Embryology of Gastrointestinal System and Hepatobiliary PDF

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CL4 : EMBRYOLOGY OF THE GITFormation of the Primitive Gut Tube:x 4th week of IUL x PRIMITIVE GUT : Cephalocaudal & Lateral folding → portion endoderm-lined yolk sac incorporated into embryo x Rest of yolk sac & allantois remain outside embryo x Lateral folding : o Mesoderm (lateral plate mes...

Description

CL4 : EMBRYOLOGY OF THE GIT Formation of the Primitive Gut Tube:

Primitive Gut: x x x x

x x x x

x

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4th week of IUL PRIMITIVE GUT : Cephalocaudal & Lateral folding → portion endoderm-lined yolk sac incorporated into embryo Rest of yolk sac & allantois remain outside embryo Lateral folding : o Mesoderm (lateral plate mesoderm) is incorporated into the gut wall:  -Splanchnic mesoderm – muscle, CT & visceral peritoneum  Somatic mesoderm – lines the body cavity o Completes the gut tube Primitive gut is suspended from the dorsal abdominal wall by the dorsal mesentery

Extends frm buccopharyngeal/oropharyngeal membrane or stomodeum/ future mouth →cloacal membrane Both membranes consist of ectoderm & endoderm only Lining of gut: Endoderm CT, muscle: Splancnic mesoderm

Blood Supply

Derivatives of the primitive gut

Foregut:

Foregut:

Oesophagus

1. Development of Oesophagus

1. Oesophagus, 2.

Stomach,

3. upper ½ of duodenum 4.

x x

hepatobiliary system,

5. pancreas

x x

Midgut: 1. Lower half of duodenum,

Respiratory diverticulum is separated from the dorsal part by tracheoesophageal septum Foregut is divided → o Ventral portion: Respiratory primordium o Dorsal portion: Oesophagus With the descent of the heart and lungs, oesophagus lengthens rapidly Oesophageal muscle (from splanchnic mesenchyme): o Upper 2/3: Striated m, Nerve: Vagus (CN X) o Lower 1/3: Smooth m, Nerve: Splanchnic plexus

2. jejunum, 3.

ileum,

4. caecum, 5. appendix, 6. asc colon, 7. transv colon (prox 2/3) Clinical Significances : Hindgut: 1. Transv colon (distal 1/3),

x

2. desc colon,

x

3. sigmoid colon,

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4. rectum, upper ½ of anal canal,

x

5. internal lining of urinary bladder & urethra

x

Oesophageal atresia &/or tracheoesophageal fistula spontaneous post deviation of tracheoesophageal septum Most common- prox part of the oesophagus ends as blind sac & distal part is connected to trachea by narrow canal above bifurcation Oesophageal stenosis – incomplete recanalization / vascular abnormalities Congenital hiatal hernia – stomach is pulled → oesophageal hiatus through diaphragm

FOREGUT-STOMACH 2. Development of Stomach x

Begins at the 4th week IUL

x

Fusiform dilatation of the foregut and enlarges & broadens ventrodorsally

x

Appearance & position change greatly due to: o

i) Diff rates of growth of its wall –

o

-Dorsal border grows faster >> than ventral border 

Dorsal border →Greater curvature



Ventral border →Lesser curvature

o

Changes in the position of the surrounding organs

o

Rotation of the stomach – Longitudinal & Ant-Post axis

Stomach – Rotation along Ant-Post Axis o o o o o

Initially cephalic & caudal ends of the stomach in the midline Rotation along Ant-Post axis causes: Caudal/pyloric part →R & slightly upward Cephalic/cardiac part →L & slightly downward Final position: Stomach long axis almost transverse to long axis of the body

Omental Bursa & Mesenteries of Stomach: o

Dorsal mesogastrium suspends stomach from the dorsal wall of the abdominal cavity

o

Ventral mesogastrium attaches the stomach & duodenum to the

o x

ventral abdominal wall

Clockwise rotation along long. axis (90°) causes: o o

Ventral border (lesser curv) →R

o

Dorsal border (greater curv) → L

o

Original L side →Ant/Ventral surface

o

Original R side →Post/Dorsal surface

o

stomach

L vagus nerve initially innervates the L side, innervates the Ant/Ventral wall R vagus innervates Post/Dorsal wall o

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Rotation along longitudinal axis pulls the dorsal mesogastrium

→L forming a space →omental bursa (lesser sac) behind the

o

FOREGUT -STOMACH

As the stomach enlarges, the omental bursa expands & acquires

o

o

Dorsal mesogastrium elongates downward forming apron-like structure (greater omentum)

o

Free margin of the Lesser omentum connecting 1 st part of

duodenum & liver contains portal triad

superior & inferior recesses o

Free margin of lesser omentum →Epiploic foramen (Foramen of Winslow) connecting the omental bursa(lesser sac) & greater sac

Layers of greater omentum fused, inferior recess disappears Mesenteries of Stomach – Dorsal mesogastrium

Mesenteries of Stomach – Ventral mesogastrium ฀Rotation of the stomach along the longitudinal axis lengthens the dorsal mesogastrium ฀Spleen develops in the dorsal mesogastrium & intraperitoneum - it connects to the kidney by lienorenal lig - it connects to the stomach by gastrosplenic lig ฀Pancreas also develops inv dorsal mesogastrium & originally o

Ventral mesogastrium ← mesoderm of the septum transversum

o

It thins out & gives rise to : o

Visceral peritoneum of the liver except bare area

o

Falciform lig - from liver →ventral body wall

o

Lesser omentum – from stomach & upper part of duodenum → liver

o

Free margin of the Falciform ligament contains the umbilical vein which after birth is obliterated to form the round ligament of liver or ligamentum teres hepatis

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intraperitoneal, but later its peritoneum fuse with post abd wall

→retroperitoneal except its tail

STOMACH

฀Dorsal mesogastrium elongates downward forming apron-like structure (greater omentum )

FOREGUT-DUODENUM Development of Duodenum Sources:

฀Greater omentum attaches to the greater curv of stomach & fuses

I. Terminal part of the foregut II. Cephalic part of the midgut

w/ mesentery of transv colon

o o o

o

Junction is located distal to the origin of the liver bud (distal to opening of bile duct) As the stomach rotates, duodenum takes the form of C-shaped loop & rotates to R Duodenum & head of pancreas are pressed against dorsal body wall, dorsal mesoduodenum fuses with adjacent peritoneum causing the duodenum & head of pancreas becomes fixed in a retroperitoneal position Small portion of duodenum (first 1” of duodenum) near pylorus remains intraperitoneal

Clinical Significances: Pyloric stenosis - circular muscle of pylorus is hypertrophied - most common anomaly in infants - presented with severe projectile vomiting, persistent hunger, weight loss

o o o

During the 2nd month, lumen of the duodenum becomes obliterated by proliferation of cells in its wall & recanalization Since duodenum develops from foregut & midgut, its arterial supply is by both Coeliac & Sup Mesenteric arteries o

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FOREGUT -LIVER Development of Liver

x x x

Appears in 3rd week IUL as outgrowth at the distal end of foregut called liver bud or hepatic diverticulum Liver bud consists of rapidly proliferating endodermal cells that penetrate septum transversum Liver bud consists of 2 parts: o Larger cranial →liver o Smaller caudal →gall bladder

Mesoderm of the septum transversum → ventral mesogastrium x Ventral mesogastrium thins out →visceral peritoneum of the liver, falciform lig & lesser omentum x Surface of the liver which is in contact with future diaphragm & not covered by peritoneum is ‘bare area of the liver’

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x

x x

During further development, endodermal cells (epithelium) give rise to: o Cords of hepatic cells (hepatocytes) & o Epithelial lining of intrahepatic biliary ducts Mesoderm of septum transversum give rise to: o Haematopoietic cells, Kupffer cells, CT cells Hepatic cords intermingle with vitelline & umbilical veins →hepatic sinusoids

Hepatobiliary

Gall bladder, Cystic & Bile duct Development of Gall bladder, Cystic & Bile ducts

Development of Hepatobiliary system x At 10th week of IUL, the weight of the liver is 10% of total body weight due to its hematopoietic function (production of RBC & WBC) x Hematopoiesis gradually subsides during the last 2 months of IUL x At birth, weight of liver is only 5% of TBW x Bile formation ←hepatocytes starts at the 12th week of IUL x x x

Caudal part the liver bud →gall bladder Stalk of the caudal part of the liver bud →cystic duct Stalk of the caudal part of the liver bud connecting the hepatic &cystic ducts to the foregut (duodenum) narrows & →bile duct

Clinical Significance: Hepatobiliary Anomalies

– – – –

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Accessory hepatic ducts Duplication of gall bladder Extrahepatic biliary atresia – 1: 15 000 live births, failure of the duct to recanalize,foetal liver infection, jaundice Intrahepatic biliary duct atresia & hypoplasia-1: 100 000 live births

FOREGUT -PANCREAS Development of Pancreas – – – – – – –

between the layers of the mesentery from dorsal & ventral pancreatic buds at the caudal part of the foregut Dorsal bud develops in the dorsal mesentery, larger, forms most of the pancreas Ventral bud develops close to bile duct

– – –

 Distal part of dorsal pancreatic duct Accessory pancreatic duct (Santorini) is formed by:  Prox part of dorsal pancreatic duct Main pancreatic duct & bile duct open at the major duodenal papilla Accessory pancreatic duct opens at the minor duodenal papilla Islets of Langerhans develops from parenchyma of pancreas at 3rd month, insulin secretion at 5th month of IUL CT of the pancreas is formed by splanchnic mesoderm Pancreas develops in the dorsal mesogastrium & originally intraperitoneal, but later its peritoneum fuse with post abd wall →retroperitoneal except its tail

CLINICAL SIGNIFICANCE :

– – –

During the rotation of duodenum to the R, the ventral bud moves dorsally & lie below & behind the dorsal bud parenchyma & duct systems of dorsal & ventral buds fused – –

– – – –

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Ventral bud →uncinate process & inf part of the head Dorsal bud → rest of the pancreas Main pancreatic duct (Wirsung) is formed by:  Entire ventral pancreatic duct

Annular pancreas- 2 ventral buds fuse in opposite direction & rotate around duodenum causing obstruction Accessory pancreatic tissue- lies anywhere from the distal end of the oesophagus to the small intestine, usually in the stomach / Meckel’s diverticulum

MIDGUT Development of the Midgut –

Physiological umbilical herniation occurs when the intestinal loop enters the extra- embryonic coelom in the umbilical cord during the 6th week IUL Rotation of the Midgut

During rapid elongation of the midgut, the primary intestinal loop also rotates 2700 counter clock wise (total) around the Sup mesenteric artery (axis) – First rotation occurs during herniation (about 900) & during retraction (re-entering) of the intestinal loop into the abdominal cavity (remaining 1800) Rotation & Herniation: –

– – –

– – – –

Midgut suspended from the dorsal abd wall by mesentery proper undergoes rapid elongation & forms primary intestinal loop (5th week IUL) At its apex, the loop is connected to the yolk sac by narrow vitelline duct

Primary loop has Cephalic & Caudal limbs Cephalic limb of the loop →Distal ½ of duodenum, jejunum & part of ileum Caudal limb of the loop →Lower portion of ileum, caecum, appendix, asc colon & prox 2/3 of transv colon

During herniation, 1st rotation occurs about 900 Even during herniation, elongation occurs -→ small intestines (jejunum & ileum) → coils – Large intestine elongates but x →coil Rotation & Retraction – –

Physiological Herniation:

– –

D/t rapid elongation of midgut loop & rapid growth of liver abdominal cavity becomes too small to contain rapidly growing midgut

– – –

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At 10th week, herniated intestinal loops begin to return to the abd cavity,known as retraction (reentry) During retraction, remaining 1800 rotation occurs During retraction, prox part of jejunum is the first to re-enter & comes to lie on the L side The later retuning loops settle more to the R side

MIDGUT – –

– – –

Mesentery:

Caecal bud, appears at 6th week at the caudal limb, is the last to re-enter Re-entry of Caecal Bud

Initially during re-entry, caecal bud is located directly below the R lobe of the liver (subhepatic position) It later descends into the RIF, placing the hepatic flexure & asc colon on the R side of the abd cavity →primitive appendix Re-entry of Caecal bud & dev of appendix

– – –

– –

–

Appendix dev during descent of the colon, its final position is frequently post to the caecum (retrocaecal) or colon (retrocolic)

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Jejunum, ileum, appendix, caecum, transv colon & sigmoid colon retain their free mesenteries Transv colon is also attached to the greater omentum Asc & desc colon become retroperitoneal Mesentery Proper : Mesentery of the intestinal loop (mesentery proper) undergoes profound changes during rotation & coiling Dorsal mesentery twists around the origin of SMA

Midgut Abnormalitie Omphalocoele Body wall defects Gastrochisis

x x

Herniation of abd viscera through enlarged umbilical ring. Due to failure of the intestines to re-enter body cavity after physiological herniation(10 week)

x

Protrusion of abd contents lat to median plane of the ant abd wall directly into the amniotic cavity & bathed by amniotic fluid (damage). Due to incomplete closure of the lat folding at 4 week. Usually occurs on the R side lat to the umbilicus

x Vitelline duct abnormalities

Meckel’s diverticulum

x x x x

th

th

Persistence of vitelline duct forming outpocketing of the ileum. 2” in length, 2 feet away from ileocaecal junction, 2% population. Usually asymptomatic. If contains heterotopic pancreatic tissue or gastric mucosa – may cause ulceration, bleeding or perforation

Vitelline cyst Vitelline fistula Malrotation

Reversed rotation of the intestinal loop:

Primary loop rotates 90 clockwise. Transv colon lies behind duodenum & sup mesenteric a

Absent or incomplete secondary rotation

Rotation stops after 90 . LI on the L side & SI on the R side of the abd

Atresia (complete occlusion) & stenosis (partial)

Abnormalities of the mesentery

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0

0

May occur anywhere along the gut. Most common in duodenum & ileum. Due to failure of recanalization or interruption of the blood supply to the loop – necrosis

Mobile caecum

Due to incomplete fixation of the asc colon to the abd wall. Caecum is extremely mobile. May cause volvulus (twisting) or possible variations in the position of appendix

Volvulus

twisting of midgut

HINDGUT Development of the Hindgut – Junction between the endoderm & ectoderm is marked by DENTATE / PECTINATE LINE CLINICAL SIGNIFICANCE:

– – – –

– –

Terminal portion of the hindgut (cloaca) & receives allantois ventrally Endoderm-lined cloaca is in contact with surface ectoderm (proctodeum) forming cloacal membrane A layer of mesoderm called urorectal septum separates the region between the allantois & hindgut Urorectal septum divides the cloaca into:  Ant portion – primitive urogenital sinus  Post portion – anorectal canal At 7th week, urorectal septum fused with cloacal membrane forming perineal body Cloacal membrane is thus divided into Urogenital membrane (antly) & Anal membrane (postly)

Hindgut Abnormalities Congenital megacolon (Hirschsprung Disease): – Part of colon dilated d/T X parasymp ganglia (myenteric plexus) in the bowel wall, neural crest cells failed to migrate to the bowel wall – Failure of peristalsis in the aganglionic segment → neonatal obstruction – Common in rectum & sigmoid colon

Anal Canal:

– – –

At the end of 8th week, anal membrane ruptures creating the anal opening of the hindgut (anus) Upper 2/3 of anal canal is derived from endoderm of hindgut, Supplied by hindgut a; Sup rectal a (br of Inf mesenteric a) Lower 1/3 is derived from ectoderm (proctodeum), Supplied by: Inf rectal a

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Urorectal fistula & rectovaginal fistula – incomplete separation of the hindgut from the urogenital sinus by the urorectal septum Rectal atresia – √rectum & anus but separated -Failure of recanalization of colon Imperforate anus –X anal opening. Anal membrane fails to rupture...