Embryology of the GI System Flashcards
What are the 3 phases of human development?
- Growth
- Morphogenesis
- Differentiation
What is growth?
Cell division and elaboration of products
What is morphogenesis?
Development of shape size of a particular organ or part of the body
What is differentiation?
Maturation of physiological processes
When can exposure to teratogens cause major congenital anomalies to an embryo?
During weeks 4-8 of development
What is gastrulation?
A formative process by which the 3 germ layers and axial orientation of embryo is established.
What happens to the bilaminar disc during gastrulation?
It is converted to the trilaminar disc
What does the embryonic ectoderm become?
- Epidermis
- CNS
- PNS
- Retina of the eye
- Other
What does the embryonic endoderm become?
-Epithelial lining of the RS
-GIT
Glands
-Liver
-Pancreas
What does the embryonic mesoderm become?
- Most of the CVS
- Connective tissues
- Muscles
- Blood cells
- Bone marrow
- Vessels associated with organs and tissues
When does the primordial gut form?
During the 4th week of embryonic development
What is the primordial gut derived from?
Endoderm lining the yolk sac
What is the primordial gut closed by at its cranial end?
Oropharyngeal membrane
What is the primordial gut closed by at its caudal end?
Cloacal membrane
What do the pharynx and oesophagus develop from?
The cranial part of the primitive gut tube
What does the laryngo-tracheal diverticulum develop from?
The ventral wall of the foregut
What does the trachea-oesophageal septum divide?
The foregut tube into the trachea and oesophagus
How do oesophageal atresia and trachea-oesophageal fistula arise?
If the tracho-oesophageal septum deviates incorrectly then there is an incomplete separation of the laryngo-tracheal tube
What happens to the foregut in the 4th week?
Dilates to form the stomach
What does differential growth of the stomach wall create?
The greater and lesser curvatures
How does the stomach fit in with the other organs?
The stomach rotates and adapts its shape
What rotation does the stomach undergo?
- A 90 degree rotation around its own longitudinal axis
- Ventral border now left side
- Dorsal border now right side
What rotation does the stomach undergo to create its final adult position?
Anterio-posterior rotation
How is the stomach attached to both the dorsal and ventral walls?
By a mesentery called mesogastrium
What alters the mesogastrium?
The rotation and disproportionate growth of the stomach
How is the omental bursa (lesser sac) created?
Rotation along the longitudinal axis pulls the dorsal mesentery to the left
What direction is the ventral mesogatrium pulled?
To the right
What is the duodenum formed from?
The caudal part of the foregut and the cranial part of the midgut
Where is the blood supply of the duodenum from?
- 2 major abdominal vessels
- Coeliac axis
- Superior mesenteric artery
What happens to the duodenum as the stomach rotates?
It becomes C shaped and also rotates
What part of the duodenum retains its dorsal mesentery?
Duodenal cap
The duodenum (exc duodenal cap) is retroperitoneal following what?
The fusion of the dorsal mesentery with the peritoneum of the posterior abdominal wall
When does the liver primordium appear?
Middle of the 3rd week
How does the liver primordium form?
Outgrowth of endoderm in the distal end of the foregut
Where does the liver bud grow into?
Septum transversum
What is the septum transversum?
Mesodermal plate between pericardial cavity and yolk stalk
How is the bile duct formed?
Connection between the liver bud and foregut narrows
What does ventral outgrowth from the bile duct form?
- Gall bladder
- Cystic duct
What develops in the dorsal mesogastrium at the end of the 4th week?
Mesenchymal condensation
What happens to the mesenchymal condensation during the 5th week?
Differentiates to form the spleen
What form of derivative is the spleen?
Mesodermal
What does the spleen act as until the 14th week?
Haematopoietic organ
What happens to the spleen during weeks 15-17?
Spleen acquires its lobular shape and is colonised by T-lymphocyte cells
What happens to the spleen during week 23?
B-cell precursors arrive
What function does the spleen have?
Lymphoid
What does the pancreas form from?
2 buds which grow form the endodermal lining of the duodenum in week 5
Where does the dorsal pancreatic bud lies?
Dorsal mesentary
Where does the pancreatic bud lie?
Close to the bile duct
What happens as the duodenum rotates in pancreas formation?
The ventral bud also moves to lie close to the dorsal bud (week 6)
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What happens if the ventral bud fails to migrate around the duodenum correctly?
Annular process
What can an annular process lead to?
Duodenal stenosis
What is it known as when pancreatic tissue forms in other areas of the foregut?
Accessory pancreatic tissue
What constitutes the midgut?
- Distal part of the duodenum
- Jejunum
- Ileum
- Caecum
- Appendix
- Ascending colon
- Proximal 2/3rds of the transverse colon
What does development of the midgut involve?
Rapid elongation of the gut tube and its associated mesentery
What is the rapid elongation of the gut tube known as?
Primary intestinal loop
What does the cephalic part of the primary intestinal loop become?
- Distal duodenum
- Jejunum
- Proximal ileum
What does the caudal part of the primary intestinal loop become?
- Distal ileum
- Caecum
- Appendix
- Ascending colon
- Proximal 23rds of the transverse colon
What must happen to the midgut for it to reach adult pattern?
Both rotation and physiological herniation
When does rotation of the midgut begin?
In the 6th week
Where does the initial rotation of the primary intestinal loop occur?
Around the axis of the superior mesenteric artery
What type of midgut rotation takes place?
- Rotations of 90 degrees anti-clockwise
- Cranial part of the midgut carried to the right
What happens to the gut tube along with the 90 degree rotation?
The gut tube herniates into the extraembryonic cavity in the umbilical cord
What does the physiological herniation of the gut tube allow?
Allows the growth of the gut tube
Why is the physiological herniation of the gut tube required?
There isn’t enough room for rapid growth of midgut loop in the abdominal cavity (massive liver and 2 sets of kidneys)
Whys is physiological herniation described as being physiological?
The process is part of the developmental process
What happens to the abdominal cavity during the 10th week?
It becomes relatively more spacious
How does the abdominal cavity become more spacious?
- Growth
- Regression of the kidneys
- Slower growth of the liver
How the herniated loop retracted?
-The intestinal loops begin to move from the umbilical cord back into the abdominal cavity
What is the first part of the herniated loop to be retracted?
The proximal part of the jejunum which enters towards the left side
What happens to the rest of the herniated loop after the jejunum is retracted?
It re-enters and lies further to the right
What is the last part of the herniated loop to re-enter the abdominal cavity?
Caecum
What happens to the gut tube as it returns to the abdominal cavity?
There is a further rotation 180 degrees anti-clockwise
What does the retraction of the herniated loop establish?
The adult pattern of the transverse colon lying in front of the duodenum
What happens to the gut tube after the retraction of the herniated loop?
Continues to elongate
Omphalocele
Herniation of abdominal viscera through an enlarged umbilical ring
What leads to omphalocele?
Failure of retraction of herniated loops
What may be included in omphalocele?
- Liver
- Stomach
- Intestinal loops
What is the viscera covered by in ompahlocele?
A layer of amnion
What is gastoschisis?
Herniation of abdominal contents directly through the body wall into the amniotic cavity
Where does gastroschisis occur?
Through a weak area right of the umbilicus
What is the prevalence of gastroschisis?
1 in 10,000 live births
What are the associations between gastroschisis and chromosomal abnormalities?
No associations
What are the organs covered by in gastoschisis?
Nothing. They are not covered by amnion or peritoneum
What is the prevalence of a persistant vitelline duct?
2-4%
What does a persistent vitelline duct form?
Meckel’s diverticulum
What can happen to the remnants of the vitelline duct?
Can form fistula or vitelline cyst/fistula
What is the hindgut formed of?
- Distal 1/3rd of the transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Upper part of the anal canal
What lining does the endoderm of the hindgut form?
- Bladder
- Urethra
What does the terminal portion of the hindgut join with?
The posterior part of the cloaca (primitive anal canal)
Where does the allantois enter?
The anterior part of the cloaca (primitive urogenital sinus)
Cloaca
Endoderm lined cavity with surface ectoderm at its ventral boudary
What is the cloacal membrane?
Endoderm/ectoderm boundary
What separates the allantois and hindgut?
Mesoderm (urorectal septum)
How is the yolk sac covered in the hindgut?
Septums merges and surrounds the allantois
Where does the tip of the urorectal septum lie as the embryo grows?
Close to the cloacal membrane
When is the opening for the hindgut formed?
End of week 7
How is the opening for the hindgut formed?
The cloacal membrane ruptures
When does the anal canal reopen?
Week 9
What kind of derivative is the caudal anal canal?
Ectoderm (proctodeum)
How is the caudal end of the hindgut closed?
Ectoderm of the anal canal proliferates
What can incomplete separation of the hindgut from the urogenital sinus by the urorectal septum result in?
- Urorectal fistula
- Rectovaginal fistula
- Rectoanal atresia
- Imperforate anus
What is the alimentary system?
Mouth to anus plus associated glands and organs
What is the primordial gut closed by at its cranial end?
Oropharyngeal membrane
What is the primordial gut closed by at its caudal end?
Cloacal membrane
What happens to the intraembryonic coelm early in the 4th week?
It becomes the embryonic body cavity
Derivatives of the foregut.
- Primordial pharynx and its derivatives
- Lower resp tract
- Oesophagus and stomach
- Duodenum (part above the opening of bile and pancreatic duct)
- Liver,biliary apparatus and pancreas
What causes tracheoesophageal fistulas?
Abnormal deviation of trachea-oesophageal septum
What causes iesphageal atresia/
Failure of recanalization
What is polyhydraminoes due to?
Atresia
What are the main events in the development of the stomach?
- Rotation of stomach, rotates 90 degrees clockwise
- Mesenteries of stomach
- Formation of omental bursa (lesser sac)
What is the prevalence of hypertrophic pyloric stenosis?
- 1 in 150 males
- 1 in 750 females
What is the cause of hypertropic pyloric stenosis?
Genetic?
High in monozygotic twins
What is the treatment for hypertrophic pyloric stenosis?
Surgical
Name an anomaly of the liver
Accessory hepatic duct
What is extrahepatic biliary atresia?
Obliteration of the bile ducts
What can annular pancreas cause?
Duodenal stenosis
What are the main events in the development of the spleen?
- End of the 4th week a mesenchymal condensation develops (dorsal mesogastrium)
- During the 5th week mesenchymal differentiates to form the spleen
What are the main events in the midgut loop?
- Herniation, physiological umbilical herniation
- Rotation of midgut loop- 90 degrees anticlockwise in the umbilical cord
- Retraction of intestinal loops, 180 degrees more anticlockwise
- Fixation of the intestines
