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SMP - GI > SDL 3 and Lecture 11 - Embryology of the GIT > Flashcards

SDL 3 and Lecture 11 - Embryology of the GIT Flashcards

1
Q

What are both the cephalocaudal folding and the lateral folding of the 3 germ layer plate driven by?

A

Growth of amniotic cavity happening faster than growth of the yolk sac

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2
Q

Describe how cephalocaudal and lateral folding contributes to the development of the GIT.

A

3-layered disc undergoes cephalocaudal folding which forms the gut tube from the yolk sac:

  1. The superior amniotic cavity enlarges relative to the yolk sac to form pockets of endoderm that pivot around a fixed anterior end (the oralpharyngeal membrane) and fixed posterior end (the cloacal membrane)
  2. Overlapping endoderm forms the foregut, midgut, and hindgut
  3. Foregut is subdivided into the pharyngeal gut in the head/neck area and the foregut
  4. Lateral folding pinches off the yolk sac by growing of the ectoderm and mesoderm on each side
  5. As this is happening there are 2 connections between the body wall and the GIT: ventral and dorsal mesenteries from mesenchyme
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3
Q

Location of dorsal mesentery?

A

From thorax to pelvis

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4
Q

Location of ventral mesentery?

A

Only in foregut region

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5
Q

What are the mesenteries associated with the stomach called? Are these ventral or dorsal?

A

Omenta

Both ventral and dorsal

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6
Q

Largest structure located in the ventral mesentery?

A

Liver

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7
Q

What does the ventral mesentery develop into? Describe each.

A
  1. Falciform ligament of the liver: connects liver to anterior abdominal wall
  2. Lesser omentum:
    - hepatogastric ligament: connects liver to stomach
    - hepatoduodenal ligament: connects liver to duodenum
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8
Q

At the level of the foregut, what structures are included in the dorsal mesentery at the end of the 5th week of the life of the embryo?

A
  1. Pancreas
  2. Spleen
  3. Celiac artery
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9
Q

Most inferior portion of the hepatogastric ligament?

A

Over superior duodenum

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10
Q

Which is larger: dorsal or ventral mesentery?

A

Dorsal mesentery

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11
Q

What does the greater omentum develop from?

A

Dorsal mesentery

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12
Q

What is found between each of the somites? What do these develop into?

A

Pair of arteries, called the intersegmental arteries branching from the dorsal aortae => arterial supply of abdomen

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13
Q

Embryological origin of blood supply of kidneys?

A

Lateral intersegmental arteries from the lateral sides of the aorta

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14
Q

Embryological origin of blood supply of epaxial and axial musculature + vertebral column ?

A

Dorsal intersegmental arteries from the posterior side of the aorta

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15
Q

Embryological origin of blood supply of GIT? How do these evolve? What is their conduit from aorta to GIT?

A

Ventral intersegmental arteries from the anterior side of the aorta

During lateral folding and fusion of dorsal aortae, the paired branches fuse into single ventral intersegmental arteries from the aorta and only three branches from the aorta are left: celiac (FG), SMA (MG), IMA (HG)

Conduit: dorsal mesentery

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16
Q

List the primitive gut derivatives that are supplied by the celiac artery.

A
  1. Pharynx
  2. Esophagus
  3. Stomach
  4. Upper duodenum
  5. Glands of pharyngeal pouches: respiratory tract, liver, gallbladder, pancreas
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17
Q

List the primitive gut derivatives that are supplied by the SMA.

A
  1. Lower duodenum
  2. Jejunum
  3. Ileum
  4. Cecum + appendix
  5. Ascending colon
  6. Cranial 2/3rds of transverse colon
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18
Q

List the primitive gut derivatives that are supplied by the IMA.

A
  1. Caudal 1/3rd of transverse colon
  2. Descending colon
  3. Rectum
  4. Superior part of anal canal
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19
Q

What is the peritoneal cavity connected with during embryonic development?

A

Umbilical vesicle

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20
Q

Which grows faster: gut tube or peritoneal cavity? What does this cause? When does this happen? What is happening during this whole process?

A

GIT grows much faster AND kidneys are taking up a lot of space

=> physiological herniation => loop of midgut covered with dorsal mesentery bulges ventrally and is forced into the body stalk (or extraembryonic coelom) for around 3 weeks (5th week of gestation) => GIT coils as it grows in the extraembryonic coelom => kidneys regress in size and hips grow => increase in space in peritoneal cavity (8th week of gestation) => intestines pulled back into the body (completely by 10th week) and as this the GIT undergoes a 270° counterclockwise rotation around the axis formed by the vitelline duct and superior mesenteric artery => rotation brings the ascending colon, transverse colon, and descending colon into their final anterior position framing the small intestines (cecum is still located more superiorly than it should near diaphragm)

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21
Q

What is the urorectal septum? Describe its development. What to note?

A

Septum made of mesoderm separating the urogenital region from the GIT (allantois from hindgut = developing bladder from developing sigmoid colon) by folds that divide the cloacal plate => creates a primitive urogenital sinus (vestibule in females and urethra in both males and females) and an anorectal canal with the perineal body in between

Urorectal septum brings vessels along with it

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22
Q

What does the allantois of the embryo develop into? Describe the development.

A

Upper portion of bladder

Develops as a dorsal most caudal tail extending from the embryo that becomes ventral and more cranial after cephalocaudal folding

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23
Q

What is the septum transversum in the developing embryo?

A

Thick mass of cranial mesenchyme, that assumes a position ventral and then caudal to the developing heart during cephalocaudal folding

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24
Q

Where do the primordia of the liver and pancreas come from?

A

From endoderm surrounding the septum transversum and cardiogenic plate in the developing heart, which send respectively FGF and BMP signals to it to cause it to become liver and the tissue that receives lower concentrations will become the VENTRAL pancreas by default

The dorsal pancreas develops from endoderm in the foregut/midgut area near the somites which receives Shh signals from the notochord to become dorsal pancreas

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25
Q

Describe the development of the liver.

A

Originally epithelial endoderm cells but then due to signal molecules and cephalocaudal folding the diaphragm presses on the GIT => these cells migrate into mesoderm of developing diaphragm and develop cords => hepatic cords grow into the ventral mesentery and space of septum transversum and give rise to the hepatic tissues

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26
Q

Describe the development of the pancreas. When does this occur?

A
  • Ventral pancreatic bud grows in the ventral mesentery
  • Dorsal pancreatic bud grows in dorsal mesentery

=> rotation of gut tube causes them to come together (6 weeks of gestation)

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27
Q

2 development issues that can occur during pancreatic development?

A
  1. Gut rotation does not take place
  2. Ventral pancreatic bud does not move appropriately and sometimes the pancreas can then come to surround the duodenum which strangles it causing a narrowing => annular pancreas
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28
Q

Can a annular pancreas be fixed?

A

Yes, if diagnosed early

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29
Q

Describe the development of the exocrine and endocrine cells of the pancreas. What is this similar to? What is unique about this? Describe the timing of the development as well.

A

Similar to branching in the developing lungs

  • Endodermal epithelium determines characteristics of tissues and their fate
  • Mesenchyme determines branching points

Unique part: some of the endodermal epithelium will bud off from the rest of the epithelium to become islets => endocrine cells of the pancreas

Difference between the 2 types of endoderm:

  1. One kind is sensitive to the notch pathway => cells receive further signals from foregut mesoderm => cells become pancreatic exocrine cells (THIS HAPPENS FIRST)
  2. The other is not => cells will receive signals from the vasculature (like neurogenin) around 2 to 3 weeks into gestation (THIS HAPPENS SECOND) => cells will either be sensitive to Pax 6 or Pax 4 => Pax 6 sensitive cells become alpha endocrine cells (glucagon) or gamma cells (pancreatic polypeptide) and Pax 4 sensitive cells become beta endocrine cells (insulin) or delta endocrine cells (SS) (THIS HAPPENS TWO WEEKS AFTER PAX 4 SENSITIVE CELL DEVELOPMENT) => all pancreatic islets => pancreatic endocrine cells
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30
Q

What other body part has a similar development to that of the branching of the pancreas?

A

Branching in the salivary glands

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31
Q

Do the islets of the pancreas come from neural crest cells?

A

NOPE

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32
Q

State of embryo at the end of the 3rd week?

A

3 germ layers have formed with amniotic cavity and yolk sac

33
Q

What part of the developing gut tube is originally open to the yolk sac?

A

Midgut

34
Q

Where is the pharyngeal gut located?

A

Above buccopharyngeal membrane

35
Q

Describe the development of the greater omentum.

A

Dorsal mesentery connecting posterior wall to stomach => posterior stomach rotates 90 degrees to the left (on the vertical axis), and pulls the dorsal mesentery with it forming a fold with its blind end on the left = a little pouch between stomach and dorsal mesentery: the lesser sac => stomach undergoes another 90 degree rotation bringing the inferior end of the stomach superiorly (horizontal axis through the middle) => this 4 layer mesentery is the greater omentum => it then grows downwards passing in front of the transverse colon => the visceral peritoneum of the transverse colon, its mesentery (transverse mesocolon), and the dorsal mesentery covering the posterior abdominal wall and the posterior layer of the greater omentum then fuse and the duplicated layers are absorbed => greater omentum is now 2 layers, stuck to the transverse colon visceral peritoneum, and hanging down below it

36
Q

What forms the lesser and greater curvatures of the stomach?

A

As the two stomach rotations occur, the posterior side, which is now on the left, grows disproportionately faster then the anterior side, which is now on the right. The greater curvature is formed by this disproportionate growth; the lesser curvature is formed by the anterior side which grows at a slower rate

37
Q

How are the left and right vagus nerves positioned with regards to the GIT? What causes this?

A

The left vagus nerve is found on the anterior aspect of the GIT and the right vagus nerve is found on the posterior aspect of the GIT

First 90 degree rotation of the stomach

38
Q

What does dorsal mesentery develop from?

A

From the mesenchyme superior to the roof of the yolk sac.

39
Q

What triggers the first 90 degree rotation of the stomach?

A

Apoptosis of dorsal mesentery forming clefts

40
Q

How does the vitelline duct develop?

A

As the yolk sac pinches off from the GIT, the narrowed portion connecting the two is the vitelline duct and connects the two once the yolk sac has been completely separated from the GIT

41
Q

What are the mesonephroi?

A

Embryological derivation of kidneys

42
Q

What are the 3 malrotations of the intestines? Are these common?

A
  1. Nonrotation: small intestine on the right, large intestine on the left causing issues with blood supply
  2. Reversed rotation (large intestine is posterior and small intestine is anterior): the individual will be fine if he presents with complete situs inversus, where everything is reversed. However, if all other organs are correctly situated and the reverse rotation occurs, major problems will ensue like constrictions of the large intestine and volvulus
  3. Subhepatic cecum: cecum stays inferior to liver and does not descend (not very serious)

NOT common

43
Q

What does the cranial limb of the GIT to the vitelline duct/SMA represent?

A

Small intestine: jejunum + part of ileum

44
Q

What does the caudal limb of the GIT to the vitelline duct/SMA represent?

A

End of ileum + cecum + large intestine

45
Q

How does the cecum assume its final location in abdomen?

A

Due to differential growth following the rotation of the GIT

46
Q

Prior to the GIT rotation, what are the branches to the SMA?

A
  1. Colic branches for caudal limb

2. Jejunoileal branches for cranial limb

47
Q

Describe the placement of the SMA branches during the GIT rotation.

A
  1. After 90 degree rotation: colic branches are to the left of the ileal branches
  2. After 180 degree rotation: colic branches above jejunoileal branches
48
Q

Where is the cecum located after the GIT has rotated 180 degrees?

A

Inferior to liver

49
Q

How does the appendix develop?

A

Vestige of incomplete development of the cecum

50
Q

What is an omphalocele? Is this common? What to note? Fixable?

A

Baby born with portion of intestines in umbilical cord at the midline with amnion around it and the organs remain enclosed in visceral peritoneum

Note: amniotic fluid is not included in the umbilical cord, as it would be detrimental so the amnion protects the intestines from it

Yes, common and fixable

51
Q

What is the difference between gastroschisis and omphalocele? Can both be fixed?

A

Gastroschisis: baby born with portion of intestines coming out of hole close to the umbilicus but are not contained in anything (NO amnion around it)

This is due to an issue with body wall closure (aka lateral folding)

Yes, both can be fixed surgically

52
Q

On what side of the body does gastrochisis occur most of the time?

A

Right side of umbilicus

53
Q

What are 4 vitelline duct related malformations? Can these be fixed?

A
  1. Meckel’s diverticulum: failure of the vitelline duct to fully regress = remnant that extends into the umbilical cord in the embryo and lies on the antimesenteric border of the ileum (NOT attached to anterior abdominal wall): chyme can get stuck in it
  2. Fibrous cord connecting Meckel’s diverticulum to umbilicus, which can make it harder for intestine to conduct peristalsis => can develop into a volvulus: loop of intestine twists around itself resulting in a bowel obstruction
  3. Umbilical fistula between ileal diverticulum and the umbilicus where chyme can come out of body wall

ALL can be fixed

54
Q

Other name for meckel’s diverticulum?

A

Ileal diverticulum

55
Q

What are 6 malformations related to the urorectal septum?

A
  1. Persistent anal membrane
  2. Anal atresia
  3. Anoperineal fistula
  4. Rectovaginal fistula (females)
  5. Rectourethral fistula (males)
  6. Rectovesical fistula (between rectum and bladder)
56
Q

What is a coelom?

A

Space derived from mesenchyme that is lined by epithelium

57
Q

How many body cavities in the human body? What do these develop from?

A 
4 cavities in females and 6 in males: 
	2 pleural cavities
	1 pericardial cavity
	1 peritoneal cavity
	2 processi vaginales (males) - derived as outpocketings of the peritoneal cavity

All develop from the embryonic coelom

58
Q

Level of diaphragm in the body?

A

12th rib in the back, and 6th rib in the midline of the chest

59
Q

Purpose of peritoneal cavity as a potential space?

A

As we eat and propel things through the intestinal tract, the intestines are continually contracting with peristalsis and moving things back and forth. The intestines themselves also move around in the peritoneal cavity during different feeding states.

60
Q

What is the extraembryonic coelom? Other name?

A

The embryo is connected to the placenta at a point where the mesenchyme is continuous with the placenta that is in contact with the mother’s uterine wall. Therefore, there is a coelom between the extraembryonic mesoderm and the chorionic plate (outermost part of fetal membranes). This is the extraembryonic coelom, or the chorionic space.

61
Q

Describe the development of the intraembryonic coelom.

A

During cephalocaudal and lateral folding, the lateral plate mesoderm gets thicker => there is a secretion of fluid between the cells that pulls them apart so that there are tiny spaces between them => these spaces coalesce to form the intraembryonic coelom, which separates the somatic and splanchnic layers of the lateral plate => as the intraembryonic coelom enlarges, and as the embryo elevates from the yolk sac, the intraembryonic coelom will eventually break free and connect with the extraembryonic coelom (only at areas where there are somites) => lateral folding separates these again and obliterates the extraembryonic coelom => intraembryonic coelom lies above the heart due to cephalocaudal folding and is U-shaped

62
Q

Describe the development of the diaphragm.

A

A region of mesenchyme which is anterior/caudal to the developing heart, the septum transversum, will form a large part of the diaphragm (placed there due to cephalocaudal folding)

As the septum transversum folds, it comes into contact with structures that are in the way of its eventual contact with the dorsal body wall: the gut tube, the kidneys and the posterior body wall itself. So the diaphragm is made up of 4 things:

  1. The septum transversum in the middle => central tendon
  2. The dorsal mesentery of the esophagus (connecting it to the posterior body wall) => esophageal hiatus
  3. The pleuroperitoneal folds associated with mesonephric kidneys (the two mesonephroi disappear and the capsule on top of them gets incorporated into the diaphragm): separates peritoneum from pleural cavities => superior portion of diaphragm inferior to pleural cavities
  4. Contributions from the posterior/lateral body wall => all lateral and posterior portions of the diaphragm
63
Q

What are the mesonephric kidneys?

A

The second in a series of 3 kidneys.

64
Q

Why do the phrenic nerves innervate the diaphragm?

A

Because when the mesenchyme of septum transversum of the diaphragm was initially high up, nerves from the neural tube in the neck region innervated the diaphragmatic musculature by traveling in a horizontal plane. When the diaphragm descended during cephalocaudal folding, the nerves stretched and traveled along with it, traversing the scalene muscles in the neck and the lateral aspects of the pericardial sac in the thorax.

65
Q

Why can pericarditis cause chronic hiccups?

A

Because the inflamed phrenic nerve is irritated causing involuntary contractions of the diaphragm

66
Q

Describe the separation of the pericardial and pleural cavities.

A

This is done by a membrane which is named by the cavities that it separates: the pleuropericardial membrane

Anterior and posterior cardinal veins come from the body wall, into the common cardinal vein, which connects them to the heart and some sort of mesenchymal sheet covers all of the veins => intussusception of the common cardinal vein in the heart occurs and the membrane starts to bow out because of its attachment to the body wall => eventually the membrane is pulled right across the opening dividing the pleural cavity from the pericardial cavity

67
Q

Other name for common cardinal vein?

A

Duct of Cuvier

68
Q

Where do most diaphragmatic hernias occur? Describe this.

A

Left side of the body where the pleuroperitoneal fold does not occur properly => the spleen and intestine may be sucked into the thorax: serious defect because it interferes with appropriate development of the lungs (can be corrected by intrauterine surgery)

On the right the liver covers this

69
Q

How do the phrenic nerves end up in the pericardial sac?

When the phrenic nerves pass the heart, which one is most anterior? What is this due to?

A

As the common cardinal veins move toward the midline, they pull tissue with them which includes the phrenic nerves that get dragged along, which get incorporated in the walls of the pericardial sac

Left one, because of the development of the pleuropericardial membrane

70
Q

How are the foregut, midgut, and hindgut defined throughout embryological development?

A

Originally based on location, and eventually based on arterial blood supply

71
Q

Is the ventral mesentery = lesser omentum?

A

Lesser omentum PLUS falciform ligament

72
Q

What vessel does the falciform ligament contain during development? What does this become?

A

Umbilical vein (from placenta, bypassing liver, and entering the ductus venosus to the heart) => ligamentum teres hepatic

73
Q

Complication of volvulus?

A

Necrosis

74
Q

Risk factors for gastroschisis?

A

Teenage mothers
Smoking
Drinking

75
Q

What do the cardinal veins of the embryo drain?

A

The body wall and head

76
Q

What is the body stalk? What does it get replaced by? Other name?

A

= Allantoic stalk = band of mesoderm that connects the caudal end of the embryo to the chorion in development

Later replaced by the umbilical cord

77
Q

What does ventral mesentery develop from?

A

Mesoderm on lateral sides of yolk sac

78
Q

Fate of common cardinal vein?

A

SVC

79
Q

What does it mean for organs to be secondarily retroperitoneal? List them.

A

Means they became retroperitoneal after gut rotation:

  1. Duodenum
  2. Ascending colon
  3. Descending colon

SMP - GI (21 decks)

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