Development of the GI tract Flashcards
When does GI tract development occur?
During weeks 3 to 12 embryonic age (5 to 14 weeks LMP)
Describe primary germ layer formation
- Primary germ layers formed during gastrulation
- At beginning of 3rd week, embry o implanted into uterine wall
- Embryo is flat disc composed of 2 cell layers:
- Epiblast
- Hypoblast
Describe the formation of the primitive streak
- Epiblast cells migrate caudal to cranial, they proliferate and condense, this forms the primitive streak
Describe gastrulation
- Starts caudal end of embryonic disc, primitive streak forms
- Cells involute, ingress, migrate, differentiate
- Gastrulation generates the 3 primary germ layers
Describe the primary germ layers
- Mesoderm
- Surrounding muscle, connective tissue and mesenteries
- Endoderm
- Sometimes known as definitive endoderm
- Epithelium of gut tube
- Epiblast gives rise to ectoderm - Ectoderm helps form skin, brain, spinal cord, neural crest cells
- Innervation of gut
- Hypoblast, sometimes known as primitive endoderm, gives rise to mainly extra-embryonic tissue
From which primary germ layer does the GI tract arise from?
Ectoderm
Describe the inital formation of the gut tube and its folding
- Embryo is initially a solid flat disk attached to the hemispherical yolk sac
- Part of yolk sac cavity is enclosed within embryo by pinching-off the yolk sac to form a yolk stalk and a balloon-like yolk sac
- The gut tube runs from the pharyngeal membrane to the clocal membrane, the rest of the yolk sac will pinch off and together with the allantois and stalk make the umbilical cord
- Within the embryo, cranial and caudal intestinal portals extend the tube towards the mouth and anus, delimited by the prochordal and cloacal plates
- Primary gut tube made of:
- Sheet of endoderm lining the yolk sac, which makes epithelia
- Surrounding mesoderm, which makes muscles and connective tissue (including mesentery)
- The gut tube primarly forms the stomach and proximal duodenum
- Gut tube formed by folding of sheets of cells in 2 directions
- Folding towards midline along cranial-caudal axis
- Folding towards the yolk sac at cranial and caudal ends
- Dorsal mesentery wraps around the gut tube to form the meseteries
What is the Yolk sac and where does it lie?
A small membranous structure outside the embryo with various functions during embryonic development, it lies underneath the endoderm
Also called umbilical vesicle
What does the yolk sac secrete?
Connective tissue called extra embroynic mesoderm, secretes out and surrounds both yolk sac and amniotic cavity
What is the name of the cavity that is developed by the somatic and splanchnic mesoderms?
Intra-embryonic coelom, allows for both mesoderms to become continuous with the extra-embryonic mesoderm. The splanchnic mesoderm will end up lining the yolk sac, and the somatic mesoderm will end up lining the amniotic cavity.
What’s the name of the outer layer of the extra embryonic mesoderm?
Chorionic cavity, connected to inner layer by connecting stalk, which goes on to form the umbilical cord.
What is above the ectoderm?
The amniotic cavity
What structures make up the umbilical cord?
- Yolk sac
- Allantois
- Stalk
What are the 2 planes in which the embryo folds? Describe the folding
Transverse plane - Amniotic cavity and ectoderm begins folding downwards, endoderm gets pushed in and pulled outwards, forming vitelline duct. As the lateral folds get closer, the vitelline duct ends up fusing, which obliterates it and forms the umbilical cord. The lateral folds should fold together.
Sagittal plane - Forms cranial and caudal ends. Cranial end helps form foregut, caudal end helps form hindgut.
- Cranial most end forms oropharyngeal membrane, forms mouth opening
- Caudal most end forms cloacal membrane, perforates to form anus and GU tract
What is the vitelline duct? What’s left when this obliterates?
Embryonic structure providing communication from yolk sac to the midgut during fetal development
After this obliterates, we’re left with the umbilical cord, composing of the yolk sac, allantois and stalk
Describe the origin of the mesenteries. What is the somatic mesoderm and what is the splanchnic mesoderm?
The mesenteries are generated from the common dorsal mesentery, it wraps around the gut tube.
The mesoderm is composed of 3 components:
- Paraxial mesoderm - Most central, helps form dermatome, myotome
- Intermediate mesoderm - Middle part of mesoderm layer, helps to form kidneys and gonads
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Lateral plate mesoderm - This is made up of 2 parts
- Each plate splits horzontally into the dorsal somatic (parietal) mesoderm, this underlies the ectoderm, and the ventral splanchnic (visceral) mesoderm, which overlies the endoderm
- The somatic mesoderm helps to form parietal peritonenum.
- The splanchnic (visceral) mesoderm wraps around the gut tube to form the mesenteries, walls of GI tract (submucosa, muscularis externa, visceral peritoneum)
- As the names suggest, these are important for the development of the peritoneal membrane
- As lateral folding (in the transverse plane) of the embryo continues once the vitelline duct has obliterated, splanchnic mesoderm begins to form walls of GI tract (Submucosa, muscularis externa, visceral peritoneum). Somatic mesoderm begins to form parietal peritoneum
- A connection is needed to connect the splanchnic and somatic mesoderms and keep the organ suspended in the peritoneal cavity (space b/w 2 mesoderms)- This is the origin of the mesenteries. On the dorsal side of the embryo, the dorsal mesogastrium joints the somatic mesoderm to the splanchnic mesoderm. On the ventral side it’s the ventral mesogastrium.
- Organs with mesenteries are very likely to be intraperitoneal organs. Organs lying outside the peritoneal cavity that are not suspended by a mesentery are retroperitoneal
Describe the attachment of the mesenteries
Dorsal wall of stomach attached to body by mesentery: dorsal mesogastrium
Ventral wall attached by ventral mesentery, which includes liver
What is the foregut composed of?
- Pharynx
- Oesophagus
- Stomach
- Cranial half of duodenum
- Ampulla of Vater
- (joining of common bile duct and pancreatic duct)
What is the midgut composed of?
- Caudal duodenum
- Jejunum
- Ileum
- Cecum
- Appendix
- Ascending colon
- Proximal 2/3 of transverse colon
What is the hindgut composed of?
- Distal 1/3 of transverse colon
- Descending colon
- Rectum
What branch of the aorta supplies the foregut?
Celiac artery
What branch of the aorta supplies the midgut?
Superior mesenteric artery
What branch of the aorta supplies the hindgut?
Inferior mesenteric artery
Describe the arterial blood supply to the GI tract
- Gut surrounded by plexus of blood vessels, joining vitelline vessels to aorta
- Plexus resolves to form arteries that supply GI tract from aorta
- Define boundaries of gut:
- Celiac artery to foregut
- Superior mesenteric artery to midgut
- Inferior mesenteric artery to hindgut
Describe the sympathetic innervation of the GI tract
Sympathetic ganglia develop next to major branches of aorta
- Post-ganglionic sympathetic axons innervate same tissues that arteries supply with blood
- Celiac ganglion - Foregut
- Superior mesenteric ganglion - Midgut
- Inferior mesenteric ganglion - Hindgut
Describe the Hox gene expression boundaries
- Boundaries of Hox gene expression along cranial-caudal axis specify position of GI tract structures
- Homeotic (Hox) genes determine position along the cranial-caudal axis
Describe the development of the foregut
- Regionalised development of the gut tube occurs simultaneously
- Gut development is a dynamic and 3D process
- Driven throughout by growth, expansion and rotation
Describe anal development - the cloaca
- The cloaca is the transient common end of digestive and urogenital system, including the base of the allantois (urogenital sinus)
- Covered by cloacal (proctodeal) membrane over ectoderm depression, the proctodeum
- Split by the urorectal septum
- This gives rise to the urogenital membrane and anal membrane (perforate at 7-8 weeks)
- Imperforate anus can be:
- Persistence of anal membrane
- Atresia of anal canal, rectum or both
Describe stomach expansion and rotation
Stomach arises by expansion and rotation of the gut tube.
Initially concave ventral, convex dorsal
- 90 degree turn about cranio-caudal axis, dorsal wall grows quickker than ventral, giving rise to greater and lesser curvatures respectively
- Pyloric end then moves cranially, cardial end moves downwards and leftwards, giving stomach its shape
- Right vagus -> posterior vagal trunk
- Left vagus -> anterior vagal trunk
What does the ventral mesogastrium become?
Lesser omentum
What does the dorsal mesogastrium become?
Greater omentum
Describe the development of the stomach
- Appears first as fusiform dilation of foregut endomderm
- As stomach rotates, dorsal mesogastrium drawn with it
- Mesogastrium encloses a space, the omental bursa
- Folded mesogastrium grows to form greater omentum, folds fusing to obliterate the bursa
How is the mature gut fixed in place?
Fusion of the mesenteries with the posterior abdominal wall fixes the mature gut in place
Describe the rotation and development of the intestines (midgut)
- Attached throughout length by dorsal mesentery (not ventral)
- Mesentery and gut grow at diff rates, leads to stereotypical folding of gut
- Ventral branch of aorta supplies midgut: SMA
- With very rapid increase in length, intestines rotate around SMA
- Abdomen too small to accommodate, herniates into umbilical stalk at 6 or 7 weeks
- By 10 weeks, abdomen is bigger, intestine return
Describe the most common abnormality of intestinal (midgut) development
- Persistence of yolk duct (normally obliterated)
- Most common intestinal tract abnormality
- Yolk duct attached to ileum, near ileo-cecal junction - apex of midgut loop
- Meckel’s diverticulum (2-4% population) usually asymptomatic
- Can contain ectopic gastric cells - Ulceration
- Can be connected to umbilicus by ligament
- Gut rotation causes volvulus
Describe an umbilical hernia
Intestines return normally, but rectus abdominis fails to fuse around umbilicus - Gut covered in skin
Describe Omphalocele
- Failure of intestinal loops to return into abdomen
- Hernia covered in amnion
- Causes unknown, but associated with maternal obesity, alcohol/tobacco, SSRI use
Describe Gastroschisis
- Failure of ventral body wall to fuse: no covering
- Increasing incidence (1 in 3000)
- Marked association with young maternal age, low maternal BMI, recreational drug use (especially cocaine)
Describe the budding of the liver
The liver begins developing in the ventral mesogastrium.
- Inducing signal: heart folding sends signal to ventral gut endoderm
- Hepatic diverticulum grows into mesenchyme of septum transversum (this is the central tendon of the diaphragm)
- The hepatic diverticulum, also known as the liver bud, is an embryonic structure that gives rise to the liver and the biliary system. It develops from the ventral wall of the primitive foregut and is the first visible sign of the liver. The diverticulum’s cranial part forms the liver and intrahepatic bile ducts, while its caudal part develops into the gallbladder and extrahepatic bile ducts
- 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 b/w liver and body wall
- Lesser omentum b/w liver and stomach
Describe the budding of the pancreas
- 2 pancreatic buds
- Dorsal from duodenal endoderm (induced by notochord)
- Ventral from hepatic diverticulum (induced by hepatic mesoderm)
- As duodenum rotates, ventral and dorsal buds meet and fuse
- If ventral bud bifold (bi-lobed), and one rotates around duodenum, annular pancreas forms, which can obstruct duodenum
Describe pyloric stenosis
- Gastric outlet obstruction caused by smooth muscle hypertrophy
- 3/1000 incidence
- Projectile vomiting shortly after feeding
- Pyloric channel elongation, “railroad track”
- L >16mm, wall >4mm, diameter >14mm
Describe Hirschsprung’s disease
- Ganglia present in dilated/hypertrophic region
- Aganglionic segment shows contraction
- Aganglionic megacolon
- Primarily affects hindgut
- Dilation of sections of colon, with lack of tone and peristalsis, leads to constipation
- Absence of parasympathetic enteric ganglia
- Caused by lack of neural crest cells
- Innervation normally inhibits contraction
- Absence > Tonic contraction
What are neural crest cells?
- Neural crest cells originate from the dorsal region of the neural tube
- Contribute to wide variety of tissues in embryo
- Parasympathetic innervation of the gut
