Development of the Gut Flashcards
Foregut goes from
pharynx to proximal 1/2 duodenum
midgut goes from
distal 1/2 duodenum to proximal 2/3 transverse colon
Hindgut goes from
distal 1/3 transverse colon to proximal 2/3 anal canal
How do we start off life?
bi-laminar disc of epiblast and hypoblast
Epiblast becomes
ectoderm
hypoblast becomes
endoderm
What creates the third layer between ectoderm and endoderm
mesoderm
oropharangeal membrane
eventually forms the mouth. At the cranial end. Ectoderm of the stomodeum. Rupture ~week 4
what does ectoderm become?
Anything skin
endoderm
the gut tract/ anything mucosa
mesoderm
blood vessels/nerves
What is mesenchyme?
undifferentiated stem cells
What are the two types of folding in the gut?
Longitudinal and transverse
Transverse folding
ectoderm and mesoderm fold laterally and ventrally, which close off endoderm forming a seperate gut tube layer.
endoderm > epithelial lining
mesoderm > supporting structures and SM
outer mesenchymal layer > outer tissue layer
Longitudinal Folding
~17-18 days. opening between gut tube and yolk sac draws closed like a drawstring bag.
Divides into foregut, midgut (open to yolk sac) and hindgut
With further folding yolk sac opening gets smaller and smaller and gut sections become more defined.
What are the two transitional regions between endoderm (mucosal) and ectoderm (skin)?
oropharangeal membrane (mouth) anal membrane (anys)
anal membrane
Eventually forms the anus. At the caudal end, ectoderm of the anal pit. ~ week 8 ruptures.
What is weird about the development of the gut tube lumen?
Initially the gut tube = oatent
As epithelium proliferates, it plugs up the lumen.
~ week 8 recanalization occurs
Arterial supply to FOREGUT
Celiac Trunk located @ T12
Arterial supply to MIDGUT
Superior Mesenteric Artery @ L1
Arterial supply to HINDGUT
Inferior Mesenteric Artery @ L3
Whats interesting about SMA
It goes all the way through the yolk sac and maintains a connection with the umbilical system
Why is the arterial supply of value?
Because it indicates the lymphatics and veins
Is the lungs endoderm or ectoderm, and why?
Endoderm. Because everything that secretes mucus is from endodermal origin.
How are the lungs formed?
Via a respiratory divaticulum, where the lungs bud of the oesophagus, eventually separating.
Oesophagus formation?
Starts as small tube immediately caudal to pharynx.
seperates from trachea to form oesophagus.
Rapidly elongates downwards.
What are some forms of oesophagus congenital malformations?
1) Tracheoesophageal Fistula (abnormal connection) = ‘punches its way into trachea
2) Tracheoesophageal Atresia (blockage) = end up with blind end, baby starts vomiting up milk
3) Congenital Hiatal hernia= short oesophagus, stomach herniates into thorax, through oesophageal hiatus
What does the stomach sit between
Ventral and dorsal mesogastrium
Stage 1 of stomach formation
Gut tube starts to dilate
Stage 2 of stomach formation
- Dilation cont.
- rotation on long axis ~90 degrees clockwise
- Ventral mesogastrium moves to right
- dorsal mesogastrium moves left
Stage 3 of stomach formation
- Dilation cont.
- rotation on coronal axis ~90 degrees clockwise
- Right boundary becomes superior (lesser curvature)
- Left boundary becomes inferior (greater curvature)
Omentum is?
Two double-sided sheets of peritoneum (4 layers). Extend from each curvature of the stomach
Formation of the greater omentum?
Dorsal mesogastrium is dragged around to the left due to stomach rotation. Begins to hang down due to WOG.
Both sides of the hanging fold fuse to make 4 layered periotneal structure
How can you go from intraperitoneal to retroperitoneal
Sometimes some viscera start off as intraperitoneal and then get pushed up against the body wall & the dorsal peritoneal layer gets obliterated.
What is the transverse colon intimately fused with?
The greater omentum
Congenital malformations of the stomach?
Congenital hypertrophic Pyloric Stenosis
- marked thickening of the muscular wall of the pylorus
- blocks exits from stomach > duodenum
- attempts to empty stomach spasming and vomiting occurs
How can you identify congenital hypertrophic pyloric stenosis?
Non-bilious vomit (SI blocked)
Are the liver and biliary tree part of the main GI tract?
No they come from the ‘hepatic diverticulum’, a ventral outgrowth that occurs ~ week 4.
Divides into 2
Larger, cranial= liver
Smaller, caudal= biliary apparatus
How does the bile duct position change?
Starts attached to ventral duodenum. As the duodenum grows/rotates it becomes dorsal
Formation of the Pancreas?
Caudal foregut, develops between both mesogastrium
- 2 buds (major= dorsal, smaller= ventral)
-ventral carried around as duodenum rotates, fuses and anastamoses with dorsal
Pancreatic duct = ventral + dorsal
But the main biliary system is connected via ventral duct / head of pancreas
How many still have the accessory duct
9%
Where are the gastrolienal and lienorenal ligaments found?
Stomach (gastrolienal) spleen (lienorenal) kidneys
The liver splits the ventral mesogastrium into two, what are they?
Falciform ligament (blood vessels and umbilival veins) lesser omentum
Interesting about the spleen?
It is not part of the foregut, it just forms in the dorsal mesogastrium
Midgut Formation
Starts straight
- forms U-shaped loop around SMA (cranial and caudal limbs)
- Herniates into umbilical cord (pysiological)
- Loop begins to rotate anticlockwise, for 270 degrees
- Eventually herniation returns and retracts to abdominal cavity.
- caecum & appendix rotate down to lower abdomen
Midgut examples of congenital malformations?
1) Non-rotation events
2) Umbilical Hernia/fistula - umbilical cord fails to close properly, gut herniates through weakened region in body wall (gut on outside)
3) Meckel’s Diverticulum
Ileal diverticulum doesn’t pinch off, common (2%), remnant of the yolk stalk (vitelline duct)
Hindgut formation
Starts as general expanded distal area ‘cloaca’
-this is divided into dorsal and ventral parts by mesenchymal unorectal septum
-As septum grows it seperates the ‘rectum’ from the ‘urogenital sinus’
-
Rectal/anal canal?
boundary between outer ectoderm and inner endoderm
Pectinate line shows boundary.
upper 2/3 IMA
lower 1/3 systemic circuit
The portal systemic anastomosis (venous drainage goes to two different sites!)
Where do anal columns terminate?
At site of anal membrane
When do anal lymphatics change
Anocutaneous line white line)
Below this is SF inguinal nodes
Congenital Malformations of the Hindgut
1) Megacolon
- segment of colon dilates, due to absense of ANS ganglion cells in the wall of gut distal to it. No nervous innervation= no peristalsis= blockage.
- normally close to anus
2) Imperforated Anus
- failure of anal membrane to rupture
3) Rectal Atresia
- anal canal (blind end) separated from rectum
- fistulas may be present into urogenital system
(MORE COMMON IN MALES)
what lies in the lienorenal lig?
tail of pancreas and splenic vessels
