Rudimentary embryology Flashcards
Three germ cell layers
Ectoderm
Mesoderm
Endoderm
Development of foregut: oesophagus
Develops from ENDODERMAL TUBE
Week 4: divides into oesophagus and laryngotracheal tube
- Failure to do this results in oesophageal atresia (8%)
OR
-Tracheooesophageal fistula (80%)
Development of foregut: stomach
~ week 4
ENDODERMAL TUBE
Foregut dilates to form stomach
ROTATES so right wall of stomach now lies posterior –> forms lesser sac behind
Vagus nerve rotates with the stomach so:
RIGHT becomes POSTERIOR
Left is anterior
Development of foregut: duodenum
Stomach rotates to the left, so the duodenum
swings to the right
Mesentery fusing with the peritoneum of the posterior abdominal wall, leaving:
–>All retroperitoneal
but the first inch (duodenal cap)
Development of the midgut
ENDODERMAL TUBE
Enlarges rapidly in early life and becomes too large for abdominal cavity
Herniates into the umbilical cord.
The apex of the herniated bowel is continuous with
the vitellointestinal duct into the yolk sac
Midgut is within the cord it rotates 90° counterclockwise around the axis of the superior mesenteric artery, bringing the third and fourth parts
of the duodenum across to the left of the midline
behind the superior mesenteric artery; this part of
the duodenum is now fixed retroperitoneally
Midgut returns to the abdomen at the 10th
week and during this time it continues to rotate
counterclockwise through a further 180°, bringing
the ascending colon to the right side of the abdomen
with the caecum lying immediately below the liver.
The caecum descends into its definitive position in
the right iliac fossa, pulling the colon with it.
The mesenteries of the ascending and descending
colon blend with the posterior abdominal wall, except
for the sigmoid colon, which retains a mesentery.
Development of the foregut SHORT
Week 5: elongates into umbilicus with vitellointestinal duct
Rotates 90 counterclockwise
Week 10: retracts back into abdomen
Extra 180 counterclokwise = total 270 counterclockwise
Atresia and stenosis of the gut
During development growth obliterates inner lumen of gut
If re-canalisation is incomplete –> atresia or stenosis
Imperforate anus
The anal membrane separates the hindgut (endoderm) from the proctodeum (anal pit) (ectoderm)
Eventually the anal membrane breaks down and
continuity is established between the anal pit and
the hindgut.
Failure of the anal membrane to rupture or anal pit
to develop results in imperforate anus
Development of anal canal
Anal canal develops from the end of the hindgut
(endoderm)
And an invagination of ectoderm, the proctodeum
Separation of structures within cloaca
Rectum, anus and genitourinary tracts develop at the end of the 9th week
Exomphalos
Persistence of midgut herniation after birth
Gastrulation
Week 3
Cells of bilaminar disc (epiblast and hypoblast) –> three germ cell layers
Orchestrated by primitive streak in epiblast
- appears during beginning of 3rd week
- primitive node at cranial end, primitve pit within this
Cells of the epiblast layer break off and migrate toward the primitive pit. Here, they detach and penetrate through the epiblast layer to form three new germ cell layers.
Endodern
Formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells
- -> epithelium lining of gastrointestinal tract and respiratory tract
- -> lining of urethra , bladder and reproductive system
- -> liver and pancreas
Mesoderm
Formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm (previous hypoblast)
- -> notocord
- ->musculoskeletal system
- -> muscular layer of stomach and intestine
- ->circulatory system
Ectoderm
Formed by the epiblast cells that remain in position
- ->epidermis of skin
- -> cornea of lens of eye
- -> nervous system
