22

What innervates the hind gut

Answer 22

Inferior mesenteric ganglion

Question 23

Abnormality of foregut development - Pyloric stenosis

Answer 23

Gastric outlet obstruction
Caused by smooth muscle hypertrophy - smooth muscle forming the pyloric sphincter overgrows
Causes projectile vomitting

Question 24

Foregut stomach development

Answer 24

• Stomach arises by expansion and rotation
• During week 4 at the level where the stomach will form the tube begins to dilate, forming an enlarged lumen
• Initially concave ventral, convex dorsal
• 90 degree turn about cranio-caudal axis
• The dorsal border grows more rapidly than ventral, which establishes the greater curvature of the stomach
• Dorsal wall of stomach attached to body by mesentery: the dorsal mesogastrium (will form greater omentum)
• Ventral wall attached by ventral mesentery, which includes the liver (will form lesser omentum)

Question 25

Mesogastrium

Answer 25

- As stomach rotates the dorsal mesogastrium is drawn with it - This mesogastrium encloses a space, the OMENTAL BURSA (lesser sac) - The folded mesogastrium grows to form the greater omentum, the folds fusing to obliterate the bursa

Question 26

Bursa

Answer 26

Protective layer, contains WBC to prevent infection if any gastric intestinal fluid spillages

Question 27

Organ budding from the foregut - forming of the liver

Answer 27

- Inducing signal: heart to ventral gut endoderm - Hepatic diverticulum grows into mesenchyme of septum transversum - Cords of hepatic endoderm, bile drainage ducts, and blood vessels proliferate, arranged as sinusoids - Liver exceeds size of septum transversum, expands into ventral mesentery - Remaining ventral mesentery gives rise to: - falciform ligament between liver and body wall - lesser omentum between liver and stomach

Question 28

Organ budding from foregut - forming of the pancreas

Answer 28

- Two pancreatic buds 1) Dorsal from duodenal endoderm (induced by notochord) 2) Ventral from hepatic diverticulum (induced by hepatic mesoderm) - As duodenum rotates, ventral and dorsal buds meet and fuse - If ventral bud bifid (bi-lobed), and one rotates around duodenum, annular pancreas forms, which can obstruct duodenum

Question 29

Formation of midgut intestines

Answer 29

- Attached throughout length by the dorsal mesentery – mesentery and gut grow at different rates leading to stereotypical folding of the gut - Ventral branch of the aorta supplies the mid gut – superior mesenteric artery (SMA) - Intestines rotate by 90 degrees around the SMA - Abdomen is too small to accommodate, so herniates into umbilical stalk at 6 or 7 weeks – They rotate by 180 degrees - By 10 weeks, the abdomen is bigger, and intestines return

Question 30

3 kinds of hernia that can occur in new borns

Answer 30

Umbilical hernia Omphalocele Gastroschisis

Question 31

Umbilical hernia

Answer 31

Intestines return normally but rectus abdominis fails to fuse around umbilicus - gut covered in skin

Question 32

Omphalocele

Answer 32

Failure of intestinal loops to return to abdomen, hernia covered in amnion

Question 33

Gastroschisis

Answer 33

Failure of the ventral body to fuse, no covering

Question 34

Examples of more abnormalities of intestinal development

Answer 34

Persistence of yolk duct Meckel's diverticulum Hirschprung's disease

Question 35

Persistence of yolk duct

Answer 35

Most common intestinal abnormality | Yolk duct attached to ileum, near ileo-cecal junction - apex of midgut loop

Question 36

Meckel's diverticulum

Answer 36

- Usually asymptomatic - May contain ectopic gastric cells: ulceration and lower GI bleeding - Can be connected to umbilicus by ligament: - Gut rotation causes volvulus - strangulation Can form umbilical fistula - contents leak out

Question 37

Hirschprung disease

Answer 37

``` Aganglionic megacolon Primarily affects the hindgut Dilatation of sections of the colon, with lack of tone and peristalsis, leading to profound constipation Absence of parasympathetic ganglia Caused by lack of neural crest cells ```

Question 38

Neural crest cells

Answer 38

- Neural crest cells populate developing gut and give rise to enteric ganglia - Neural crest cells from occiptocervical region populate the entire gut - Neural crest cells from sacral region populate the distal gut

Question 39

Anal development - the CLOACA

Answer 39

- Cloaca – transient common end of the digestive and urogenital systems, including the base of the allantois (urogenital sinus) - Covered by the cloacal membrane over an ectoderm depression, the proctodeum - Split by the urorectal septum – give rise to urogenital membrane and anal membrane - Imperforate anus can be: - Persistence of an anal membrane - Artesia of anal canal, rectum or both

Question 40

3 steps to forming of the anal

Answer 40

1) Invagination 2) Septum formation 3) Separation