Embryology

Question 1

Stomach Development

Answer 1

 Undergoes 2 rotations and asymmetrical growth
• Rotates 90 degrees on vertical axis so everything on right now lies posterior (reason why left vagus nerve is found on anterior aspect of GI system and right vagus nerve is found on posterior aspect; result of cell death on right side
• Rotates 90 degrees on horizontal axis through middle which swings inferior portion of stomach to a more superior/right position so stomach is curved backwards C-shaped
 Posterior/dorsal side (now on left) grows disproportionately faster and forms greater curvature

Question 2

Dorsal mesentery/Greater Omentum

Answer 2

 Tissue connecting gut tube with posterior abdominal wall is dorsal mesentery
 Develops from mesenchyme superior to roof of yolk sac as the embryo undergoes cephalocaudal folding and pinches off at yolk sac
 Carries blood vessels from aorta to gut tube
 Greater Omentum – four-layered pocket hanging below stomach as a result of folding; lies superficial to abdominal organs

Question 3

Abdominal BLood Supply

Answer 3

 Paired dorsal intersegmental arteries – supply the somites and muscle mass; right beneath the nerves and give rise to posterior intercostals arteries
 Paired lateral intersegmental arteries – supply the mesonephric kidneys; give rise to renal arteries
 Paired ventral intersegmental arteries – supply gut tube; pair either fuses or one drops out so left with three unpaired ventral segmental artery
• Celiac artery – supply abdominal foregut
• Superior mesenteric artery – supply midgut and ½ of transverse colon
• Inferior mesenteric artery – supply hindgut

Question 4

Formation of Intestines

Answer 4

 Investment of mesenchyme that will give rise to mesenteries that hold organs in place and acts as conduit for vasculature to supply organs
 Endodermal gut tube grows quicker than peritoneal cavity
 Bulges ventrally and pulls dorsal mesentery ventrally and forced into body stalk for 5 weeks (physiological umbilical hernia); kidneys are very large relative to rest of body
 Vitelline duct combines gut tube to yolk sac
 Pulled back into peritoneal cavity once mesonephric regress and create more room; undergoes 270 degree counterclockwise rotation around the vitelline duct that brings the cecum, ascending colon, transverse colon, and descending colon into their final anterior position
-small intestine undergoes rapid growth during the rotation of developing intestines

Question 5

Ventral Mesentery/Lesser Omentum

Answer 5

 Only in abdominal foregut, cranial to entry of pancreatic and hepatic ducts
 2 pieces of ventral mesentery divided by growth of liver
 Ventral mesentery between liver and anterior wall is falciform ligament
 Lesser omentum is segment from liver to gut tube; 2 regions (hepatogastric ligament and hepatoduodendal ligament)

Question 6

Urorectal Cloaca

Answer 6

 Urorectal septum develops outside of yolk sac; grows down towards cloacal plate and separates urogenital sinus and rectum

Question 7

Liver

Answer 7

 Liver forms hepatic diverticulum

 High concentration of FGF and BMP signals cells to become pancreas; low concentration area becomes ventral pancreas

Question 8

Pancreas

Answer 8

 Pancreas starts out dorsal and ventral pancreatic buds; dorsal bud stays in place and ventral bud rotates to join dorsal bud; most of pancreas is from dorsal bud but pancreatic duct is formed from ventral bud
• Annular pancreas occurs when the pancreatic buds rotate in manner to surround the gut tube (potentially strangling the small intestine)
• Exocrine pancreas (digestive enzymes) develops from Notch-sensitive endoderm; surrounding mesenchyme releases signal molecules (Follistatin, FGFs) that activate the Notch pathway
• Endocrine pancreas (Islet of Langerhands) develop from Notch INsensitive endoderm (develops in hindgut)
• Pax-6 sensitive cells develop in weeks 8-9 (alpha and gamma cells – glucagon and pancreatic polypeptide)
• Pax-4 sensitive cells develop later (beta and delta cells – insulin and somatostatin)

Question 9

Formation of Intraembryonic Coelom

Answer 9

 paraxial mesoderm (somites), Intermediate mesoderm (gonads and kidneys), lateral plate mesoderm (secretion of fluid to separate somatic and splanchnic layers to form intraembryonic coelom)
o intraembryonic coelom forms as result of LATERAL PLATE MESODERM pinching in and is space between yolk sac and amniotic cavity; ONLY OCCURS WHERE YOU HAVE SOMITES; forms a U shaped tube that gets brought anteriorly by cephalocaudal folding resulting in septum transversum attaching to anterior body wall via central tendon of diaphragm (forms U shaped diaphragm)
o extraembryonic coelom (chorionic space) derived from mesoderm between the extraembryonic mesoderm and the chorionic plate

Question 10

Formation of Pleuropericardial Membrane

Answer 10

o Anterior and posterior cardinal veins come from body wall into the common cardinal vein (Duct of Cuvier) which connects them to the heart
o Intussusception results in pulling in of the cardinal veins into where heart is which is going to pull the body wall in with it forming the pleuropericardial cavity

Question 11

Development of Muscular Diaphragm

Answer 11

o Region of mesenchyme which is anterior to the developing heart (septum transversum) will form large part of diaphragm during cephalocaudal folding (reason why phrenic nerve from high up in body innervates it)
- the esophageal mesentery is incorporated into the diaphragm
-the mesonephric kidneys disappear and the capsule that covered them will become the posterior and lateral diaphragm (pleuroperitoneal membrane)
o Diaphragm made up of 4 parts: septum transversum in middle; mesentery of esophagus; pleuroperitoneal membrane associated with mesonephric kidneys; contributions from posterior body wall

Question 12

formation of foregut, midgut, hindgut

Answer 12

• foregut and hindgut are result of cephalic and caudal amniotic overgrowth and pinching
• midgut stays in open connection with yolk sac which becomes yolk stalk and then umbilicus

-lateral folding is what actually seals the gut tube so it is its own tube

Question 13

dorsal/ventral mesentary and greater/lesser omentum

Answer 13

• greater omentum has 4 layers as a result of folding; around the greater curvature of stomach
• lesser omentum contains the hepatoduodenal and hepatogastric ligaments and located around the lesser curvature of the stomach
• ventral messentery is ONLY LOCATED IN THE FOREGUT

Question 14

vitelline duct

Answer 14

• connects the yolk sac to the gut tube

- this runs through the umbilicus

Question 15

Definition of coelom and communication in formation of one

Answer 15

• coelom - space lined by an epithelium that was derived from mesenchyme
• extraembryonic to intraembryonic communication within developing embry ONLY occurs in presence of somites