Embryology of the GI Flashcards
When does fertilisation to implantation occur?
Week 1
When does the bilaminar germ disk form?
Week 2
When does the trilaminar germ disk form?
Week 3
When does organogenesis occur?
Week 3 to 8
What is organogenesis?
The production and development of the organs of an animal or plant
What is responsible for establishing the left sideness of an organ or body body?
Genes such as the PTX2 gene
What does mutations in the PTX2 gene result in?
Dextrocardia because it is responsible for establishing the left sideness of an organ
What are the general 3 phases of embryological development?
1) Growth/proliferation (cell division and elaboration of products)
2) Morphogenesis (development of shape, size of a particular organ or part of the body)
3) Differentiation (maturation of physiological processes)
What does exposure to teratogens during weeks 4 to 8 lead to?
Major congenital conditions
What is gastrulation?
Process of bilaminar embryonic disc being converted into trilaminar disc
What are the 3 layers of the trilaminar disc?
Ectoderm
Mesoderm
Endoderm
What does the ectoderm go onto form?
Skin (epidermis, hair, nails)
CNS
PNS
Sensory epithelia of the eye
Ear
Nose
Liver (except parenchyma)
Urinary bladder
What does the endoderm go onto form?
Epithelial lining of renal system
GI tract
Liver
Pancreas
Thyroid
Parathyroid parenchyma
Urinary bladder
What does the mesoderm go onto form?
Muscle
Bones
Cartilage
Dermis
Vascular system
Most of the CVS connective tissue
Blood cells
Bone marrow
Urogenital system except bladder
Spleen
Suprarenal gland cortex
What are the 3 parts of the digestive system?
Foregut
Midgut
Hindgut
What gives rise to the parietal and visceral mesoderm?
Lateral plate mesoderm
When does the gut tube form?
During the 3rd and 4th week from the endoderm lining the yolk sac
What is the gut tube closed by at the cranial end?
Oropharyngeal membrane
What is the gut tube closed by at the caudal end?
Cloacal membrane
What does the foregut form?
Oesophagus
Stomach
Omental bursa (lesser sac)
Duodenum
Liver and bilary apparatus
Spleen
Pancreas
Explain the development of the oesophagus?
1) Develops from cranial part of primitive gut tube
2) Laryngo-tracheal diverticulum (lung bud) develops from the ventral wall of the foregut
3) Tracheo-oesophageal septum divides the foregut tube into the trachea and oesophagus
What is the lung bud also known as?
Laryngo-tracheal diverticulum
What divides the foregut tube into the trachea and oesophagus?
Tracheo-oesophageal septum
When does the stomach develop?
4th week
Explain the development of the stomach?
1) Foregut dilates to form the stomach
2) Differential growth of the stomach wall creates the greater and lesser curvatures
3) Stomach rotates and adapts its shape to fit in with the other developing organs
4) Undergoes a 90o rotation around its own longitudinal axis
5) Undergoes an anterio-posterior roation to create final adult position
What is the stomach attached to the dorsal and ventral wall by?
Mesogastrium (a mesentery)
Explain the development of the omental bursa (lesser sac)?
1) Stomach is attached to dorsal and ventral wall by mesogastrium
2) Rotation along its longitudinal axis pulls the dorsal mesentery to the left creating the omental bursa (lesser sac)
When the stomach rotates on its longitudinal axis, what direction is the ventral mesogastrium pulled in?
Right
What is the duodenum formed from?
Caudal part of the foregut and cranial part of the midgut
Explain the development of the duodenum?
1) Formed from the caudal part of the foregut and the cranial part of the midgut
2) As the stomach rotates the duodenum becomes c-shaped and also rotates
3) Duodenal cap retains its dorsal mesentery but the rest of it is retroperitoneal following the fusion of the dorsal mesentery with the peritoneum of the posterior abdominal wall
What supplies blood to the duodenum?
Coeliac trunk and superior mesenteric artery
When does the liver bud appear?
Middle of the 3rd week
What is the liver bud also known as?
Hepatic diverticulum
Explain the development of the liver and gallbladder
1) Liver bud appears as outgrowth of endoderm in the distal end of the foregut
2) Liver bud (hepatic diverticulum) grows into septum transversum
3) Connection between the liver bud and foregut (duodenum) narrows to form bile duct
4) Ventral outgrowth from bile duct forms the gallbladder and cystic duct
What forms the gall bladder and cystic duct?
Ventral outgrowth from the bile duct
When is the spleen formed?
5th week
Explain the development of the spleen?
1) Mesenchymal condensation develops in the dorsal mesogastrium near the body wall
2) Mesenchyme differentiates to form the spleen
What layer of the trilaminar disc is the spleen formed form?
Mesoderm
What does the spleen act as up to week 14?
Haematopoitic organ (produces blood cells)
When is the spleen colonised by T cells?
Weeks 15 to 17
Wheen does the precurser for B-cells arrive at the spleen so it can begin its lymphoid function?
Week 23
What is mesenchymal condensation?
Occurs when previously dispersed mesenchymal cells gather together to differentiate into a single tissue type
What are mesenchymal cells?
Multipotent stromal cells that can differentiate into a variety of cell types
Explain the development of the pancreas?
1) Forms from 2 buds which grow from the endodermal lining of the duodenum
2) As the duodenum rotates the ventral bud also moves to lie close to the dorsal bud
3) Dorsal and ventral bud fuse to form pancreas
When does the pancreas form?
Week 6
What is the main pancreatic duct formed from?
Combination of the ventral duct and distal part of the dorsal duct
What does the proximal part of the dorsal duct of the pancreas form?
Can form an accessory duct
What are examples of foregut developmental anomalies?
Oesophageal atresia and trachea-oesophageal fistula
Annular pancreas
How does oesophageal atresia and trachea-oesophageal fistula happen?
If the trachea-oesophageal septum deviates incorrectly then there is an incomplete separation of laryngo-tracheal tube (lung bud)
How does annular pancreas occur?
If the ventral bud of the pancreas fails to migrate around the duodenum correctly it may cause duodenal stenosis and pancreatic tissue may form in other areas of the foregut (accessory pancreatic tissue)
this is the most common birth defect seen in the pancreas: a ring of extra pancreatic tissue covers the first part of duodenum.
What does the midgut form?
Distal part of duodenum
Jejunum
Ileum
Caecum
Appendix
Ascending colon
Proximal 2/3rds of the transverse colon
Explain the development of the midgut?
1) Enlongation of gut tube and its associated mesentery to form primary intestinal loop
2) Cephalic part of primary intestinal loop forms distal duodenum, jejunum and proximal ileum
3) Caudal part of primary intestinal loop forms distal ileum, caecum, appendix, ascending colon and proximal 2/3rds of transverse colon
4) Undergoes rotation and physiological herniation to reach adult pattern
What does the cephalic part of the primary intestinal loop go onto form?
Distal duodenum
Jejunum
Proximal ileum
What does the caudal part of the primary intestinal tube go onto form?
Distal ileum
Caecum
Appendix
Ascending colon
Proximal 2/3rds of the transverse colon
When does rotation of the primary intestinal loop occur?
Week 6
Explain the rotation and herniation of the primary intestinal loop?
1) Initial rotation pccirs around the axis of the superior mesenteric artery
2) Rotation is 90 degress anti-clockwise
3) Gut tube herniates into the extraembryonic cavity in the umbilical cord
4) Allowing the growth of the gut tube which never had room before due to liver and kidneys
5) Abdominal cavity becomes more spacious during 10th week to intestinal loop moves from the umbilical cord back ino the abdominal cavity
What direction is the rotation of the primary intestinal loop?
90 degrees anti-clockwise
What direction is the cranial part of the primary intestinal loop carried during rotation?
Right
Why does the abdominal cavity become more spacious during the 10th week?
Growth
Regression of kidneys
Slower growth of liver
What are some midgut developmental anomalies?
Abnormal rotation of primary intestinal loop
Reversed rotation
Omphacocele (abdominal wall birth defect)
Gastrochisis (intestines outside babys body)
Remnants of vitelline duct
What is omphacocele?
Herniation of abdominal viscera through an enlarged umbilical ring due to failure of the umbilical ring to retract
What may omphacocele involve?
Liver
Stomach
Intestinal loops
What covers omphacocele?
Layer of amnion
What is amnion?
Layer that closely covers the embryo when it is first formed
What is gastrochisis?
Herniation of abdominal contents directly through the body wall into the amniotic cavity
What are remnants of vitelline duct?
Small vitelline duct persists forming Meckel’s diverticulum, may form fistula or vielline cyst/ligament
What does the hindgut include?
Distal 1/3 of transverse colon
Descending colon
Sigmoid colon
Rectum
Upper part of anal canal
What is the bladder and urethra formed from?
Endoderm of hindgut
Explain the development of the hindgut?
1) Terminal portion of hindgut joins posterior part of the cloaca (primitive anal canal)
2) The allantois enters the anterior part of the cloaca
What is the primitive anal canal called?
Cloaca
What is the anterior part of the cloaca?
Primitive urogenital sinus
What is the endoderm/ectoderm boundary of the hindgut development?
Cloacal membrane
What is cloaca?
Endoderm lined cavity with surface ectoderm at its ventral boundary
What happens in terms of arteries supplying the gut tube as the body elongates?
They subdicide into celiac, superior mesenteric and inferior mesenteric
What does the coeliac artery supply?
Foregut
it supplies oxygenated blood to the liver, stomach, abdominal esophagus, spleen, and the superior half of both the duodenum and the pancreas.
What does the superior mesenteric artery supply?
Midgut
the midgut from the ampullary region of the second part of the duodenum to the splenic flexure of the large intestine.
What does the inferior mesenteric artery supply?
Hindgut
the distal 1/3 of the transverse colon, splenic flexure, descending colon, sigmoid colon and rectum
What are some examples of hindgut developmental abnormalities?
Urorectal fistula
Rectovaginal fistula
Rectoanal atresia
Imperforate anus
What is imperforate anus?
Failure of anal membrane to breakdown
What is a fistula?
Abnormal connection between two hollow spaces
What is an atresia?
Absence or abnormal narrowing of opening in the body
