GI Embryology Flashcards
Arteries and Gut Regions
Foregut
Esophagus
Stomach
Duodenum
Liver
Pancreas
Hindgut
Colon
Rectum
Anal Canal
Sigmoid
Midgut
Duodenum
Jejunum
Ileum
Colon
Cecum
Appendix
Pharyngeal Gut
Oral Cavity
Pharynx
Embryo Dev
Week 3 & 4 folding occurs into 3 disc layer
Endodermal lined gut tube connected to vitelline duct
Buccopharyngeal m. breaks down week 4 to form mouth= stomodeum
Cloacal m. ruptures in 7th week, creats opening for anus and urethra
Mesenteries
2x peritoneum that enclose organ & connect it to body wall
Peritoneaum simple squamous epith that secretes mucus and lines abdominal cavity
Dorsal Mesentary
extends from lower end of esophagus to cloacal region of hindgut.
Moditifed during dev to become:
Dorsal mesogastrium
mesoduodenum
proper
mesocolon
Ventral Mesentery
only in terminal esophagus, stomach & upper duodenum
Modified during development to become lesser omentum & falciform ligament.
Mucosa
Epithelial lining & glands (from endoderm)
Lamina propria
Muscularis mucosae
Splanchnic Mesoderm
Submucosa
Muscularis externa
Adventitia/serosa
Foregut Expanded
Esophagus- week 4 respiratory diverticulum is outgrowth from primitive gut tube.
Located caudal to primitive pharynx
Separated from dorsal part of foregut by tracheoesoph septum eventually.
Esophageal segment grows & eleongates during 2nd month.
Esophageal Atresia
Results from deviaton of tracheoesophageal septum in posterior direction associated w/ polyhydraminios
Short Esophagus
Failure of esophagus to elongate in proportion to dev of neck & thora.
Results in stomach being displaced cranially into thorax forming congenital hiatal hernia.
Stomach Dev
4TH week, dilation in foregut occurs
dorsal border grows fater than ventral border which yeild lesser & greater curvatures
Stomach develops in midline then rotates 90 CW then CCW
R & L vagus innervate post & ant walls
Omental Bursa
dorsal mesogastrim continues to expand during dev as 2x layered sac over both small intestines & transverse colon
Congenital Pyloric Stenosis
5x more common in males than in females
Hypertrophy of m. layer in pyloric region of stomach
After feeding stomach is distended= projectile vomiting!
Spleen Formation
Not foregut
Mesodermal origin- b/t layers of dorsal mesentery
Duodenum Formation
Week 4, distal foregut & proximal midgut join to form C shaped tube
Stomach rotation brings duodenum ultimately to lie on R side of abdominal cavity
Duodenum & pancrease head pressed dorsally against body wall.
R surface of dorsal mesoduodenum fuses with peritoneum.
Retroperitoneal position
Duodenal Formation 2
During 2nd month, duodenum epithelial proliferation, obliteration of lumen & recanalization
Duodenal Stenosis & Atresia
Stenosis partial occlusion of lumen due to incomplete recanalization of duodenum
Foreceful vomiting like in pyloric stenosis
Duodenal atresia complete closure of lumen & results in polyhydraminos
Liver & Gallbladder Development
Glands of GT formed by complex interaction b/t endodermal outpocketings of primitive gut tube & surrounding mesenchyme (mesoderm)
*Use SHH & HOX
Liver Bud
End of 3rd or beginning of 4th week, liver bud appears out of foregut
Caudal portion of liver bud differentiates into bile duct
overgrowth from bile duct gives rise to gallbladder & cystic duct.
As growth continues, liver bud extends into septum trasnversum
Septum Transversum
Plate of splanchnic mesoderm lies b/t thoracic cavity & stalk of yolk sac. Appears on day 22
Ultimately becomes: central tendon of diaphragm & ventral mesentery
Hepatic cords (foregut endoderm) diff into hepatocytes & lining of biliary ducts
Umbilical & vitelline v.–> hepatic sinusoids
CT, hematopoietic tissue & Kupffer cells derived from mesoderm of septum transversum
Bare A of Liver
Mesoderm on surface of liver fomrs visceral peritoneum of liver, except A where it lies in contact with diaphragm (bare A of liver)
Functions of Developing Liver
Hematopoiesis- 5 to 6 week
Bile production- 12th week, bile drains down newly fomred bile duct & stored in intestines.
As bile released into intestines it gives meconium its characteristic green color
Gallbladder & Extrahepatic Duct Development
Bile Duct- narrow channel b/t liver bud & foregut
Gllbladder & cystic duct- formed from outgrowth of bile duct
gallbladder also undergoes mitosis whereby lumen is obliterated
Accessory Hepatic ducts & duplication of gallbladder
Also extrahepatic biliary atresia
Intrahepatic biliary atresia- from maternal infection
Pancreas Formation
In 5th week
Two buds originating from endodermal lining of duodenum
Duodenum rotates 90 CW, the ventral bud carried dorsally (along with bile duct), eventually located posterior to dorsal bud.
2 buds fuse in 6th week
Dorsal bud- head, body & tail
Ventral bud- uncinate process
Exocrine and Endocrine Pancreas
Exocrine- none
Endocrine-
Islets of Langerhan start to develop in 3rd month.
A cells diff first, followed by B cells or D cells
Not known when F cells are formed.
Glucagon first detected in fetal plasma by 15th week, insulin in 5th month.
Ectopic Pancreatic Tissue
Can be found anywhere from distal esoph to tip of primary intestinal loop
Most freq in duodenum or mucosa of stomach
5% Meckel’s Diverticulum contains pancreatic tissue
Annular Pancreas
2 portions of ventral bud rotates in opp direction & encircle the duodenum from both sides
Midgut Dev
Midgut rapidly expands in 5th week resulting in fomraiton of U shaped loop
Same time as primary intestinal loop rotates 90 CCW around an axis formed by SMA
Physiological Herniation
In 6th week all intestinal loops enter extraembryonic cavity in umbilical cord
Due to rapid growth of cephalic limb
Expansion of liver
Retraction of Herniated Loops
In 10th week, intestine re enter abdominal cavity
As it returns it undergo a further 180 CCW rotation
Due to regression of 2nd kidnye sys, reduced growth of liver, expansion of abdominal cavity
During dev midgut undergoest 270 degree CCW rotation!
Fixation of Intestines
Mesenteries of ascend & descend colon are fixed against peritoneum & later fuse, therefore they become retroperitoneal.
Transverse mesocolon fuses w/ post wall of greater omentum, maintainign its mobility
Atresia or stenosis of gut
would cause considerable backup in gut leading to excessive GI distension & excessive vomiting in newborns.
Most commonly caused by vascular accidents
Apple Peel Atresia
10% of bowel atresia, occurs in jejunum
Recanalization
Omphalocele
Failure of return of intestinal loop into body cavity after phys herniation 6-10 weeks
Associated w/ high rate mortality, sever cardiac & NTDs & chrom abnormalityes
Gastroschisis
Herniation of abdominal contents through defect in abdominal wall directly into amniotic cavity.
Not covered by amnion & peritoneaum, occurs lateral to the umbiliucs
Not associated w chrom abnormalites or othere severe defects.
Excellent survival rates.
*ventral wall defect*
Vitelline Duct Abnormalities
As intestines reenter abdomen, vitelline duct obliterated
Except:
Remnant of vitelline duct, may contain ectopic pancreatic or gastric tissue
Abnormal Rotation
Hindgut Dev
In embryonic period of cloaca is subdivided into anorectal canal (post) & urogenital sinus (ant)
Accomplished by means of urorectal septum.
Anal Canal
Upper 2/3 formed from anorectal canal (hindgut)
Lower 1/3 formed from invagination of ectoderm
Junction is known as pectinate line.
At pectinate line epith undergoes a sudden transition from simple columnar to stratified squamous.
BV: cranial portion- sup rectal a.
Caudal portion- inf rectal a.
Imperforate Anus
No anal opening, lack of recanalization of anal canal, failure of anal mem to break down .
Rectoanal Atresia & fistula
ectopic positioning of anal opening, due to incomplete separation of anorectal canal from urogenital sinus
Differentiation
Primite gut diff into diff organs depending on reciprocal inteaction b/t endoderm & mesoderm.
Mesoderm HOX dictates type of structure that will fomr
Endoderm SHH induces HOX expression in mesoderm
Appendix
Anal Canal
Upper 2/3= simple columnar
Lower 1/3= stratified squamous non keratinized
Anus- stratified sqamous keratinized of skin
Anal Canal Sphincters
Internal Anal sphincter- thickened inner layer of muscularis externa
External anal sphincter- skeletal m.
