Embryology of the GI Tract Flashcards
What is gastrulation?
Formation of the trilaminar disc:
Ectoderm, endoderm & mesoderm
What occurs in week 1?
Fertilisation and implantation
What occurs in week 2?
Bilaminar disc formation
What occurs in week 2-3?
Establishment of body axes (anterior-posterior/left-right) due to gene expression
What occurs in week 3?
Gastrulation: bilaminar disc –> trilaminar disc
What occurs in week 3-8?
Organogenesis (formation of organs)
In what week does the primordial gut tube form and from what layer?
4th week and from the endoderm lining the yolk sac
What two membranes close the ends of the primordial gut tube?
Cranial end: oropharyngeal membrane
Caudal end: cloacal membrane
Where does the oeosophagus form from?
Cranial end of the primitive tube. The trachea-oesophageal septum (ridge) divide the foregut into the trachea and oesophagus
In what week does the stomach form?
4th week, the foregut dilates to form stomach
What causes the stomach to form a lesser and greater curvature?
Differential growth rates
What two rotations does the stomach undergo?
Longitudinal
Anterio-posterior
Describe the longitudinal rotation the stomach
90 degree rotation so that the ventral border now lies on the left side (LC) and the dorsal border now on the right (GC) - faces different direction
Describe anterio-posterior rotation of the stomach
Creates final position: tilts backwards so that LC is superior to GC
How is the omental bursa (lesser sac) created?
Mesogastrium (mesentery connecting stomach to body walls) repositioned during stomach rotation.
Longitudinal rotation pulls dorsal mesentery to the LEFT to create the omental bursa (and ventral mesogastrium pulled to right)
From which part of the gut tube is the duodenum formed from?
Caudal part of foregut and cranial part of the midgut
How is the duodenum peritonised?
The 1st part is intraperitoneal as the duodenal cap retains its dorsal mesentery, but the rest of the duodenum is retroperitoneal as its mesentery fuses with the peritoneum of the posterior abdominal wall
When does the liver primordial bud begin to develop and from what layer?
In the 3rd week as an outgrowth of endoderm in the distal end of the foregut
It grows into the septum transverse (primitive diaphragm)
How does the bile duct, Gallbladder and cystic duct form?
Connection between liver bud and foregut (duodenum) narrows to form the bile duct.
Ventral outgrowth from the bile duct forms the gall badder and cystic duct
When does the spleen begin to develop and how?
Mesenchymal condensation in the dorsal mesogastrium (L side of body) but develops in the 5th week when the mesenchyme differentiates
What is mesenchyme?
Mesodermal embryonic tissue which develops into connective and skeletal tissues, including blood and lymph (forms spleen)
What is the role of the spleen in weeks 14, 15-7 and 23?
W14: haematopoietic organ (producing blood cells)
W15-17: becomes lobular and colonised by T-lymphocytes
W23: development of B-cell precursors and spleen begins lymphoid function
In what week does the pancreas develop and from what layer?
In week 5 from two buds:
Ventral (which outgrows from the bile duct
Dorsal from the duodenum
How does the pancreas develop?
As duodenum rotates (due to stomach repositioning) the ventral bud moves to lie directly below the dorsal bud.
How does the main pancreatic duct form?
The ventral and dorsal buds fuse, with the ventral + distal part of dorsal duct forming MPD.
The proximal part of the dorsal duct forms an accessory duct.
What structures are found in the midgut?
Distal part of duodenum Jejunum Ileum Appendix Caecum Ascending colon 2/3 transverse colon
What is the primary intestinal loop?
Part of the midgut, after rapid elongation of the gut tube and mesentery
What does the cephalic part of the primary intestinal loop become?
Distal duodenum, jejenum and proximal ileum
What does the caudal part of the primary intestinal loop become?
Distal ileum, caecum, appendix, ascending colon and proximal 2/3 of transverse colon
What two movements must the primary intestinal loop undergo to reach adult pattern?
Rotation and herniation
How does rotation of the primary intestinal loop occur?
Rotates around the axis of the superior mesenteric artery (cranial part shifted to the right side of abdomen- goes under the caudal part) - 90 degrees anticlockwise
In what week does rotation of the primary intestinal loop occur?
6th week
Describe the process of herniation of the primary intestinal loop
As 90 rotation occurs, the gut tube herniates into the extra embryonic cavity in the umbilical cord
Retraction occurs in the 10th week (as abdominal cavity becomes more spacious) and as the loops move back into the abdomen, there is a further 180 degree rotation anti-clockwise
What does the hindgut consist of?
Distal 1/3 transverse colon, descending colon, sigmoid colon, rectum and upper part of anal canal
What embryonic layer forms the urethra and bladder?
Endoderm of the hindgut
What is the primitive anal canal called?
Cloaca
What embryonic layers is the cloaca derived from?
It is an endoderm lined cavity, however the clocal membrane (at the ventral surface of cloacal on body wall) is ectoderm
What embryonic layer separates the allantois and hindgut?
Mesoderm (becomes the ureorectal septum)
What is the primitive urogenital sinus?
When the allantois enters into the anterior part of the cloaca
What happens in to the clocal membrane in the 7th week?
Ruptures, creating an opening for the hindgut
How is the anal canal reformed after rupturing of cloacal membrane?
Ectoderm of anal canal proliferation closing the caudal end - so causal anal canal is ectoderm derived with a different blood supply
In what week does the anal canal reopen?
Week 9
Name three conditions caused by abnormal development in the foregut
Oesophageal atresia
Trachea-oesophageal fistula
Annular pancreas
What is annular pancreas?
Developmental condition caused by failure of ventral pancreatic bud to migrate around the duodenum correctly (could cause duodenal stenosis)
Name three conditions cause by abnormal development of the midgut
Omphalocele
Gastrochisis
Remnant of Vitelline Duct
What is omphalocele?
Herniation of abdominal viscera through enlarge umbilical ring but still covered by layer of amnion - may include liver, stomach, intestinal loops
What is gastrochisis?
Herniation of abdominal content directly though the body wall into amniotic cavity (no covering) through week area in umbilicus
What happens if there are remnants of the Vitelline duct?
Forms Meckel’s Diverticulum: remnants of ileum goes into the umbilical cord and stays intact, so faecal matter passes though the umbilicus
What abnormal developmental issue causes conditions in the hindgut?
Failure of separation of the hindgut from the urogenital sinus by the urorectal septum
What conditions arise due to lack of urorectal septum formation?
Urorectal fistula
Rectovaginal fistula
Rectoanal atresia
Imperforate anus
