GI Embryology - development of the midgut and hindgut Flashcards
Name the blind diverticula of the caudal primitive gut tube.
Cloacal membrane.
List the midgut structures.
Duodenum (distal to the bile duct), small intestines, caecum, appendix, ascending colon and proximal 2/3 of the transverse colon.
What is the vitelline duct?
The tube that connects the midgut to the yolk sac
What is the primary intestinal loop?
When the midgut elongates massively, it forms a loop which has the SMA as its axis and is connected to the yolk sac by the vitelline duct.
It has cranial and caudal limbs (in relation to the SMA)
Describe the physiological herniation that happens in the 6th week of development.
The intestines herniate into the proximal part of the umbilical chord because the primary intestinal loop is elongating rapidly and the liver is taking up so much of the abdominal space.
If you took a section through the proximal umbilicus during the 6th week of development what structures would you see?
Covered by the amnion. Umbilical arteries Umbilical vein Loop of intestines Allantois (sac-like structure that helps the embryo exchange gases and handle liquid waste)
How many 90 degrees midgut rotations occur?
3
When does the first 90 degrees of the midgut occur?
During herniation of the primary intestinal loop into the proximal umbilicus.
What happens to the cranial limb of the proximal intestinal loop after the first 90 degrees rotation?
It becomes very convoluted.
What happens during the second 90 degrees rotation of the primary intestinal loop?
The cranial and caudal limbs cross. Loops of jejunum and ileum are pushed to the left hand side. The cecal swelling appears.
What are the position of the cranial limb and distal limb of the primary intestinal loop to each other before any rotations occur?
Cranial limb is directly cranial to the caudal limb
What are the position of the cranial and caudal limb of the primary intestinal loop to each other after the first rotation?
The cranial limb is directly to the right of the caudal limb.
After the second rotation of the primary intestinal loop what is the relation of the cranial and caudal limb to each other?
The cranial limb is caudal to the caudal limb! An exact reverse has occurred due to the 180 degrees rotation
What is the effect of the third rotation?
The small and large intestines are now in their correct anatomical positions and back inside the abdomen.
What happens to the cecal bud/ caecum after the third rotation?
It descends
What are the derivatives of the cranial limb of the primary intestinal loop?
Distal duodenum, jejunum and proximal ileum
What are the derivatives of the caudal limb of the primary intestinal loop?
The distal ileum, cecum, appendix, ascending colon and proximal 2/3 of transverse colon.
What is the correct direction of rotation for the midgut?
Anti-clockwise
What are the two types of malrotation of the midgut that can occur?
- Incomplete rotation
2. Reversed rotation
What occurs and what are the consequences of incomplete rotation of the midgut?
The midgut rotates only 90 degrees anti-clockwise.
Crossing of the limbs of the loop doesn’t occur (second rotation), therefore the caudal limb remains at the left hands side and you get a colon that is fully on the left (ascending, transverse and descending colon).
What happens during reverse rotation and what are the consequences?
The midgut makes one 90 degrees rotation clockwise (instead of anti-clockwise).
The transverse colon passes posterior to the duodenum, instead of anterior.
What is the major complication of midgut defects?
Volvulus - when a loop of bowel is twisted around a focal point (where the mesentery attaches to the intestinal tract)
What are the major complications of a volvulus?
Strangulation and therefore ischaemia (if blood supply to that bowel section compromised).
GI tract obstruction - constriction of lumen
Name three abnormalities that can occur if the vitelline duct persists after development
- Vitelline cyst
- Vitelline fistula
- Meckel’s diverticulum
What is a vitelline cyst?
When the midsection of the vitelline duct remains patent, with fibrous sections connecting it to the abdominal wall and bowel loops. This is at risk of volvulus.
What is a Vitelline fistula?
When the vitelline duct remains patent. This means direct communication between the intestines and umbilicus remains and intestinal contents can leak out of the umbilicus.
What is Meckel’s diverticulum?
It is the most common GI abnormality. A blind ended tube remains projecting out from the intestines, due to incomplete closure of the vitelline duct. The diverticulum can/ cannot have fibrous strands that connect it to the anterior abdominal wall
State the rule of 2s for Meckel’s diverticulum
2: % of the population
2: feet from the ileocecal valve
2: inches long
2: usually detected (sometimes asymtomatic) in under 2’s (as in all congenital abnormalities)
2: 1 male:female ratio
What can be the clinical consequence of Meckel’s diverticulum?
Can be asymptomatic or can cause inflammation and infection.
What atypical tissue can be found in Meckel’s diverticulum?
Ectopic pancreatic or gastric tissue
In which gut structures can growth of tissue be so rapid that the lumen is obliterated?
Oesophagus, bile duct and small intestine
What occurs to restore obliterated GI lumens later in development?
Recanalisation
What can be the consequences if recanalisation is partially or wholly unsuccessful?
Atresia - lumen obliterated
Stenosis - narrowing of lumen
Of the structure
Where is the most common location for stenosis and atresia?
The duodenum
What are the reason for stenosis and atresia in the duodenum?
Most likely cause is incomplete canalisation, but “vascular accidents” can also happen - impairing the blood supply so recanalisation doesn’t occur?
Pyloric stenosis is a common abnormality of the stomach in infants. What is a sign of PS?
Projectile vomiting
Why is projectile vomiting characteristic of pylorus stenosis?
Movement of stomach contents into the duodenum is slowed -> stomach dis tends -> projectile vomiting.
What is the cause of pylorus stenosis?
Hypertrophy of the circular muscle in the pyloric sphincter. NOT a recanalisation failure.
Put the following structure of the GI tract in order of which has the highest -> lowest incidence of atresia and stenoses:
Ileum/jejunum
Colon
Duodenum
Duodenum > ileum/ jejunum > colon
What is the most common cause of upper duodenum atresias?
Failure of recanalisation.
What is the most likely cause of lower duodenum atresias?
Often vascular accidents - caused by malrotation, volvulus, body wall defects (occasionally e.g. Umbilical hernia).
List 4 defects of the anterior abdominal wall
- Gastroschisis
- Omphalocoele
- Umbilical hernia
- Inguinal hernia
What causes Gastroschisis (“split-stomach”)
Failure of closure of the abdominal wall during folding of the embryo. This leaves the gut tube and derivative outside the body cavity.
Most cases of this are picked up on the 20 week abdominal scan of the embryo. Prognosis is good as long as bowel loops are healthy (they can be damaged by moving around).
What causes omphalocoele?
Persistence of physiological herniation.
What can omphalocoele get confused with, and how does it differ?
It can get confused with an umbilical hernia. Unlike an umbilical hernia an omphalocoele is not covered in a layer of skin and subcutaneous tissue.
Also an umbilical hernia will normally resolve itself over time, whereas an omphalocoele is normally a sign of a more complex congenital abnormality.
What structures does the hindgut give rise to?
Distal 1/3 of transverse colon, descending colon, rectum, upper anal canal and urinary epithelium of bladder and urethra.
The anal canal is divided into superior and inferior parts by the pectinate line. The two parts are histologically different. Describe some other differences.
Differences in:
Arterial and venous supply
Lymphatic drainage
Innervation
What is the cloaca?
A blind diverticulum at the far end of the hind gut.
What separates the cloaca from the outside of the embryo?
The cloacal membrane
What type of partitioning does the cloacal undergo?
Anteroposterior subdivision
In the sixth week a wedge of mesoderm begins to grow (finishes in the 7th week) which partitions the cloaca into what two structures?
Anterior - urogenital sinus
Posterior - anorectal canal
As the mesoderm grows in the hindgut what depression is formed?
Anal pit
Name the blind diverticula of the cranial primitive gut tube.
Buccopharyngeal membrane.
What is the function of the allantois and where is it found?
A hindgut sac-like structure, which is one of the embryonic membranes along with the yolks and amniotic sac.
The function of the allantois is to collect liquid waste from the embryo, as well as to exchange gases used by the embryo
What is the perineal body?
or central tendon of perineum) is a pyramidal fibromuscular mass in the middle line of the perineum at the junction between the urogenital triangle and the anal triangle. It is found in both males and females. In males, it is found between the bulb of penis and the anus; in females, is found between the vagina and anus, and about 1.25 cm in front of the latter.
What is the periumeum?
The area between the anus and the scrotum in the male and between the anus and the vulva (the labial opening to the vagina) in the female.
Why does ectoderm form part of the anal canal?
Because before the epithelial plug ruptures there is a depression formed called the anal pit, where ectoderm invaginates inside what will soon be the anal canal.
What is the blood supply to the anal canal above and below the pectinate line?
Above - IMA (inferior mesenteric artery)
Below - Pudenal artery
What is the “white” line?
Where the non-keratinised stratified squamous epithelium becomes continuous with the keratinised stratifed squamous epithelium of the perinanal skin
What is the nerve supply above and below the pectinate line?
Above - S2,3&4 pelvic parasympathetics
Below - S2,3&4 pudenal nerve
What type of epithelium is found above and below the pectinate line?
Above - simple columnar epithelium
Below - non-keratinised stratified squamous epithelium
What lymph drainage vessels are found above and below the pectinate line?
Above - internal iliac nodes
Below - superficial inguinal nodes
Where is stratified columnar epithelium found?
Rare type of epithelium, found in ocular conjunctiva of the eye parts of the pharynx and anus the female's uterus male urethra and vas deferens Lobar ducts in salivary glands.
What type of embryonic tissue contributes to the anal canal above and below the pectinate line?
Above - endoderm
Below - ectoderm
What type of sensation can be felt in the anal canal above the pectinate line?
Above - only stretch
Below - temperature, touch and pain
How does the localisation of visceral pain differ from that of somatic pain?
Visceral pain is more poorly localised
Where does visceral pain of the foregut and its derivatives localise?
epigastrium
Where does visceral pain of the midgut localise?
periumbilical
Where does visceral pain of the hindgut localise?
suprapubic
What type of innervation does the parietal peritoneum receive?
Somatic innervation
Why does an appendicitus initially cause periumbilical pain which later on localised to the right iliac region?
Initially the inflamation of the appendix stimulates the visceral nocireceptors which are innervated by midgut derived nerves and therefore localise to the periumbilical region. Later on the inflammation spreads so that it is stimulating nocireceptors in the parietal peritoneum near the appendix. The parietal peritoneum is innervated by somatic nerves which localies pain more accurately, therefore pain is felt close to the appendix at the right iliac region
List 3 hindgut abnormalities
- Imperforate anus
- Anal/ anorecatal agenesis
- Hindgut fistulae
What is the hingut abnormality, imperforate anus?
A failure of the anal membrane to perforate causing a persistent blind diverticula at the caudal end of the hindgut (anal canal)
What is anal/ anorectal agenesis?
Failure for the anus or anus and rectum to develop entirely
What is a fistula?
A fistula is an abnormal or surgically made passage between a hollow or tubular organ and the body surface, or between two hollow or tubular organs.
What is a hindgut fistulae?
An abnormal passage between hindgut structures: vagina, urethra, rectum…
Which midgut and hindgut mesenteries are retained?
jejunum ileum appendix transverse colon sigmoid colon
Which midgut and hindgut mesenteries become fused?
Duodenum
Ascending colon
descending colon
rectum (no peritoneal covering in distal 1/3)
What are the names of the structures that the dorsal mesentery of the gut become?
greater omentum gastrolienal ligament - stomach to spleen Lienorenal ligament - spleen to kidney mesocolon mesentery proper
What are the names of the structures that the ventral mesentery of the gut become?
lesser omentum - foregut to liver
Falciform ligament - liver to ventral body wall
Define mesocolon
A mesentery joining the colon to the dorsal abdominal wall
Which structure does the greater omentum attach to?
It is attached to the greater curvature of the stomach up to the beginning of the duodenum. It then covers the anterior surface of the transverse colon and small intestines and then reflects on itself to go up posterior to the transverse colon and attach to the posterior abdominal wall
Which structures does the lesser omentum attach to?
Liver to lesser curvature of stomach
Liver to first part of the duodenum
Which structures does the falciform ligament attach to?
liver to ventral body wall
Which structures does the lienorenal ligament attach to?
Spleen to kidney
Which structures does the gastrolienal ligament attach to?
Stomach to spleen
What is the name of the ligament that is part of the lesser omentum that attaches the liver to the lesser curvature of the stomach?
Hepatogastric ligament
What is the name of the ligament that is part of the lesser omentum that attaches the liver to the first part of the duodenum?
Hepatoduodenal ligament
What is the left border of the greater omentum continuous with?
Gastrosplenic/ gastrolienal ligament
How many layers make up the greater omentum?
4 - peritoneum is 2 layers and the omentum has a fold where it reflect
What is meant by the mesentery proper?
Loops of mesentery found at the jejunum and ileum
Which arteries and veins supply the blood to the midgut?
SMA and SMV
Which arteries and veins supply the blood to the hingut?
IMA and IMV
How is the midgut innervated?
Parasympathetic - vagus nerve
Sympathetic - superior mesenteric ganglion and plexus
How is the hindgut innervated?
Parasympathetic - pelvic (S2,3&4)
Sympathetic - inferior mesenteric ganglion and plexus
What developmental defect involving the anterior abdominal wall can lead to urine leaking out of the umbilicus?
Patent urachus - the urachus connect the allantois to the dome of the bladder during fetal development. It should obliterate after the first trimester. If it remains patent then urine can be transferred from the bladder to the umbilicus and leak out.
What developmental defect involving the anterior abdominal wall can lead to meconium leaking out of the umbilicus?
Patent vitellointestinal duct - would allow the transfer of new-born faeces (meconium) from the small intestine to the umbilicus.
What developmental defect can cause abdominal contents that are not covered in peritoneum to protrude through the abdominal wall?
Gastrochisis
What developmental defect can cause abdominal contents to herniate into the umbilical chord (contents covered by peritoneum)?
Exomphalos
