Lecture 6 - Embryology 1 Flashcards
where does the primitive gut extend from?
oropharyngeal membrane to cloacal membrane
what are the 3 parts of the gut tube and what where do each part extend from?
foregut - mouth to 1st half of duodenum
midgut - 2nd half of duodenum to 2/3rds along transverse colon
Hindgut - distal 1/3rd transverse colon to superior 2/3rd rectum
At what point is the midgut continuous with the yolk sac?
vitelline duct
What are the components of the primitive gut tube and what is each component derived from?
epithelial lining - endoderm
smooth muscle and connective tissue - visceral mesoderm
visceral and parietal peritoneum - visceral and parietal mesoderm
What is the primitive gut tube suspended from the posterior abdominal wall by?
dorsal mesentery
What is a mesentery
double fold of peritoneum that encloses an organ and connects to the body wall. these organs are intraperitoneal
retroperitoneal
Organs that are not surrounded by peritoneum are called retroperitoneal
what does the dorsal mesentery suspend?
from lower oesophagus to cloaca
what does the dorsal mesentery suspend?
– from lower oesophagus to 1st part of duodenum
- forms lesser omentum and falciform ligament (umbilical vein)
what is the function of the mesentery?
Carry blood supply, lymphatics and nerve supply to and from organs
What undergo remodelling to give rise to the arteries of the GIT?
the vitelline arteries
Which are the 3 arteries that supply the GIT?
- Coeliac trunk
- Superior Mesenteric - midgut
- Inferior mesenteric - hindgut
How does the definitive gut tube lumen form?
week 6
- proliferation of endoderm derived epithelial lining occludes gut tube
- apoptosis occurs for 2 weeks - creates vacuoles - recanalisation
- vacuoles coalesce - fully recanalise by week 9
- epithelium undergoes further differentiation
What can abnormal recanalisation cause?
abnormal - duplications of the GI tract
incomplete recanalisation - stenosis or blocked gut tube
What are the structures most commonly affected if there is abnormal recanalisation?
ileum followed by duodenum
if there is a duplication problem what else can happen
formation of cysts
high incidence of complications - bowel obstruction or intussusception
Development of the foregut
Foregut extends from the oropharyngeal membrane to the first part of the duodenum
Gives rise to respiratory diverticulum
Separates from respiratory diverticulum - tracheoesophageal septum – pharynx and oesophagus
oesophageal atresia
can happen independently of tracheooesophageal fistula
displacement of septum can separate proximal and distal ends of oesophagus
prevents foetus from swallowing amniotic fluid - returns to mother through placental circulation -
What is polyhydramnios
atresia prevents foetus from swallowing amniotic fluid - returns to mother through placental circulation -
surgical repair - 85% survival rate
When and how does the oesophagus form?
Forms in week 4 caudal to the lung bud
oesophagus has endodermal epithelial lining and smooth muscle layer from visceral mesoderm.
initially very short
What other muscle does the oesophagus have apart from smooth muscle?
skeletal muscle - from paraxial mesoderm
congenital hiatal hernia
oesophagus rapidly lengthens in weeks 4-7 to descent to abdomen
- insufficient elongation - results in part of stomach positioned supradiaphragmatically
- differs from common acquired hiatal hernia - irreducible
How and when does the stomach form?
The stomach appears in the 4th week as a dilation of the foregut
suspended in abdomen - dorsal and ventral mesenteries
Differential growth in week 5 forms the greater curvature i.e. the dorsal wall grows faster
In which direction does the stomach rotate in weeks 7-8?
rotates around 2 axes
90° clockwise rotation around the craniocaudal axis - lesser curvature moves from ventral position to the right.
greater curvature moves from dorsal position to the left
What happens to the vagus nerves on the gut tube ?
initially located on left and right sides of gut tube
left vagus trunk - becomes anterior
right vagus trunk - posterior (dorsal)
what does the rotation around the ventrodorsal axis do?
greater curvature - faces slightly caudally
lesser curvature - slightly cranially
Formation of the Lesser Peritoneal Sac
as stomach rotates - creates space behind it - lesser sac
what is the epiploic foramen?
The epiploic foramen (of Winslow) is the narrow opening that connects greater and lesser sacs
formation of the greater omentum
dorsal mesentery attached to stomach - greater curvature - continues to grow
reflects back on itself to form lesser sac
both fuse before birth
what does the posterior layer of the greater omentum also fuse with
mesentery of the transverse colon
Congenital Pyloric Stenosis
Narrowing of pyloric sphincter caused by hypertrophy of smooth muscle
more common in males
1 in 500
restricts gastric emptying
signs of pyloric stenosis
Palpable pyloric mass
Projectile vomiting
Visible peristalsis
Heterotopic Gastric Tissue
-Inappropriate epithelial differentiation of the gut tube can result in ectopic gastric tissue
-acid production can cause inflammation and ulceration in the surrounding area
Damage can result in strictures due to scarring or rupture of the gut wall
What are the 2 origins of the duodenum and how does it form?
proximal half is foregut, distal half is midgut.
The duodenum elongates in week 4 resulting in a ventrally projecting C-shape
This is then dragged to the right by the rotating stomach
What happens to the dorsal mesentery attached to the duodenum during development
The dorsal mesentery attached to the duodenum degenerates so that the duodenum lies against the posterior abdominal wall - secondarily retroperitoneal
where does the foregut become the midgut
The boundary is distal to the entrance of the common bile duct
