Development of the GI system Flashcards
what occurs in the fourth week in the primordia GI tract?
Embryonic folding pulls the endoderm of the yolk sac into the cranial and caudal ends of the embryo forming the foregut, midgut and hundgut with the narrow opening to yolk sac forming the vitelline duct (yolk stalk)
how is the gut tube closed on either end?
The gut tube is closed on both ends by the buccopharyngeal (cranial) and cloacal membranes (caudal)
at both these locations the endoderm and ectoderm are in direct contact with each other
in the primitive gut, what makes up the foregut
Includes the oral cavity, pharynx, lower respiratory tract, esophagus, stomach, liver, gall bladder, pancreas, upper duodenum,
with all those being below the diaphram they are supplied by the celiac trunk
in the primitive gut, what makes up the midgut?
incudes the lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, and proximal two thirds of transverse colon,
all being supplied by the superior mesenteric artery (SMA)
in the primitive gut, what makes up the hindgut?
Hindgut includes the distal one third of transverse colon, descending colon, sigmoid colon, rectum, and upper anal canal
-all being supplied by the inferior mesenteric artery
what are the germ line derivitives of the GI tract wall? when is it matured?
The endoderm forms the epithelial lining and associated glands of the GI tract
Splanchnic mesoderm generates connective tissue, vasuculature, and smooth muscle walls
ectoderm forms the enteric ganglia, nerves, and glia of the GI tract (via neural crest cells) as well as the epithelium of the lower 1/3 anus
all the GI gastroepithelium is well underway by 8-10 weeks and peristalic contractions begin as early as week 10
development of the dorsal and ventral mesenteries
By the end of the 4th week, the gut tube caudal to the developing diaphragm is suspended from the posterior abdominal wall by a dorsal mesentary
this mesentary extends from the lower esophagus to the caudal end of the hindgut
adult derivitives include:
- greater omentum
- small intestinal mesentary
- transverse mesocolon
- and sigmoid mesocolon
A ventral mesentary develops between the ventral body wall and the foregut running fro, the septum transversum and stomach to the umbillicus
adult derivitives include:
- lesser omentum
- falciform ligament
- coronary ligament
- and triangular ligaments of the liver
difference between intraperitoneal and retroperitoneal organs
Intraperitoneal organs are suspended in the mesentary
retroperitoneal organs are those organs exculded from the peritoneal cavity
secondarily retroperitoneal organs were initially suspended within the cavity but after in development become fused to the body wall
- includes the ascending and descending colon
- much of the pancreas
what happens in stomach development and rotation
While suspended by both the ventral and dorsal mesentaries, the developing stomach elongates and rotates 90 degrees
- ventral border stomach moves to the right and dorsal border of stomach moves to the left
- left stomach surface becomes ventral and right side becomes dorsal
the portion of the dorsal mesentary between the body wall and the stomach is referred to as the dorsal megogastrium
as the stomach enlarges and rotates the dorsal mesogastrium enlarges forming a large draping double fold mesentary that eventually anteriorly overlies the lower abdomen contents
-forming the greater omentum
this rotation also seperates the abdominal cavity into a smaller lesser sac lying behind the stomach and a greater sac lying anteriorly and continuous with the greater abdominal cavity
rotation also changes the orientation of the right and left vagus nerves to one consisting of anterior and posterior trunks
Defects in the stomach development: Hypertrophic pyloric stenosis
Narrowing of pyloric lumen thereby obstructing food passage
-inabillity for the sphincter to relax due to faulty neural crest cell migration so ganglion cells of enteric nervous system are not properly populated
Muscularis externia in the region hypertrophies forming a palpapal mass at the right costal margin
condition is associated with projectile non-bilious vomiting after feeding, fewer and smaller stools and failure to gain weight
process of liver formation
Begins as diverticulum from gut endoderm that grows into the septum transversum
Endoderm differentiates into hepatocytes, bile ducts, and hepatic ducts and bile formation by hepatic cells begins at week 12
splanchnic mesoderm differentiates into stromal cells, kupffer and stellate cells
liver also functions as an embryonic hematopoietic organ
Congenital anomalies of the liver are very rare
Process of the pancreas formation
Endodermal buds sprout into dorsal and ventral mesentaries
Cystic diverticulum and ventral pancreatic bud grow into the ventral mesentary where as the dorsal pancreatic bud grows into dorsal mesentary
Pancreatic buds develop both exocrine and endocrine portions
During the 5th week the ventral pancreas migrates around posteriorly and fuses with dorsal pancreas
- dorsal pancreas forms head, body, and tail
- ventral pancreas forms uncinate process
the dorsal pancreatic duct connection is lost as it reconnects to the ventral pancreatic duct
-but can be retained as an accessory pancreatic duct
Pancreatic anomalies
Accessory pancreatic duct
Pancreas divisum: having two totally separate ducts; patients prone to pancreatitis
Annular pancreas:
-duodenal obstruction or stenosis can occur with annular pancreas, bilious vomiting can occur if the annulus develops inferior to bile duct
development of the gall bladder and the bile ducts
Formed by outgrowths from cystic endoderm
bile duct is initially closed but opens via canalization
what is biliary atresia
defined as obliteran of extrahepatic and or intrahepatic ducts
ducts are replaced by fibrotic tissues due to acute or chronic inflammation
- symptoms include progressive neonatal jaundice with onset soon after birth, white clay stools, and dark colored urine
- 12-19 month survival time but can be treated with a transplant of the liver
development of the spleen
During the 4th week, a mesenchymal condensation develops in the dorsal mesogastrum where the site of the spleen formation
spleen is derived from mesoderm and will be eventually populated by myeloid lymphoid cells
Mudgut formation: gut rotation
Because of the limited size of the abdomen, the lengthening midgut loop must hernia out into the umbilicus (out at 6 weeks and brought back at 10 weeks)
First there is a 90 degreee counter clockwise rotation bringing the cecum back into the left side of the abdominal cavity
then there is a 180 degree counterclockwise rotation of the gut loop as it continues reentering the abdomen
-this brings the cecum to upper right quadrant and the ascending colon to lie anterior to duodenum
as the last rotation occurs bringing the rest of the midgut into the cavity, the cecum descends to the lower right quadrant carrying the ascending colon along with it so the ascending colon ends up on the right side of the abdomen and the transverse colon overlies the duodenum and the pancreas
what is omphalocele?
Herniation of the gut into the umbilicus remains and is still covered by peritoneum
- increased risk with trisomy 13 and 18
- herniated bowel does not fully retract
- lateral body folding and fusion is abnormal creating aa wall weakness allowing the bowel to herniate
- CT of the skin and hypaxial musculature of the body wall do not form normally thereby creating a wall weakness
what is Gastroschisis
herniation of abdominal contents through abdominal wall into the exterior
there is no peritoneal covering
Meckels diverticulum
Failure of the yolk stalk connection to the mid gut to regress so the midgut rmains connected to the umbilicus
- lead to abdominal swellings
- intestinal obstruction
- bowel sepsis
- and GI bleeding
Rule of 2s
- 2% incidence
- 2x more common in males
- 2% have medical symptoms
- 2 feet proximal to terminal ileum
- 2 inches long
- 2 years of age
what is volvulus?
rolled up twisted intestines
symptoms include acute abdominal pain, vomiting and GI bleeding
increased risk with gut rotation
what is a non-rotation of midgut loop?
completes first 90 degree counterclockwise rotation but does not do the 18- counterclockwise rotation
ends up with a left sided colon and a right sided small intestines
what is a reverse gut rotation?
Completes the initial 90 degree counterclockwise rotation but then does a 180 rotation in the colockwise direction
now the transverse colon lies posterior to the duodenum
risk of ischemia and stenosis of the colon
what is intestinal stenosis and atresia?
during the 5-6th month the lumen of the intestines becomes nearly completely obliterated due to endodermal proliferation and villi formation
with development the lumen is normally reopened but intestinal stenosis ad atresia is commonly associated with trisomy 21
Duodenal stenosis: partial occlusion due to ineffective recanalization
-usually involves the horizontal and or ascending part
duodenal atresia: complete occlusion of the lumen remains
Hindgut and separation of the cloaca
Urorectal septum partitions cloaca into a dosral anorectal canal and a ventral urogenital sinus
septum develops fork like extensions of the lateral cloacal walls that grow toward one another dividing the cloaca into urogenital and anal canal
Cloacal membrane ruptures thereby opening both the urogenital sinus and anal canal to the exterior
formation of the anal pt and the pectinal line
Anal rectal lumen is temporally closed by epithelial anal plug
mesenchyme surrounding the anal canal proliferates forming an anal pit with ectodermal lined walls
anal plug eventually opens via apoptotic death
the anal canal then consists of a superior 2/3 endodermal epithelium seperate BV and innervation
and lower 1/3 ectodermal epithelium with separate BV and innervation
the pectineal line is the line dividing the origin of hindgut and anal pit
what is an imperforated anus?
persistent anal membrane with various types that are distinguished on whether they are low, intermediate or high relative to the levator ani and pelvic landmarks
Hirchsprungs disease
absence of ganglionic plexus due to absence of neural crest derived cells
lack peristalsis
increase wall thickness due to hypertrophy in the intestines proximal to the aganglionic segment
abnormal colonic dilation or distension
treated with surgical removal of constricted aganglion distal segment
Abnormal urorectal septum
Rectovaginal, rectovestibular, rectoperineal
rectovesical, rectourethral, rectoperineal
