Embyro: Digestive System Flashcards
Describe the relationship of the cephalocaudal and lateral folding of the embryo to development of the foregut, midgut, and hindgut regions.
A gradient of retinoic acid within the embryo (from high in the caudal to low in the cranial end) initiates production of transcription factors (segmental homeobox genes) and Sonic Hedge Hog (SHH) that specify regional differentiation along the length of the gut tube and prime the gut for further development.
- Trascription factors affect the relationship between the endoderm of the gut and the surrounding mesoderm (which will form the organs and surrounding connective tissue)
Define the term mesentery
In general, a fold of tissue that attaches organs to the body wall.
Mesenteries of the gut tube:
The thoracic esophagus and anus are anchored within the body wall and are therefore retroperitoneal
The stomach and liver are suspended in a mesentery that is attached to the dorsal AND ventral body walls:
- the dorsal mesentery of the stomach becomes the greater omentum
- the ventral mesentery of the liver becomes the falciform ligament
- the mesentery between the stomach and liver becomes the lesser omentum
The rest of the GI tract is suspended by a dorsal mesentery, named according to the organ to which it is attached (mesoduodenum, mesoappendix, mesocolon, etc.)
Some portions of the GI tract remain intraperitoneal in the adult and are therefore suspended by a mesentery.
Some portions of the GI tract, however, are applied against the body wall during development and the dorsal mesentery becomes incorporated into the body wall, making the organ secondarily retroperitoneal.
List the derivatives of the fore-, mid-, and hindgut regions. Describe their morphogenesis and summarize the role that gut mesodermal homeobox genes play in regional organ differentiation.
Regions of the gut:
- Pharynx
- Foregut
- esophagus
- stomach
- proximal duodenum
- liver, gall bladder (green), pancreas - Midgut (distal duodenum, jejunem, ileum, proximal colon)
- Hindgut (distal colon, anorectal canal)
Describe the formation of the liver in relation to the developing cardiac mesoderm.
The entire gut has the potential to form liver, but the expression of this potential is blocked except where cardiac mesoderm lies next to the gut. There, factors* produced by the cardiac mesoderm unblock the inhibition and instruct the gut cells to begin expressing liver-specific genes.
*Gut endoderm originally induced the development of the primary heart fields and now the cardiac mesoderm is inducing the development of the hepatic field (secondary herat field was created by the neural crest)
Describe the process of gut rotation and its consequences for gut development.
Intestine:
- The posterior loop starts to rotate counter-clockwise around the superior mesenteric.
- Dorsal mesogastrium rotates with it
- Complete rotation is 270 degrees which occurs progressively as that region develops.
- Rapid growth of the gut leads to temporary herniation into the umbilicus.
- 90 degrees of rotation occurs in the loop;
- The remaining 180 degrees occurs during return to the abdomen.
- Rapid growth of the gut leads to temporary herniation into the umbilicus.
Stomach
- During later development, the stomach will rotate, carrying the dorsal mesogastrium with it to form the omental bursa.
- Rotates 90 degrees
- Rotation of the dorsal mesogastrium carries the pancreas and spleen to the left side, with the pancreas becoming secondarily retroperitoneal as the mesogastrium presses it into the posterior abdominal wall
- Spleen goes with the omentum and not get pushed up against the body wall
Describe the vascular supply of the primitive gut tube. How does this arrangement compare to that in the adult?
Define physiological umbilical herniation and the timing and reason for its occurrence.
During the 10th week, herniated intestinal loops begin to return to the abdominal cavity.
Describe the embryological origin of the following defects: congenital umbilical hernia, left sided colon, Meckel’s diverticulum, gut stenosis, gastroschisis, and omphalocele.
- Congenital umbilical hernia: An umbilical hernia is a bulge of intraabdominal organs through an opening in at the base of the umbilicus (belly button). This occurs when abdominal muscles fail to come together forming an opening called an umbilical ring.
- Left sided colon: Rotation of the gut amounts to only 90 degrees and the colon/cecum are the first portions of the gut to return from the umbilitcal cord and they settle on the left side of the abdominal cavity. The returning loops then move more and more to the right, resulting in left-sided colon.
- Meckel’s diverticulum: Small portion of the vitelline duct persists, forming an outpocketing of the ileum
- Gut stenosis: Vascular “accidents” during development result in compromised blood flow and tissue necrosis in a section of the gut resulting in the defects.
- gastroschisis: Protrusion of the abdominal contents through the umbilicus directly into the amniotic cavity.
- Intestinal loops not protected by amnion so exposed to amniotic fluid which can compromise their development.
- omphalocele: Failure of intestinal loops to return to the abdominal cavity
- Intestinal loops still covered by amnion so their development is not affected
Describe the role of the urorectal septum in hindgut formation.
The formation of a urorectal septum divides the cloaca ventrally into urogenital sinus and dorsally into the rectoanal canal:
- urogenital sinus contributes to the lower urogenital tract:
- bladder (except trigone), urethra, and vagina in the female
- bladder (except trigone), prostate gland, and prostatic and membranous urethras in the male
- rectoanal canal: forms the rectum and upper anal canal
- urorectal septum: develops into the perineal body
The portion of the cloaca where the hindgut endoderm is up against the ectoderm of the skin breaks down to allow the formation of the anus.
