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Gastrointestinal > Development of GI Tract > Flashcards

Development of GI Tract Flashcards

1
Q

What is the significance of folic acid during development?

A

Folic acid helps fusion and closure of spinal cord

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2
Q

When are the primary germ layers formed?

A

The primary germ layers are formed during the process of gastrulation

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3
Q

How long after does an embryo implant into the uterine wall?

A

At the beginning of the third week, the embryo has implanted into the uterine wall

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4
Q

Describe the structure of an embryo

A

The embryo is a flat disc, comprised of two cell layers
Epiblast
Hypoblast

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5
Q

Explain how the embryo initially begins to develop

A

Epiblast cells in the midline of the embryo begin to ingress, starting from the caudal end
Visible as the primary streak

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6
Q

What do the ingressing cells differentiate into?

A

Ingressing cells differentiate into Mesoderm

Surrounding muscles, connective tissue & mesenteries and blood vessels

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7
Q

What does the epiblast give rise to?

A

Epiblast gives rise to Ectoderm (neural crest)

Innervation of the gut (enteric NS), skeletal muscles

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8
Q

What does the hypoblast and epiblast develop into?

A

Hypoblast (and epiblast) gives rise to Endoderm

Epithelium of the gut tube and glands

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9
Q

When does gastrulation occur?

A

Gastrulation generates the three primary germ layers within the 3rd week

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10
Q

How is the gut tube formed?

A

The gut tube is formed by folding of sheets of cells in two directions

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11
Q

Explain the folding that occurs to form the gut tube

A

Folding towards the midline along the cranial-caudal axis

Folding towards the yolk sac at the cranial and caudal ends

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12
Q

What does the cloacal membrane form?

A

Cloacal membrane - urogenital sinus; anus and genitals

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13
Q

What does the buccopharyngeal membrane form?

A

Buccopharyngeal membrane - head and neck

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14
Q

What is the septum transversum?

A

Septum transversum is the area where the diaphragm forms

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15
Q

What is the mesentery?

A

Mesentery = folded membrane that anchors on posterior axis (peritoneum)

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16
Q

Where is the GI tract anchored to/?

A

GI tract is anchored to the posterior abdominal wall

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17
Q

Describe the initial structure of the embryo

A

The embryo is initially a solid flat disc attached to the hemispherical yolk sac (& similarly to amnion)

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18
Q

How is part of the yolk sac cavity enclosed in the embryo?

A

Part of the yolk sac cavity is enclosed within the embryo by pinching-off the yolk sac to form a yolk stalk and balloon like yolk sac

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19
Q

How does the gut tube link from the mouth to the anus in the embryo?

A

Within the embryo, the cranial and caudal intestinal portals extend the tube towards the mouth and anus, delimited by the procordal and cloacal plates

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20
Q

What is the primary gut tube composed of?

A

Sheet of endoderm which makes the epithelia and glands

Surrounding mesoderm, which makes muscle and connective tissue (including mesentery)

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21
Q

What is the foregut composed of?

A 
Pharynx
Oesophagus
Stomach
Cranial half of duodenum
Ampulla of Vater
(joining of common bile duct and pancreatic duct)
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22
Q

What is the midgut made up of?

A 
Caudal duodenum (from duodenal papilla)
Jejunum
Ileum
Caecum
Appendix
Ascending colon 
Proximal 2/3 of transverse colon
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23
Q

What composes the hindgut?

A

Distal 1/3 of transverse colon
Descending Colon
Rectum

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24
Q

Describe the atrial blood supply of the gut

A

Inferior mesenteric artery - hind gut
Superior mesenteric artery - midgut
Celiac artery - foregut & abdominal organs

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25
Q

How well perfused is the gut?

A

Gut surrounded by plexus of blood vessels, joining vitelline vessels to aorta

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26
Q

What is the significance of the plexus of arteries around the gut?

A

Plexus resolves to form the arteries that supply the GI tract from the aorta
These define the boundaries of the gut

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27
Q

Describe the arterial blood supply of the GI tract

A

~ 5 arterial branches to the thoracic oesophagus (from the descending aorta)
Celiac artery to the foregut
Superior mesenteric artery to the midgut
Inferior mesenteric artery to the hindgut

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28
Q

Where do sympathetic ganglia form?

A

Sympathetic ganglia develop next to major branches of the aorta

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29
Q

Where do the postganglionic sympathetic axons innervate?

A

Postganglionic sympathetic axons innervate the same tissues that the arteries supply with blood

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30
Q

Explain the sympathetic innervation of the gut

A

Celiac ganglion - foregut
Superior mesenteric ganglion - midgut
Inferior mesenteric ganglion - hindgut

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31
Q

How does the stomach develop from the gut tube?

A

Stomach arises by expansion and rotation

During week 4 at the level where the stomach will form the tube begins to dilate, forming an enlarged lumen

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32
Q

Explain the development of the stomach from the gut tube

A

Initially concave ventral, convex dorsal
90°turn about craniocaudal axis
The dorsal border grows more rapidly than ventral, which establishes the greater curvature of the stomach

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33
Q

What is the sciatic nerve?

A

Sciatic Nerve - largest nerve in body, supplies bum

34
Q

How is the stomach held in place?

A

Dorsal wall of stomach attached to body by mesentery; the dorsal mesogastrium (will form greater omentum)
Ventral wall attached by ventral mesentery, which includes the liver (will form lesser omentum)

35
Q

What is the significance of the vagus nerve in the GI tract?

A

Vagus nerve help supply intricate parts of GI tract

36
Q

What is the role of the peritoneum?

A

Peritoneum membrane protects

Mesentery attached to stomach liver and abdominal organs

37
Q

What is the falciform ligament?

A

Falciform ligament - thin white fibro structure attached to abdominal wall to anchor organs to the front and back

38
Q

What is the mesogastrium?

A

the middle region of the abdomen between the epigastrium and the hypogastrium

39
Q

Explain how the mesogastrium forms the greater omentum

A

As the stomach rotates, the dorsal mesogastrium is drawn with it
The mesogastrium encloses a space, the omental bursa (lesser arc)
The folded mesogastrium grows to form the greater omentum from the greater curvature, the folds fusing to obliterate the bursa

40
Q

How is the mature gut folded in place?

A

Fusion of the mesenteries with the posterior abdominal wall fixes the mature gut in plac

41
Q

Where are retroperitoneal structures of the GI tract located?

A

Retroperitoneal structures are sitting on the abdominal wall

42
Q

What is the role of the Cystic Artery?

A

Cystic artery allows communication between organs behind the liver
Intraperitoneal

43
Q

What is pyloric stenosis?

A

An abnormality of foregut development - pyloric stenosis (narrowing)
Gastric outlet obstruction caused by smooth muscle hypertrophy

44
Q

What is the incidence of occurence of pyloric stenosis?

A

3 in 1000 incidence

45
Q

What are the symptoms of pyloric stenosis?

A

Projectile vomiting shortly after feeding (not bile stained)
Pyloric channel elongation ‘railroad track’
L >16mm, wall > 4mm, diameter > 14mm

46
Q

How does the liver bud from the foregut?

A

Inducing signal:
Heart to ventral gut endoderm
Hepatic diverticulum grows into mesenchyme of septum transversum, expanding into ventral mesentery

47
Q

How is a blood supply and drainage system formed around the liver?

A

Cords of hepatic endoderm, bile drainage ducts, and blood vessels proliferate arranged as sinusoids

48
Q

How does the liver move to its position within the body?

A

Liver exceeds size of septum transversum (diaphragm area), expands into ventral mesentery

49
Q

What does the remaining ventral mesentery give rise to?

A

Remaining ventral mesentery gives rise to:
Falciform ligament between liver and body wall
Lesser omentum between liver and stomach

50
Q

What is the liver diverticulum?

A

The hepatic diverticulum (or liver bud) is a primordial cellular extension of the embryonic foregut endoderm that gives rise to the parenchyma of the liver

51
Q

How does the liver diverticulum form?

A

The liver diverticulum arises as a bud from the most caudal portion of the foregut

52
Q

What are the 2 Pancreatic buds?

A
  • Dorsal from duodenal endoderm (induced by
    notochord)
  • Ventral from hepatic diverticulum (induced by hepatic
    mesoderm)
53
Q

How do the pancreatic buds form the pancreas?

A

As duodenum rotates, ventral and dorsal buds meet and fuse

54
Q

What is an annular pancreas?

A

Rare condition where the second part of duodenum is surrounded by a ring of pancreatic tissue continuous with pancreatic head
This portion of pancreas can constrict duodenum and block/impair flow of food to rest of intestines

55
Q

How does an annular pancreas form?

A

If ventral bud bifid (bi-lobed), and one rotates around duodenum, annular pancreas forms, which can obstruct duodenum

56
Q

What kind of gland is the pancreas?

A

Pancreas is an endocrine and exocrine gland

57
Q

How are the intestines (midgut) attached to the body?

A

Attached throughout length by dorsal mesentery (but not ventral mesentery)

58
Q

What causes the folding of the gut?

A

The mesentery and gut grow at different rates, leading to stereotypical folding of the gut

59
Q

Describe the blood supply to the intestines

A

A ventral branch of the aorta supplies the midgut: superior mesenteric artery (SMA)

60
Q

Explain the development of the intestines around the superior mesenteric artery

A

With a very rapid increase in length, the intestines rotate around SMA
Abdomen is too small to accomodate, so herniates into umbilical stalk at 6/7 weeks
By 10 weeks, abdomen is bigger, and intestines return

61
Q

Describe the midgut development

A
  1. 90 degree rotation of midgut
  2. Elongates into yolk sac and comes out
  3. Does another 180 rotation
  4. Starts to form large bowel as it comes back in around
    10-12th week (small intestine + main parts of large
    intestine forming)
62
Q

List some abnormalities of intetsinal (midgut) development

A
  • Umbilical hernia
  • Omphalocele
  • Gastroschisis
  • Persistance of yolk duct
  • Meckel’s Diverticulum
63
Q

What is an umbilical hernia?

A

Intestines return normally, but rectus abdominis fails to fuse around umbilicus: gut covered in skin

64
Q

What is omphalocele?

A

Failure of intestinal loops to return into abdomen

Hernia covered in amion (causes unknown, but associated with maternal obesity, alcohol / tobacco, SSRI use)

65
Q

What is gastroschisis

A 
Failure of ventral body wall to fuse: no covering 
Increasing incidence (1 in 3000). Marked association with young maternal age, low maternal BMI, recreational drugs (especially cocaine)
66
Q

Describe features of a persistent yolk duct

A

Most common intestinal abnormality

Yolk duct attached to ileum, near ileocecal junction - apex of midgut loop (about 2 ft from ileocecal junction)

67
Q

What is meckel’s diverticulum?

A

Out-pouching / bulge in lower part of small intestine

Bulge is congenital and is a leftover of umbilical cord

68
Q

Describe the features of meckel’s diverticulum

A

(2-4% population) usually asymptomatic
Can get inflamed (clinically indistinguishable from acute appendicitis - pain)
May contain ectopic gastric cells; ulceration + lower GI bleeding
Can be connected to umbilicus by ligament
- Gut rotation causes volvulus
- Can form umbilical fistula

69
Q

What is Hirschsprung’s disease?

A

Aganglionic megacolon - not able to pass + digest food due to lack of neural cells
Primarily affects hindgut
Dilatation of sections of colon, with lack of tone + peristalsis, leading to profound constipation
Absence of parasympathetic ganglia

70
Q

What causes Hirschsprung’s disease?

A

Caused by lack of neural crest cells
Ganglia present in dilated/hypertrophic region
Aganglionic segment shows contraction

71
Q

What is the significance of neural crest cells?

A

Neural crest cells populate the developing gut and give rise to enteric ganglia - parasympathetic supply

72
Q

What gives rise to the enteric ganglia?

A

A subpopulation of the neural crest cells which migrate through the embryo from the lateral edges of the neural plate gives rise to the enteric ganglia

73
Q

Describe the location of different neural crest cells in the gut

A

Neural crest cells from the occipitocervical (vagal) region populate the entire gut
Neural crest cells from the sacral region populate the distal gut

74
Q

What are the consequences of failure of enteric gut development ?

A

Hirschsprung’s disease results from failure of development of the enteric ganglia
normally :
Neural crest cells give rise to enteric ganglia
Normal rectal innervation is inhibitory

75
Q

What are the effects of Hirschsprung’s disease?

A

Failure of neural crest cells to migrate to the correct location leads to absence of ganglion cells
Absence of inhibitory innervation results in tonic contraction and colonic obstruction

76
Q

Explain the significance of the cloaca

A

The cloaca is the transient common end of digestive and urogenital systems, including the base of the allantois (urogenital sinus)

77
Q

Describe the development of the anus

A

Covered by cloacal (proctodeal) membrane over ectoderm depression, the proctodeum
Split by the urorectal septum
Gives rise to urogenital membrane and anal membrane (perforate at 7-8 weeks)

78
Q

What is an imperforate anus?

A

defect that is present from birth (congenital)

79
Q

Describe the effect of an imperforate anus

A

Imperforate anus can be:

  • Persistence of anal membrane
  • Atresia of anal canal, rectum or both
80
Q

What does the cloaca give rise to?

A

Cloaca gives rise to the rectum and urogenital sinus

Gastrointestinal (18 decks)

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