Embryology 7 Flashcards
In what week does the primordial gut tube form?
week 4
During the formation of the primordial gut tube, the yolk sac is incorporated into the embryo via lateral folding. How does the gut tube remain connected to the yolk sac?
omphaloenteric duct
The omphaloenteric duct connects what two structures?
connects gut tube to yolk sac
What membranes are responsible for closing the primordial gut tube?
oropharyngeal membrane (cranial end)
cloacal membrane (caudal end)
What are the three distinct regions of the primordial gut tube?
foregut
midgut
hindgut
What is the arterial supply to the foregut? (4)
celiac trunk (to most of the foregut)
exceptions include:
derivatives of pharyngeal arches, lower respiratory system, large portion of esophagus
What is the arterial supply to the midgut?
superior mesenteric artery
What is the arterial supply to the hindgut?
inferior mesenteric artery
The developing gut tube lies within what cavity?
peritoneal cavity
The developing gut tube is covered by
peritoneum (because it lies within the peritoneal cavity)
What are mesenteries?
double layers of peritoneum that suspend the gut tube between the ventral and dorsal abdominal walls
The mesogastrium connects what two structures?
connects stomach with body wall
The mesocolon connects what two structures?
colon with body wall
Describe the function of the ventral mesentery.
suspends foregut + cranial part of midgut (up to umbilicus) from anterior body wall
Where is the dorsal mesentery found?
stretches along the entire gut tube
Give three examples of abdominal organs that grow between the two layers of mesentery.
liver
gallbladder
pancreas
What is a consequence of portions of the dorsal mesentery retracting during development?
as a result of the dorsal mesentery retracting, some organs become adhered to the dorsal wall of the abdominal cavity
What are secondarily retroperitoneal organs?
organs that become adhered to the dorsal abdominal wall because of dorsal mesentery retraction
In what region(s) are secondarily retroperitoneal organs covered with peritoneum?
secondarily retroperitoneal organs are covered with peritoneum on their anterior wall
What are primarily retroperitoneal organs?
organs that develop retroperitoneally from the beginning (in contrast to secondarily retroperitoneal organs)
Give an example of a primarily retroperitoneal organ.
kidneys
What are the derivatives of the foregut? (8)
primitive pharynx + derivatives
lower respiratory system
esophagus
stomach
part of duodenum cranial to opening of common bile duct
liver
biliary apparatus
pancreas
[PPPLLESB]
In what week does the esophagus begin to form?
week 4
Describe how the primitive esophagus is formed in week 4.
laryngotracheal tube branches off from primitive gut and is separated from gut by tracheoesophageal septum
Describe the muscle composition (and origin of muscle) of the growing esophagus. (2)
cranial part of esophagus = striated muscle from pharyngeal arches 4 and 6
caudal part of esophagus = smooth muscle from visceral lateral mesoderm
What provides innervation to the two types of esophageal muscle?
vagus nerve (CN X)
The stomach develops from
the caudal part of the foregut tube
Describe the positioning of the stomach during the early stages of its development.
stomach positioned in midline with gastro-esophageal junction cranial and pyloric-duodenal junction caudal
Describe the positioning of the vagus nerve during the early development of the stomach.
vagus nerves run laterally on both sides of developing stomach
What is the underlying cause for the formation of the greater and lesser curvatures of the stomach?
dorsal side of stomach grows faster than ventral side → greater curvature on dorsal side + lesser curvature on ventral side
How does the stomach further develop following the formation of the greater and lesser curvatures?
stomach rotates 90° clockwise around longitudinal axis → ventral border/lesser curvature moves to right side + dorsal border/greater curvature moves to left side
Differentiate between the right and left vagus nerves in terms of what they innervate. Why does this dichotomy arise? (3)
right vagus nerve → posterior wall
left vagus nerve → anterior wall
arises because of clockwise 90° rotation of stomach
Following the first 90° rotation, how does the stomach further develop?
another rotation around the dorsal-ventral axis occurs → gastro-esophageal junction moves caudally + pyloric-duodenal junction moves cranially
What is the fate of the dorsal and ventral mesenteries that suspend the foregut tube?
dorsal mesentery → dorsal mesogastrium
ventral mesentery → ventral mesogastrium
The duodenum develops from (2)
the caudal portion of the foregut
the cranial portion of the midgut
The duodenum receives blood supply from (2)
celiac trunk (because it is partially derived from the foregut)
superior mesenteric artery (because it is partially derived from the midgut)
Describe how the duodenum changes during the rotation of the stomach.
duodenal loop rotates to right
becomes pressed against posterior abdominal wall
dorsal mesenterium regresses
most parts of duodenum become secondarily retroperitoneal
Describe the fate of the duodenal lumen.
in weeks 5-6, duodenal lumen is temporarily obliterated but then recanalizes by the end of week 8
The liver and biliary apparatus develop from
the caudal portion of the foregut
What is another name for the liver bud?
hepatic diverticulum
The liver bud originates from
the ventral portion of the caudal portion of the foregut, in week 4
After the liver bud forms, it branches into (2)
the mesoderm of the septum transversum
the cranial portion of the ventral mesentery
The mesoderm of the septum transversum develops into
the diaphragm
Within the ventral mesentery, the liver bud divides into what two parts?
larger/superior portion = liver primordium
smaller/inferior portion = gallbladder
The connection between the liver bud and foregut develops into
the common bile duct
What is the origin of the common bile duct?
the connection between the liver bud and foregut
The stalk that connects the gallbladder to the liver bud becomes
the cystic duct
What is the origin of the cystic duct?
the stalk that connects the gallbladder to the liver bud
The ventral mesentery is divided by the liver into what two structures?
lesser omentum
falciform ligament
What forms the visceral peritoneum of the liver?
ventral mesentery
What structures comprise the lesser omentum, and what is their anatomical function? (3)
hepatogastric ligament
hepatoduodenal ligament
function: connect liver with stomach + duodenum
What is the anatomical function of the falciform ligament?
connects anterior body wall with liver
What structures does the umbilical vein?
umbilical vein connects placenta to liver
Where is the umbilical vein located in the embryo?
umbilical vein is located at free border of falciform ligament
What is the remnant of the umbilical vein in the adult?
round ligament
The pancreas develops from
1 dorsal + 2 ventral pancreatic bud
Describe the growth of the pancreatic buds. (2)
pancreatic buds grow from foregut tube into dorsal and ventral mesenteries
two ventral buds merge into one bud
List the steps in the formation of the pancreas. (2)
duodenum rotates to right
ventral and dorsal buds merge to form pancreas
Describe how the head of the pancreas is formed.
after duodenal rotation, smaller ventral pancreatic bud is superior to dorsal pancreatic bud and forms the head of the pancreas
Is the pancreas primarily or secondarily retroperitoneal?
rotation of the stomach + duodenum pushes pancreas in dorsal direction → secondarily retroperitoneal
(T/F) The spleen is a derivative of the foregut.
False. The spleen forms in the dorsal mesogastrium from the visceral mesenchyme and is therefore not a derivative of the foregut.
The spleen is derived from
the visceral mesenchyme in the dorsal mesogastrium
Describe how the spleen forms. (2)
mesenchymal lobules appear at week 5
mesenchymal lobules coalesce to form spleen
What is the final location of the spleen?
left upper quadrant
What determines the final location of the spleen?
rotations of the stomach and its adjacent organs
When does hematopoiesis occur in the spleen?
hematopoiesis occurs in the spleen until late fetal life
List three types of anomalies of foregut derivatives.
esophageal atresia
duodenal atresia = stenosis
atresia of upper GI tract = polyhydramnios
What is a common term for esophageal atresia?
blind-ended esophagus
Esophageal atresia is caused by
abnormal development of laryngotracheal tube
Abnormal development of the laryngotracheal tube can lead to (2)
esophageal atresia
esophageal stenosis
Duodenal atresia or stenosis is cuased by
insufficient recanalization of the duodenum at the end of the embryonic period
Duodenal atresia/stenosis is commonly found in what grouping of patients?
commonly found in children with Down syndrome or other chromosome abnormalities
What is polyhydramnios?
excess of amniotic fluid in the amniotic cavity
What causes polyhydramnios?
atresia of upper GI tract
List the derivatives of the midgut. (4)
small intestine (including most of duodenum)
cecum + vermiform appendix
ascending colon
right 1/2 to 2/3 of transverse colon
The midgut derivatives are supplied by
the superior mesenteric artery
Describe the formation of the midgut loop.
midgut elongates and forms ventrally-oriented hairpin loop with cranial + caudal limb
Describe the positioning of the superior mesenteric artery with respect to the midgut loop.
superior mesenteric artery runs down long axis of midgut loop
The cranial portion of the midgut loop develops into
most of the small intestine
The caudal portion of the midgut loop develops into
ascending colon
transverse colon
Where does the cecal diverticulum form?
forms at apex of caudal portion of midgut loop
What is physiologic umbilical herniation?
when both limbs of the midgut loop get pushed into the proximal part of the umbilical cord because the abdominal cavity can’t accommodate the fastly-growing midgut
In what week does physiologic umbilical herniation occur?
week 6
Describe the rotations of the midgut loop.
while herniated to umbilical cord, midgut loop rotates 90° CCW (viewed from anterior) along axis of superior mesenteric artery
in week 10, midgut returns to abdomen and makes an additional 180° CCW rotation
How do the jejunum and ileum form from the midgut loop?
during its first 90° CCW rotation, the cranial part of the midgut further elongates to form the jejunum and ileum
How does the 180° rotation of the midgut loop affect the cecal diverticulum?
180° CCW rotation places cecal diverticulum below the right lobe of the liver
Describe the fate of the cecal diverticulum.
cecal diverticulum extends caudally and becomes cecum + vermiform appendix
What is the most common position of the appendix relative to the cecum?
posterior to the cecum (an arrangement called retrocecal appendix)
What is a retrocolic appendix?
appendix posterior to colon
What is a pelvic appendix?
appendix positioned ventrally over pelvic brim
Describe whether the duodenum, jejunum, ileum, transverse colon, and ascending colon are primarily retroperitoneal, secondarily retroperitoneal, or intraperitoneal. (@)
secondarily retroperitoneal = most of duodenum + ascending colon (mesentery resorbed)
intraperitoneal = jejunum + ileum + transverse colon (all retain mesenteries)
What is omphalocele? (2)
protrusion of abdominal contents through umbilicus into umbilical cord
protruding bowel covered by membranes of umbilical cord
What causes omphalocele?
occurs when physiological umbilical hernia (forms by week 6 and should retract by week 10) persists
How is omphalocele treated?
surgery
How are umbilical hernias different from omphaloceles?
in umbilical hernias, herniating abdominal contents covered by skin
umbilical hernias resolve without surgery
What is gastroschisis?
protrusion of abdominal contents through defect in (most typically, right) abdominal wall, lateral to the umbilicus
no protective membrane around abdominal contents
What is a common consequence of gastroschisis?
gangrene (death of tissue due to infection)
What is the genetic predisposition of omphalocele? What diseases is omphalocele associated with?
minimal genetic predisposition
associated with syndromes, such as trisomy 18
What is the genetic predisposition of gastroschisis? What diseases is gastroschisis associated with?
minimal genetic predisposition
rarely occurs in syndromes
What is malrotation?
when midgut rotation is less than a total of 270°
