Development of the GI tract Flashcards
Give an overview of the development of the GI tract
-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
What happens at gastrulation
At gastrulation, the human embryo is arranged as a flat disc that contains three primary germ layers
What are the primary germ layers
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)
Explain how initial gut folding and tube formation occurs
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
Giveaway summary of the formation of the gut tube
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.
What is the primary gut made up of?
the primary gut is made up of:
-sheet of endoderm, which makes the epithelia.
-surrounding mesoderm, which makes the muscle and connective tissue
Explain the sub divisions of the foregut
Foregut:
-pharynx
-oesophagus
-stomach
-cranial half of duodenum
-ampulla of Vater
(Joining of common bile duct and pancreatic duct)
What are the subdivisions of the midgut
Midgut:
-caudal duodenum
-jejunum
-ileum
-cecum
-appendix
-ascending colon
-proximal 2/3 of transverse colon
What are the subdivisions of the hindgut
Hindgut:
-distal 1/3 of transverse colon
-descending colon
-rectum
What do Hox genes determine
Homeotic (hox) genes determine position along the cranial-caudal axis Which specifies the position of GI tract structures
what are different arteries that supply blood to the foregut, midgut, hind gut?
Foregut:
-aorta
-celiac artery
Midgut:
-superior mesenteric artery
Hindgut:
-inferior mesenteric artery
Explain arterial blood supply to the GI tract
-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
What defines the boundaries of the gut?
these define the boundaries of the gut:
-celiac artery to the foregut
-superior mesenteric artery to the midgut
-inferior mesenteric artery to the hindgut
Explain where sympathetic ganglia develop
Sympathetic ganglia develop next to major branches of the aorta
Explain the role of post ganglionic sympathetic axons
post ganglionic sympathetic axons:
-innervate the same tissues that the arteries supply with blood
Explain the role of the celiac ganglion (Foregut)
-controls upper digestive organs like stomach, liver, pancreas etc.
Explain the role of superior mesenteric ganglion (midgut)
-controls the small intestine and part of the large intestine
Explains the role of inferior mesenteric ganglion (hindgut)
-controls the lower part of the large intestine and rectum
Explain characteristics of the foregut development
-regionalised development of the gut tube occurs simultaneously.
-gut development is dimensional and 3D process
-driven throughout by growth, expansion and rotation
How does the stomach expand and rotate during GI development
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).
What is an abnormality of foregut development?
Pyloric Stenosis is an abnormality of foregut development
Explain characteristics of pyloric stenosis
-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
What is the origin of the mesenteries
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)
Explain mesentery attachment
mesenteries hold together the stomach and liver of the embryo
Explain the stomach development (foregut)
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.
What fixes the mature gut in place
Fusion of the mesenteries with the posterior abdominal wall fixes the mature gut in place
Explain organ budding from the liver- foregut
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
Explain organ budding from the pancreas- foregut
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
Explain how intestines are formed (midgut)
-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
Explain midgut development
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.
What are abnormalities of intestinal (midgut) development
-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
What is an umbilical hernia
-intestines return normally, but rectus abdominis fails to fuse around umbilicus
what is omphalocele
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.
What is gastroschisis
-failure of ventral body wall to fuse
-increasing incidence (1 in 3000)
-marked association with young maternal age, low maternal BMI, recreational drugs
Describe Hirschsprung’s disease
-ganglia present in dilated/hypertrophic region
-aganglionic segment shows contraction
Explain characteristics of Hirschsprung’s disease
-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
What are neural crest cells
-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
Explain anal development- the cloaca (hindgut)
-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
Explain how the cloaca gives rise to the rectum and urogenital sinus
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
What does the dorsal, ventral, pelvic and phallic part become
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)
