Development and congenital abnormalities of the GI tract

Question 1

When does the primitive gut start to form?

Answer 1

Week 4 during the cephalocaudal and lateral folding

Question 2

What structures are at the top and end of the gut system

Answer 2

THe buccopharyngeal membrane is the top

The cloaca is the bottom

Question 3

What are the three derivatives of the primitive gut

Answer 3

Foregut

Midgut

Hindgut

Question 4

What structures make up the foregut

Answer 4

Oeosphagus
Trachea and lung buds
Stomach
Duodenem up to entrance of bile duct
Liver, biliary system, pancreas

Question 5

What structures make up the midgut

Answer 5

Duodenum distal to opening of bile duct
rest of small intestine
Caecum and appendix
Ascending colon
Proximal 2/3 of transverse colon

Question 6

What strucutres make up the hindgut

Answer 6

Distal 1/3 of the transverse colon
Descending colon
SIgmoid colon
Rectum and superior part of anal canal
Epithelium of the urinar bladder and most of the urethra

Question 7

Outline the formation of oesophagus, including how the foregut divides into a respiratory and gut division

Answer 7

In week 4 = A respiratory diverticulum (or lung bud) appears on ventral wall of foregut.

Therefore at this point = Foregut is divided into a central respiratory primordium and a dorsal oesophagus.

The oesophagus then lengthens as the heart and lungs descend.

Question 8

Describe how the muscle coat of the oesophagus changes with differing nerve supply

Answer 8

Upper 2/3 = Striated muscle with vagus nerve

Lower 1/3 = Smooth muscle with splanchnic nerves

Question 9

The three main germ layers that help contribute to the gut are; Endoderm, splanchnic mesoderm, ectoderm. Describe how each of these contirbute

Answer 9

Endoderm = Epithelium of GI tract, parenchyma of the viscera

Splanchnic mesoderm = Connective tissue, muscular components, peritoneal components

Ectoderm = Distal part of the anal canal

Question 10

Describe the arterial supply to each portion of the primitive gut

Answer 10

Foregut = Branches of the coeliac trunk

Midgut = Branches of superior mesenteric artery

Hindgut = Branches of the inferior mesenteric artery

Question 11

Outline when and how the stomach is formed, including the rotation and how the lesser/greater curvatures are formed

Answer 11

Starts to form in week 4 = As a fusiform dilation of the foregut. It undergoes 2 types of rotation to get into final position

Longitudinal rotation = This means rotation in a clockwise fashion (if looking down on stomach from above). The lesser curvature of the stomach is originally on the antrior surface, with greater curvature being posterior. The rotation is 90 degrees. So that the original left side now faces anterior, and original right side faces posterior

Anteroposterior rotation = So that the cephalic and caudal ends of the stomach are no longer in the midline. The caudal/pyloric part moves to the right, and the cephalic/cardiac portion moves to the left and slightly down.

The original posterior wall (now on left) grows faster then the anterior portion (now on right) = This means that you get the greater and lesser curvatures forming.

Question 12

Outline the formation of the mesenteries of the stomach and how it is affected by the rotation

Answer 12

The stomach is suspended by the embryological equivalent of mesentery = Known as mesogastrium

There is both ventral and dorsal mesogastrium.
As the stomach rotates, these rotate also

Dorsal mesogastrium = Is pulled to the left. This creates a space behind the stomach called the omental bursa (less peritoneal sac).
The dorsal mesogastrium continues to grow to gorm the greater omentum

Ventral mesogastrium = Pulled to the right and forms the lesser omentum which passes from liver to lesser curvature of the stomach. It also forms the falciform ligament which runs from liver to ventral abdo wall

Question 13

Describe the course of the lesser omentum

Answer 13

From liver to stomach

Question 14

Describe course of falciform ligament

Answer 14

From liver to ventral abdo wall

Question 15

Describe course of greater omentum

Answer 15

This runs from stomach to dorsal abdo wall. It hangs like a drape of mesogastrium.

It actually drapes over the transverse colon. And later on in development it fuses with the mesentery of the transverse colon.

Question 16

Outline for the formation of the duodenum including how it is affected by rotation of the stomach, and its vasculature

Answer 16

Also starts developing in the 4th week. Remember it forms from both the terminal part of the foregut and proximal midgut. Therefore supplied by both coeliac and SMA

As the stomach rotates = The duodenum take son a C shaped loop. It rotates to the right and becomes retroperitoneal. But the duodenal cap remains intraperitoneal.

Question 17

Outline the formation of the liver and biliary apparatus

Answer 17

There is an ventral outgrowing of the distal foregut = Known as the hepatic diverticulum or (Liver bud)

Liver bud = Penetrates through septum transversum. It goes on to form the liver here. The bile duct forms from the connection between the hepatic diverticulum/liver bud and the foregut (duodenum). This connections subsequently narrows and forms the bile duct.

Gallbladder = The bile duct develops a small ventral outgrowth that becomes the gallbaldder and cystic duct.

Liver = Grows rapidly in size, and by week 9 is about 10% of body weight. Initially both lobes are same size, but right lobe then gets bigger.

Question 18

What is the septum transversum and how is it located in relation to the liver, and the central tendon of diaphragm

Answer 18

The septum transversum gives rise to parts of the thoracic diaphragm.

Cranial part of the septum transversum = Forms into the central tendon of the diaphragm.

Question 19

When does haematopoiesis begin

And bile formation

Answer 19

Haematopoiesis begins by week 6

Bile formation by the heaptic cells = Begins by week 12.

Question 20

List the various origins of the hepatic cells and sinusoids.

Answer 20

Mesoderm of the septum transversum = 1) Haemapoeitc cells

2) Kupffer cells
3) Connective tissue cells

Liver cords =
1) Parenchyma (liver cells) lining the biliary ducts

Intermingling of epithelial cords with vitelline and umbilicla veins =
1) Hepatic sinusoids.

Question 21

Outline the development of the pancreas

Answer 21

The ventral and dorsal pancreatic buds = Origintate from the endodermal lining the duodenum.

These grow in size and also undergo rotation

Pancreatic duct together with bile duct drain into duodenaum at the major papulla

Accessory duct (where present) enters duodenum through minor papilla which is about 2cm cranial to the major papilla.

In 10% of cases the duct system fails to fuse and you get a doublee system

Question 22

Name the 4 origins of the pancreatic system and what their final structures are

Answer 22

Ventral pancreatic bud = Uncinate and inferior part of the pancreatic head

Dorsal pancreatic bud = Entire pancreas except the above

Ventral pancreatic duct and distal part of dorsal pancreatic duct = Main pancreatic duct

Proximal part of dorsal pancreatic duct = Accessory pancreatic duct (if present)

Question 23

When does insulin production start?

Answer 23

At about 5 months or 35 weeks in

Question 24

Outline the development of the spleen

Answer 24

Starts to develop at week 5

Derived from a mass of mesenchymal cells between the layers of the dorsal mesogastrium

Function = In the foetus works as a haemopoietic centre

Question 25

Name the 2 origins of the pancreatic cells and connective tissues, and what cells/tissues they form

Answer 25

Parenchymatous pancreatic tissue 1) Pancreatic islets (of langerhans) 2Glucagon and somatostatin secreting cells Splanchnic mesoderm sorrounding the pancreatic buds = 1) forms the connective tissues

Question 26

What is the significant of the vitelline duct and the midgut

Answer 26

The vitelline duct connects the central part of the midgut to the yolk sac. Normally it fully obliterates at 5-6 weeks. But its persistance is a vitelline fistula which causes dishcarge of meconium from the umbilicus. Some vitelline fistulas = Can have some of the vitelline duct retained in the midgut and this becomes meckles diverticulum.

Question 27

Describe the process of physiological umbilical hernation and subsequently how the herniated loops retract back into the abdo cavity

Answer 27

During week 6 = The primary loop elongates rapidly. Along with the quickly rowing liver the abdo cavity is too small so loops enter the extraembryonic cavity via the umbilical cord During week 10 = Mesonephric kidneys regresses, liver slows, abdo cavity enalrges, so that loops can return. First to re-enter is the proximal part of jejunum that lies on the left side. The other loops enter on the right.

Question 28

Outline the rotation of the midgut, including the angles

Answer 28

In total the midgut rotates about 270 degrees anticlockwise. It does this around the SMA which acts as an axis. Remember the SMA is what supplies the midgut, which is what is rotating Initial 90 degree rotation = This happens first while the middgut is still herniated out. This means that it is like you are pulling the gut out of umbilicus and rotating it 90degrees anticlockwise the way you are facing it. Remaining 180 degree rotation = Then the physiological hernia re-enters, and once it has done this is rotates the rest of the 180 degrees.

Question 29

Describe the caudal and cranial limbs of the physiological umbilical hernia, and what they go on to form

Answer 29

Cranial limb = This is the proximal part of the gut tube that herniates out. The bit that is going out to the hernia This forms = distal duodeneum, jejunum , and part of ileum Caudal limb = This is the distal part of the herniating limb. The bit that is returning back from the hernia to the abdo. This forms = The rest of the ileum, the ascending colon, and the proximal 2/3 of the transverse colon

Question 30

Outline the timeline and out the caecum and appendix form

Answer 30

At 6 week = The caudal limb of the primary instestinal loop (that has herniated) forms a caecal bud. This bud is the last part to then re-enter the abdomen. First lies in RUQ, then descends into RIF. Here, its distal process forms a small diverticulum which is the appendix. Location of the appendix = Usually due to unequal growth of the caecum the appendix lies medially to the caecum. But obviously cna sometimes be posterior.

Question 31

Name the retro and intra peritoneal structures in the gut

Answer 31

Retroperitoneal = 1) Duodenum (except first 2.5cm) 2) Ascending colon 3) Descending colon Intraperitoneal, and therefore retain their free mesenteries 1) Other derivatives of the midgut loop, being the jeunum and ileum 2) Lower end of caecum and appendix 3) Transverse colon 4) Sigmoid colon

Question 32

Describe the structure of the cloaca

Answer 32

Remember that the cloaca is the expanded terminal part of the hindgut. It is lined with endoderm, and is in contact with the cloacal membrane which is surface ectoderm Cloacal membrane = This has the endoderm of cloaca on the inside, and the ectoderm of the proctodeum (anal pit) on the outside.

Question 33

Outline how the cloaca is partioned to eventually form the urinary bladder and the anorectal canal

Answer 33

At week 5 = The allantois (future urogenital sinus, prev outpouching of the yolk sac) enters the cloaca. It enters the anterior portion of the cloaca At the same time = The hindgut enters the posterior portion of the cloaca and becomes the future anoretal canal. There is a layer of mesoderm that separates these 2 structures = Called the urorectal septum At 6 weeks = Urorectal septum progresses forward to cloacal membrane to divide in half At end of 7th week = The clocal membrane breaks down. This creates the anal opening for the hindgut, and the ventral opening for the urogenital sinius.

Question 34

What is the allantois

Answer 34

It is a caudal outpouching of the yolk sac, that invades half of the cloaca in order to form the bladder system

Question 35

What does the urorectal spetum go on to form

Answer 35

Remember urorectal septum is the bit of mesoderm that seperates the division of the cloaca into urogenital sinus, and anorectal canal. This later becomes = The perineal body

Question 36

Which and how much of each embryological layers contirbute to the anal canal? What is the name of the junction that seperates them

Answer 36

Endoderm = Forms superior 2/3 Ectoderm of the proctodeum = Forms the distal 1/3 The junction is the pectinate line just below the anal columns.

Question 37

What are the differences in the vasculature, lymph drainage and nerve supply of the areas of the anal canal below and above the pectinate line

Answer 37

Above the pectinate line = 1) arterial = superior rectal artery, which is continuation of IMA 2) venous = Superior rectal vein, that drains into IMV 3) lymph drains = Inferior mesenteric lymph nodes 4) nerve supply = Autonomic nerves, hence painless carcinomas Below the pectinate line = 1) arterial = Inferior rectal arrteries that come off the internal pudendal 2) venous = Inferior rectal veins that drain into pudendal, then internal iliacs 3) lymph drains = Superficial inguinal lymph nodes 4) nerve supply = Inferior rectal nerve, hence painful as somatic.

Question 38

Outline how an oesophageal atresia is formed and the consequneces

Answer 38

Occurs in 1 in 3000-4500 live births When assoicated with traceho-oesophageal fistula = Occurs because of a posterior deviation of the traceho-oesophageal septum. When it occurs as an isolated abnormality = Due to failure of reanalisation of the oesophagus during week 8 Clinical = Mnifests as polyhydramnios as cannot swallow. And as excessive drooling and regurgitation after birth. Confirmed by radiology after trying to pass an NGT.

Question 39

Outline how a congenital hiatal hernia forms

Answer 39

Failure of the oesopahgus to elongate enough as the neck and thorax develops. This means = Stomach gets pulled up into chest through diaphragm

Question 40

Outline how a congenital hypertrophic pyloric stenosis occurs. Is it more common in boys or girls?

Answer 40

Occurs in = More common in men. 1 in 150 in men, 1 in 750 in females. Cause = Muscular thickening of the circular and longitudinal muscles of the pylorus Present = Projectile comiting, marked distension of stomach. Management = Surgical with pyloromyotomy.

Question 41

Outline how a duodenal stenosis and atresia occurs

Answer 41

Duodenal atresia = Incomplete recanalisation of the duodenum. Generally invovles the horizontal (third part) and/or fourth part of duodenum Duodenal atresia = Same as above, but almost always occurs at junction of bile and pancreatic ducts. It also has an autosomal recessive inheritance. 1/3 of infants have Down syndrome. Presentation = Also with polyhydramnios in feotal life. And bilious vomiting and distension of epigastrium ater birth. Diagnosis = Double bubble sign on X ray or USS

Question 42

Outline how extrahepatic biliary atresia occurs

Answer 42

Occurs in 1 in 10,000 to 15,000 live births Cause = Infections, immunological reactions, or failure of the remodelling process at the hepatic hilum Location = Usually just above porta hepatis. Signs = Pathological jaundice after birth, with clay coloured stool Management = Kasai hepatoportoenterostomy or liver transplant

Question 43

What causes intrahepatic biliary atresia

Answer 43

Occurs in 1 in 100,000 live births Can be caused by foetal infections as well

Question 44

Outline how an annular pancreas forms

Answer 44

Cause = When the migration of the left portion of the ventral bud is in the wrong direction. Produces an ring (annulus) around the pancreas. Aetiology = Can be associated with Down syndrome, intestinal atresia, imperforate anus, apncreatitis, malrotation of the gut. Male saffected more. Duodenal obstruction = This is the most severe complciation of n annular pancreas.

Question 45

Outline what accessory pancreatic tissue is

Answer 45

THis is just extra pacnreatic tissue, that most commonly can lie in the mucosa of the stomach or in the Meckle diverticulum

Question 46

Outline what an accessory spleen is

Answer 46

Can just get accessory spleens in about 10% of people. They are usually isolated, but can be attached by thin bands.

Question 47

Outline the key differences between gastroschisis and omphalocele for definition, defect, incidence, aetiology, genetics, associations, management, and prognosis

Answer 47

Gastroschisis 1) Defintiion = Herniation not covered by peritoneum or amnion. 2) Defect = It is para-umbilical, usually right side 3) Incidence = 05-4.5 / 10,000 live births 4) Aetiology = R paraumbilical area weakened by regression of R umbilical vein 5) Genetics = Only 1.2% of cases have chromosomal abnormalities 6) Associations = Young mothers, nicotine, alcohol, males, IUGR, low socio-economic status 7) Management = Surgical repair after birth 8) Prognosis = Generally very good unless a volvulus forms Omphalocele 1) Definition = Herniation of abdo viscera (which can include organs like liver, bowel, spleen) covered in amnion through umbilical ring 2) Defect = Central 3) Incidence = 2-3 / 10,000 live births 4) Aetiology = Failure of the bowel to actually return to body cavity after its physiological umbilical herniation 5) Genetics = 50% have associated chromosomal abnormalities. 6) Associations = 50% also have cardiac defects, 40% neural tube defects. Mortliaity rates of 25% 7) Management = Surgical repair 8) Prognosis = Depends on what associated abnormalities there are.

Question 48

Outline how a congenital umbilical hernia forms and how this is different from gastroschisis and omphalocele

Answer 48

This is herniation of abdo contents through an imperfectly closed umbilicus. Because there is a defect in the linea alba. Protrusion = Usually greater omentum/small bowel that is ocvered in subcut tissue and skin

Question 49

Outline what happens in abnormal/incomplete rotation of the gut, and what its effects are.

Answer 49

When the gut only rotates 90 degrees = The colon and caecum are actually on the left, with small bowel on the right. The caecul is just inferior to the pylorus of stomach and connected to the posterolateral abdo wall by peritoneal bands. These pass over the duodenum and can cause duodenal obstruction. I Infants present with bilious vomiting. Malrotation of the gut = Gut remains in the wrong place. Can lead to catastrophic twisting of the gut (midgut volvulus). SM vessels can get obstructed causing infection and gangrene of gut. Rotation of gut 90 degree clockwise (wrong way) = Causes the transverse colon to lie posterior to SMA. This can cause the transverse colon to actually block off.

Question 50

Outline what a Meckel diverticulum is, and how it forms

Answer 50

Occurs in 2-4% of population. Most common anomaly of GI tract. 3-5 times more common in males Cause = A small part of the vitelline duct remains, forming a pocket in the ileum. In adults it is usually 40-60cm from the illeocaecal valve. Inflammation here = Can cause Meckels diverticulum which mimics appendicitis. This vitelline remnant can contain small patches of gastric and pancreatic mucosa that can secrete acid, ulcerate and bleed. Vitelline cysts and fistulaes = Can also form and cause volvulus and faecal discharge at umbilicus respectively

Question 51

Outline what duplications of intestinal loops and cysts are

Answer 51

Duplications can occur anywhere in gut = Most common around ileum. 33% associated with other defects = Intestinal atresias, imperforate anus, gastroscisis and omphalocele. Duplicated loops can contain = Ectopic gastric mucosa.

Question 52

What are the differences in the cause of atesias in the upper and lower GI tract

Answer 52

Upper GI tract = Usually due to lack of recanalisation Lower GI tract = Usually due to deficient blood supply

Question 53

What is apple-peel atresia?

Answer 53

This accounts for 10% of atresia Intestine is short and atresia is the proximal jejunum, with a portion distal to the lesion coiled around the mesenteric remnant. Cause = Low birth weight and other abnormalities.

Question 54

Outline what a subhepatic caecum and appendix is, and how it forms

Answer 54

More common in males. Caecum re-enters the abdo cavity and lies next to liver normally. But there is adheres to inferior surface of liver and is therefore in wrong position. Can cause difficulties in diagnosis of appendicitis and during appendicectomy.

Question 55

Describe what a mobile caecum is and why it occurs

Answer 55

Occurs in 10% of population. Cause = Failure of the ascending colon to fuse with post abdo wall. This means the mesocolon mesentery remains as there is no fusion The long mesentery = Can cause abnormal movements of gut, and even volvulus or caecum/colon.

Question 56

What is a retrocolic hernia

Answer 56

When portions of small intestine are trapped behind mesocolon Generally asymptomatic

Question 57

Describe rectoanal atresia and fistulae

Answer 57

Occur in 1 in 5000 live births. Cause = Abnormalities in formation of the cloaca

Question 58

Describe imperforate anus

Answer 58

1 in 5000 live births also, like rectoanal atresia and fistula Cause = Lack of recanalisation of lower portion of anal canal. Needs surger

Question 59

Describe anal stenosis

Answer 59

Anus is in normal position, but anal canal is narrow. Cause = Slight dorsal deviation of the urorectal septum that divides the cloaca

Question 60

Describe membranous atresia of the anus

Answer 60

Anus is in normal position, but thin layer of tissue seperates anal canal from exterior. Cause = Failure of the anal membrane to perforate at week 8

Question 61

Describe congenital megacolon

Answer 61

Known as aganlionic megacolon or Hirschsprung disease Incidence = 1 in 5000 live births (same as rectanal atresia/fistula and imperforate anus also). Males are 4 times more likley to get Congenital megacolon accounts for 33% of all neonetal obstruction. Genetics = Autosomal dominant. RET proto-oncogene is major gene involved. Cause = Absence of parasymphathetic ganglia in bowel wall. Occurs because neural crest cells fail to migrate into wall of colon in weeks 5-6.

Question 62

Describe anorectal agenesis

Answer 62

This can occur with or without a fistula is actually most common type of anorectal anomaly What happens = Rectum just ends blindly, often with a fistula to the bladder, urethra, or vestibule of the vagina. Cause = Incomplete seperation of the cloaca by the urorectal septum. Symptoms = Passage of meconium or flatus through urine or in vestibule of vagina

Question 63

Describe rectal atresia

Answer 63

When rectum and anus are separated. Sometimes two segments of bowel are connected by a fibrous cord Cause = Abnormal recanalisation or deficient blood supply. These are the two general causes of atresia also. Remember Upper vs lower GI tract atresia.