Two Flashcards
Describe the formation of the primordial gut and which parts are supplied by which vasculature.
a) As a result of cephalocaudal
and lateral folding of the
embryo, a portion of the
endoderm-lined yolk sac is
incorporated into the embryo
to form the primitive gut at
wk 4. All three germ layers
contribute to the gut, which
is divided into foregut,
midgut and hindgut based on
the arterial supply. Each
region and tissue has distinct
functions and specialized
morphology. The boundaries
of the 3 regions are
determined by the territores
of the 3 supplying arteries.
i) Foregut – Celiac artery;
differentiates into gut and
other organs, tissues
ii) Midgut – Superior
mesenteric artery;
differentiates into gut
iii) Hindgut – Inferior
mesenteric artery,
differentiates into gut and
other organs, tissues
What does the endoderm give rise to? Describe what happens with it from weeks 6 to 9?
b) Endoderm gives rise to the
epithelium, glands,
parenchyma including liver
and pancreas. At wk 6, the
endodermally derived
undifferentiated cells of the
gut proliferate and occlude the
gut lumen. During the next 2
wks, the lumen recanalizes.
By 9 wks, definitive mucosal
epithelium differentiates from
the endoderm (See duodenal
atresia and intestinal duplication)
What is the part of the mesoderm that gives rise to the gut? What parts of the gut does it give rise to?
c) Lateral plate splanchnic mesoderm coating the endodermal tube gives rise to muscularis propria, lamina propria, submucosa, blood vessels and connective tissue of the gut.
How are epithelial mesenchymal interactions important? What does the neural crest give rise to? When does gut SM innervation begin and why is it important?
d) Epithelial-mesenchymal interactions are essential in directing morphogenesis and
regional specific cellular differentiation of the developing GI tract.
e) The neural crest gives rise to the enteric neurons of myenteric (Auerbach) and submucosal (Meissner) plexii. Innervation of the gut smooth muscle begins by wk 10 and is necessary for peristalsis. (see Hirschsprung’s Disease).
What does the foregut give rise to?
a) Primordial pharynx (oral cavity, pharynx, tongue,
tonsils, salivary glands, upper respiratory system).
b) Respiratory diverticulum (lung bud) of the foregut
forms the lung.
Esophagus
Stomach
Duodenum proximal to ampulla of vater and common bile duct
Liver, biliary tree, ventral pancreas
Gallbladder
Common bile duct
Describe how the esophagus, stomach, and duodenum (when?) are formed from the foregut.
c) Esophagus and stomach
i) Tracheoesophageal septum separates trachea
from esophagus (see tracheoesophageal
fistula)
ii) Stomach begins as a fusiform (spindled-
shaped) enlargement of foregut and shifts to the left as it grows. Differential growth
of the dorsal and ventral walls of the stomach produce the greater and lesser
curvatures.
d) Duodenum proximal to ampulla of Vater and common bile duct
i) During week 5 - 6, the lumen is obliterated because of
proliferation of epithelial cells. The lumen is
recanalized by end of embryonic period (see duodenal
atresia).
Describe the development (when it begins) of the liver, biliary tree, and pancreas from the foregut. What parts of the foregut lead to the different parts of these organs ? What is annular pancreas? What does it cause?
e) Liver, biliary tree, pancreas arise from ventral buds off
caudal foregut (duodenum) at 4th week
i) Hepatic diverticulum (liver bud) gives rise to the liver
and biliary tree. It grows into the inferior septum
transversum (mesoderm between heart and midgut,
mesogastrium) and divides into a cranial and caudal part.
(1) The cranial portion of hepatic diverticulum becomes
the liver.
(a) Endodermal cells form hepatic cells and intrahepatic biliary tree.
(b) Mesenchyme derived connective tissue, hematopoietic, and Kupffer cells
(c) Hematopoiesis begins in the 6th week and contributes to the relative increase in liver size. In the embryo, the liver’s primary function is hematopoietic.
(d) Hepatocytes make bile at 12th week. Meconium is green by 13th week
(2) The caudal portion of hepatic diverticulum becomes gallbladder and ventral pancreas.
(3) Stalk connecting the liver bud to the duodenum becomes common bile duct (see biliary atresia)
ii) Pancreas develops from dorsal pancreatic bud and
ventral bud of endoderm from foregut at level of
duodenum. Dorsal pancreatic bud give rise to head,
body and tail of pancreas; ventral bud gives rise to
uncinate process and a portion of the head
(1) The ventral bud normally moves dorsally and the two buds fuse.
(a) Annular pancreas may be result of bilobed ventral buds that more dorsally on both sides of the duodenum and fuse with the dorsal bud. Can cause duodenal
obstruction, pancreatitis and peptic ulcer.
(2) Pancreatic acinar and islet tissue differentiate
from endoderm of the buds and insulin
secretion begins in early fetal period.
What is derived from the midgut?
Midgut
a) Small intestine, including distal duodenum, cecum,
vermiform appendix, ascending colon, and proximal
transverse colon.
Describe the process of physiologic umbilical herniation. When does it occur? What happens in non-rotation? What happens if it doesn’t return?
b) “Physiologic umbilical herniation” At wk 6, the liver is
relatively large and there isn’t enough space in the
abdominal cavity for the growing, elongating midgut,
so it herniates as a U-shaped “primary intestinal loop”
into the yolk stalk in the proximal umbilical cord
(which communicates with the intraembryonic coelom
at the umbilicus).
i) When in the umbilical cord, the midgut rotates 90 degrees counterclockwise.
ii) At wk 10-11, intestine returns to abdomen, and rotates another 180 degrees
counterclockwise. Why the gut returns to the abdomen is uncertain. (See
omphalocele, umbilical hernia).
(1) If the midgut loop doesn’t rotate as it returns to the abdomen get “Nonrotation”
with small bowel on right and colon on left.
(a) Increased risk of volvulus, obstruction
(2) The apex (leading edge) of the loop (at the ileum) is attached to the umbilical yolk
sac endoderm by the vitelline (oomphalomesteric) duct. (See Meckel’s
diverticulum in Lecture 9 and 13b).
What is derived from the hindgut?
a) Distal transverse, descending and sigmoid colon, rectum, superior anal canal, lining of urinary bladder and most of urethra.
Describe the development of the allantois and the cloaca. What is patent urachus?
b) Cloaca is the caudal end of the hind gut, in contact with ectoderm at cloacal membrane
i) The allantois (a finger-like diverticulum of the roof of the cloaca that extends into the connecting stalk) joins the superior ventral cloaca and contributes a small portion of the endoderm of the bladder. Normally, the allantois becomes fibrotic, the lumen disappears and it forms the urachus (median umbilical ligament).
(1) If the lumen fails to obliterate, patient has a “patent uracus, a uracal sinus or cyst”. Symptoms include urine leakage from the umbilicus, UTI, peritonitis.
Describe what happens with the urorectal septum.
ii) The urorectal septum is a wedge of mesodermal tissue between the allantois and hindgut and includes the two folds, the superior Tourneaux and the lateral Rathke
folds. The urorectal septum grows caudally and splits the cloaca into two parts by wk 7. The cloacal membrane is then divided
into the posterior anal membrane and the
anterior urogenital membrane:
(1) Rectum and cranial anal canal
(posterior)
(2) Urogenital sinus forms the epithelial
lining of the urinary tract (anterior)
Describe how the inferior anal canal is developed? What is imperforate anus? What is the pectinate line and what is its significance?
5) Proctoderm (anal pit) gives rise to the inferior anal
canal. The anal pit is created when mesenchyme
around the anal membrane proliferates and the anal
membrane (separates the endoderm and ectoderm of the anal canal) breaks down at wk 8. (see imperforate anus)
Imperforate anus is a defect that is present from birth (congenital). The opening to the anus is missing or blocked. The anus is the opening to the rectum through which stools leave the body.
a) The pectinate line (an area of irregular mucosal folding in anorectal canal) is where anal membrane was in the embryo, i.e, the junction between the endoderm and ectoderm. Superior to the pectinate line, inferior mesenteric arteries perfuse. Inferior to the pectinate line, internal iliac perfuses.
