Development of the GI tract Flashcards
The gut is divided into
Foregut
Midgut
Hindgut
When does GI tract development occur
3rd to 12th weeks embryonic age
Describe the formation of the blastocyst from the zygote
Zygote undergoes mitosis -> 1 cell to 8-16 cells to form morula.
Embryo undergoes compaction, cells form epithelial tight junctions between cells on the outer side. Outside cells form trophectoderm, inner cells form inner cell mass to form the embryo.
Inner cell mass cells differentiate into epiblast and hypoblast layers.
Where does the primitive streak form
Forms at caudal of the embryo, moves towards cranial end.
Describe gastrulation
Epiblast cells condense towards middle of embryo. These cells undergo epithelial to mesenchymal transformation. They then fill the space between the epiblast and hypoblast layers (involution and ingress). The cells then migrate and differentiate.
Describe the primary germ layers
Mesoderm - Surrounding muscle, connective tissue and mesenteries
Endoderm - Epithelium of the gut tube
Ectoderm - Innervation of the gut
The primitive streak is surrounded by
the primitive groove
The ectoderm is patterned by
the mesoderm
What are notochordal cells
Mesodermal cells which form a rod like structure, which patterns the above layer (ectoderm)
Describe initial gut folding and tube formation
Gut tube formed by folding of sheets of cells in two directions
Folding towards midline along the cranial-caudal axis
Folding towards the yolk sac at the cranial and caudal ends
Gut tubeis closed by
Pharyngeal membrane at cranial end, cloacal membrane at caudal end.
What are mesenteries
Important in anchoring gut tube to body wall. Neural ectoderm folds upwards , so edges come together to form closed neural tube. Lateral folding of lateral sides to the midline of the embryo to enclose the yolk sac. The endoderm sides fuse to form an enclosed tube. The mesoderm attaches the gut to the body wall.
The foregut includes
Pharynx
Oesophagus
Stomach
Cranial half of duodenum
Ampulla of Vater
The midgut includes
Caudal duodenum
Jejunum
Ileum
Cecum
Appendix
Ascending colon
Proximal 2/3 of transverse colon
The hindgut includes
Distal 1/3 of transverse colon
Descending colon
Rectum
Hox gene expression boundaries
Boundaries of hox gene expression along the cranial-caudal axis specify the position of GI tract structures
Which arteries supply blood to each part of the gut
Celiac -> Foregut
Superior mesenteric -> Midgut
Inferior mesenteric -> Hindgut
Arterial blood supply to the GI tract
Gut surrounded by plexus of blood vessels, joining vitelline vessels to aorta. Plexus resolves to form aerteries that supply the GI tract from the aorta.
These define the boundaries
5 branches to the thoracic oesophagus.
Innervation of the gut tube
Sympathetic ganglia develop next to major branches of the aorta
Post-ganglionic sympathetic axons innervate the same tissues that the arteries supply with blood.
What are the principles of foregut development
Regionalised development of the gut tube occurs simultaneously
Gut development is a dynamic three-dimensional process
Driven throughout by growth, expansion, rotation
Describe stomach expansion and rotation
Gut tube undergoes swelling. Left side of vagal nerve lies ventral, right side lies dorsal. (anterior and posterior). Curvature of gut tube occurs, more rotation along craniocordal axis.
What is the pylorus
A construction at the outlet of the stomach.
What are the mesenteries attaching the stomach to the body walls
Dorsal mesogastrium (as the stomach rotates, this is drawn with it) which extends and encloses the omental bursa. Folds to form the greater omentum, obliterating the bursa
Ventral mesogastrium -> Lesser omentum (stomach and liver)
Falciform ligament -> Liver and ventral body wall
What fixes the mature gut in place in the body
Fusion of the mesenteries with the posterior abdominal wall
What is pyloric stenosis
Obstruction of the pylorus, caused by overgrowth of smooth muscle that surrounds the opening (smooth muscle hypertrophy)
Results in projectile vomit after feeding
How does the liver bud from the foregut
Heart induces signal for liver to bud. Hepatic diverticulum grows into mesenchyme of septum transversum. Cords of hepatic endoderm, bile drainage ducts and blood vessels proliferate, arranged as sinusoids. Liver expands into ventral mesentery.
How does the pancreas bud from the foregut
Two pancreatic buds - Dorsal from duodenal endoderm, ventral from hepatic diverticulum
Duodenum rotates, ventral and dorsal buds meet and fuse
Ventral bud bifid, one rotates around duodenum, annular pancreas forms, which can obstruct duodenum
How are the intestines arranged in the midgut
Attached by dorsal mesentery
Mesentery and gut grow at different rates
Ventral branch of the aorta supplies the midgut: superior mesenteric artery
Intestines rotate around SMA, herniates into umbilical stalk at 6/7 weeks. At 10 weeks, intestines return and unherniate.
What is umbilical hernia
Intestines return normally, but body wall has not closed properly over intestines (failed fusion). Gut is covered in skin.
What is omphalocele
Failure of intestinal loops to return into abdomen: heria coverd in amnion. Associated with materal obesity, alcohol/tobacco, SSRI use.
What is gastroschisis
Failure of ventral body to fuse, no covering around the gut or intestines. Associated with young maternal age, low BMI and drug misusage.
What is the persistence of yolk duct
The yolk duct is normally obliterated
Yolk duct attached to ileum, near ileo-cecal junction, apex of midgut loop
What is Meckel’s diverticulum
Asymptomatic defect
Can contain ectopic gastric cells: ulceration
Can be connected to umbilicus by ligament
What is Hirschsprung’s disease
Ganglia present in dilated/hypertrophic region
Aganglionic segment shows contraction
Affects hidngut, dilatation of sections of the colon, lack of tone, peristalsis, leading to constipation
Absence of parasympathetic enteric ganglia
Lack of neural crest cells - > Tonic contraction
What are neural crest cells
Originate from dorsal region of the neural tube
Contribute to parasympathetic innervation of the gut
