Development of the GI Tract Flashcards
In what week is the primitive gut tube formed in embryo
4th-5th weeks
State the derivatives of the foregut
- Stomach
- Liver
- Pancreas
- Oesophagus
- Proximal duodenum (proximal to sphincter of oddi)
State the derivatives of the midgut
- Distal duodenum
- Jejunum
- Ileum
- Caecum
- Ascending colon
- Proximal 2/3 of transverse colon
State the derivatives of the hindgut
- Distal 1/3 of transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Upper anal cana
- Lining of bladder and urethra
State the blood supply to the foregut, midgut and hindgut structures
- Foregut - celiac trunk
- Midgut - superior mesenteric artery
- Hindgut - inferior mesenteric artery
Describe how the coelomic cavity and peritoneal cavity develop
- Intraembryonic coelom formed as the embryo folds
- Begins as one large cavity – all cavity have same membrane
- Later subdivided by the future diaphragm into abdominal and thoracic cavities
- One membrane lining the whole intraembryonic cavity
- Includes specialized pericardium and pleural membrane
- Peritoneum and peritoneal cavity
- Peritoneal membrane lines the abdominal cavity and invests the viscera
- During development it grows, changes shape and specialises
- Peritoneal cavity is a potential space only
- Normally contains nothing other than serous fluid
- Peritoneal membrane lines the abdominal cavity and invests the viscera
Define mesentery
Double layer of peritoneum suspending the gut tube from the abdominal wall
Describe the regions of the gut tube where dorsal and ventral mesenteries are found
- Dorsal mesentery suspends the entire gut tube from the dorsal body wall
- Ventral mesentery only in the region of the foregut
Describe how the greater and lesser sacs are formed
- Dorsal and ventral mesenteries in the region of the foregut divide the cavity into left and right sacs only in the foregut
- Left sac contributes to the greater sac
- Right sac becomes the lesser sac - lies behind the stomach
- Caudal to the foregut, only the greater sac present
- Greater and lesser peritoneal sacs connected through epiploic foramen
- Formed from the rotation of the stomach during development
Define omentum
Double layer of peritoneum that connects the stomach with other abdominal organs
Describe how the greater and lesser omentum are formed
- Greater omentum formed from the dorsal mesentery
- First structure seen when abdominal cavity is opened anteriorly
- Lesser omentum formed from the ventral mesentery
- Free edge conducts the portal triad
- Goes from liver to lesser curve of stomach to duodenum
- Free edge conducts the portal triad
- Formed due to the rotation of the stomach during development
List the peirtoneal structures within the body
- Possess mesenteries
- Stomach, jejunum, ileum, appendix, transverse colon, sigmoid colon, rectum
Describe retroperitoneal structures and give examples
- Were never in the peritoneal cavity and never had a mesentery
- Eg. Kidney, aorta, inferior vena cava
Describe secondarily retroperitoneal structures
- Had a mesentery but with successive growth and development, mesentery is lost through fusion at posterior abdominal wall
- Eg. Pancreas, duodenum, descending colon, ascending colon
What is fusion fascia
- Movement of structure causes it to lose mesentery and pushed against abdominal wall
- Previous peritoneal space becomes fusion fascia (secondary retroperitoneal structures)
Describe the basic development of the foregut structures (stomach, liver, pancreas, duodenum)
- Liver and biliary system developed from ventral mesentery
- Pancreas developed from both dorsal and ventral mesentery (has fusion fascia)
- Duodenum
- Shape determined by rotation of the stomach
- Rotation of the stomach pushes duodenum to right, then against posterior abdominal wall
- Becomes secondarily retroperitoneal (has fusion fascia)
- Stomach
- Mid week 4, slight dilation of distal foregut indicates position of stomach primordium
- Faster growth of dorsal border creates the greater curvature
- Rotates, changing the position of the dorsal and ventral mesenteries
Around what point is the axis for the midgut loop rotation
Superior mesenteric artery
Describe the physiological herniation that occurs in GI development
- During the 6th week, growth of the primary intestinal loop is very rapid
- Liver is also growing rapidly - abdominal cavity is too small to accommodate both
- Intestine herniate into the umbilical cord during first rotation
State the direction and number of rotations of the GI tract
- Rotation involves 3 counter clockwise 90˚ turns
- Cranial derivatives end up on the left and caudal derivatives end up on the right
State the importance of the pectinate line in development
- Anal canal is divided by the pectinate line into superior and inferior parts
- Differences in arterial supply, venous and lymph drainage, and innervation
- Proctodeum - junction between two embryonic germ layers
Describe the differences in blood supply, lymph drainage, embryonic germ layer and innervation above and below the pectinate line
- Above pectinate line
- Blood supply from inferior mesenteric artery
- S2-S4 pelvic parasympathetics
- Columnar epithelium
- Lymph drainage to internal iliac nodes
- Made from endoderm - visceral
- Can only sense stretch
- Below pectinate line
- Blood supply from pudendal artery
- S2-S4 pudendal nerve
- Stratified epithelium
- Lymph drainage to superficial inguinal nodes
- Made from ectoderm - somatic
- Can feel pain, temperature, touch
Describe the appearance of GI tract due to malrotation
- Incomplete rotation
- Midgut loop makes only one 90˚ rotation
- Left-sided colon
- Reserved rotation
- Midgut loop makes one 90˚ rotation clockwise
- Transverse colon passes posterior to the duodenum
Describe the consequences of a volvulus in the GI tract
- Obstruction in the intestines due to twisting or knotting of the GI tract
- Structures that should not be mobile become mobile
- Leads to strangulation and ischaemia
- In adulthood, presents with bloated abdomen, constipation
Describe the abnormalities that can happen due to vitelline duct remnants
- Vitelline cyst - vitelline duct forms fibrous strands
- Connect stomach to anterior abdominal wall
- Vitelline fistula
- Direct communication between umbilicus and intestinal tract
- Meckel’s diverticulum
- Bulge in the small intestine due to remnant of vitelline duct which can cause volvulus
Outline the effects of recanlisation in the GI tract
- Happens in oesophagus, bile duct, small intestine
- In some gut structures, cell growth becomes so rapid that the lumen is partially or completely obliterated
- Recanalisation acts to restore the lumen
- If recanalisation is wholly or partially unsuccessful, atresia or stenosis of the structure can occur
- Most occur in duodenum
Describe the characteristics of pyloric stenosis
- Hypertrophy of the circular muscle in the region of the pyloric sphincter
- Not a recanalistion failure
- In infants, narrowing of the exit from the stomach causes characteristic projectile vomiting
- Vomit does not contain bile (not bilious)
Differentiate between gastroschisis and omphalacoele
- Gastroschisis
- Failure of closure of the abdominal wall during folding of the embryo
- Leaves gut tube and derivatives outside the body cavity
- Omphalacoele
- Persistence of physiological herniation
- Differs from umbilical hernia because hernias have covering of skin and subcut tissue
- Associated with other defects, including chromosomal defects
- Less serious than gastroschisis as peritoneum present over gut
What hindgut abnormalities can occur
- Imperforate anus
- Failure of anal membrane (cloacal membrane) to rupture
- Anal/anorectal agenesis
- Hindgut fistulae
