Embryology Flashcards
1
Q
Describe lateral embryological folding in the 4th week
A
Creates ventral body wall and the primitive gut becomes tubular
2
Q
Describe craniocaudal embryological folding in the 4th week
A
Creates cranial and caudal pockets from the yolk sac endoderm (the beginning of primitive gut development)
3
Q
How do the foregut/midgut/hindgut begin?
A
Foregut and hindgut begin as blind diverticula
Midgut has an opening at first and is continuous with the yolk sac
4
Q
When does development of the primitive guts tube begin?
A
Begins in the 3rd week (‘pinches off’ from yolk sac cavity)
5
Q
Where is the primitive gut tube initially found?
A
Runs from stomatodeum/future mouth to proctodeum/future anus, caudally with an opening at the umbilicus.
6
Q
What is the internal lining of the gut tube derived from embryologically?
A
Endoderm
7
Q
What is the external lining of the gut tube derived from?
A
Splanchnic mesoderm
8
Q
What suspends the future gut tube in the intraembryonic coelom?
A
A double layer of splanchnic mesoderm
9
Q
What is the blood supply to the foregut?
A
Celiac trunk
10
Q
What is the blood supply to the midgut?
A
SMA
11
Q
What is the blood supply to the hindgut?
A
IMA
12
Q
What is key about structures close to the junctions between foregut/midgut or midgut/hindgut
A
They have a mixed blood supply
13
Q
Describe the blood supply to the duodenum
A
Proximal entry of bile duct via gastrodudenal and superior pancreaticoduodenal (CT)
Distal to entry of bile duct via inferior pancreaticoduodenal (SMA)
14
Q
Describe what happens to the intraembryonic coelom
A
Begins as one cavity, later subdivided by the future diaphragm into abdominal and thoracic cavities.
15
Q
What lines the abdominal cavity?
A
The peritoneal membrane (invests the viscera)
16
Q
What are the mesenteries?
A
A double layer of peritoneum suspending the gut tube from the abdominal wall, allowing a conduit for blood and nerve supply, and mobility where required.
17
Q
What surrounds the new, embryonic, gut?
A
Splanchnic mesoderm
18
Q
What suspends the entire gut tube from the dorsal body wall?
A
Dorsal mesentery
19
Q
Where is ventral mesentery found?
A
Only in the foregut region
20
Q
What does the left sac from the foregut region become?
A
Contributes to the greater sac
21
Q
What does the right sac from the foregut region become?
A
Becomes the lesser sac (comes to lie behind the stomach)
22
Q
What are omenta?
A
Specialised regions of peritoneum
23
Q
What is the greater omentum formed from?
A
Formed from the dorsal mesentery
24
Q
What is the lesser omentum formed from?
A
The ventral mesentery
25
What is the first structure seen when the abdominal cavity is opened anteriorly?
The greater omentum
26
Describe the rotation of the stomach
Primitive stomach rotates in two directions (around longitudinal and anteroposterior axis)
Results in the greater and lesser curvature coming to lie first on
Puts vagus nerves anterior and posterior to the stomach instead of left and right
Shifts Cadiz and pelorus from the midline, stomach lies obliquely
Contributes to moving the lesser sac behind the stomach
Creates the greater omentum
27
What does the liver develop into?
The ventral mesentery
28
What does the spleen develop into?
The dorsal mesentery
29
Describe peritoneal reflection
A change in direction from...
- Parietal peritoneum to mesentery
- Mesentery to visceral peritoneum
- Visceral peritoneum..
30
What happens when there is no mesentery?
Structures that are not suspended in the abdominal cavity are retroperitoneal
31
What defines a retroperitoneal structure?
A structure that was never in the peritoneal cavity and never had a mesentery
32
What is a secondary retroperitoneal structure?
Structures that began development invested by peritoneum (intraperitoneally) and had a mesentery BUT, with successive growth and development the mesentery is lost through fusion st posterior abdominal wall (duodenum, pancreas)
33
Name 2 retroperitoneal structure
Pancreas
| Duodenum
34
Where does the primitive foregut extend from/to
Extends from the lung bud to the liver bud
35
In what week embryologically does the respiratory diverticulum form in the ventral wall of the foregut at the junction with the laryngeal gut?
4th week
36
What creates the greater curvature of the stomach?
Faster growth of the dorsal border
37
What are the foregut-derived glands (ventral mesentery)?
Liver, biliary system, part of the pancreas (uncinate process and inferior head).
38
What are the foregut-derived glands (dorsal mesentery)?
Pancreas (superior head, neck, body and tail)
39
What is the earliest GI associated gland?
The liver
40
Where does the liver develop from?
Hepatic bud in the ventral mesentery. Occupies a large proportion of the abdomen during development.
41
What does the duodenum develop from?
Causal foregut and cranial midgut.
42
When does the duodenum form its c-shaped loop?
When the stomach rotates
43
What effect does rotation of the stomach have upon the duodenum?
Pushes duodenum to the right, then against posterior abdominal wall.
44
What happens to the duodenum in the 5th and 6th weeks?
It's lumen is obliterated, the recanalysed by the end of the embryonic period.
45
What does the midgut give rise to?
The small intestine (including most of the duodenum), caecum and appendix, ascending colon, and proximal 2/3 of the transverse colon.
46
What forms the axis of the loop formed by the developing midgut?
The SMA
47
What connects the loop formed by the rapidly developing midgut?
It is connected to the yolk sac by the vitaline duct, and has cranial and caudal limbs.
48
What happens to the intestines during the 6th week of growth?
It herniates out into the umbilical cord, as there is insufficient room in the abdominal cavity to accommodate the rapid growth of both the liver and the primary intestinal loop.
49
Describe the rotation of the midgut
1st - 90 degrees anti-clockwise, positions cranial limb to the right of the embryo, and the caudal limb to its left.
2nd - 90 degrees anti-clockwise, bringing the caudal limb over the cranial limb
3rd - 90 degrees anti-clockwise, as the intestines return to the abdominal cavity
50
What happens to the cranial limb of the midgut post rotation?
It begins to undulate, creating the small intestine
51
What happens to the caudal limb of the midgut post rotation?
Caudal limb forms a bud which will become the caecum
52
What are the derivatives of the cranial limb of the midgut?
Distal duodenum, jejunum, proximal ileum
53
What are the derivatives of the caudal limb of the midgut?
Distal ilium, caecum, appendix, ascending colon, proximal 2/3 transverse colon
54
What is incomplete rotation of the midgut?
The midgut only makes one 90 degree rotation, so there is a left sided colon
55
What is reversed rotation of the midgut?
Midgut loop makes one 90 degree rotation CLOCKWISE, so transverse colon passes posterior to the duodenum.
Issue - SMA may compress transverse colon, obstructing/strangulating it.
56
What are some possible major complications of midgut defects?
Volvulus (resulting in strangulation and ischaemia)
| Could be due to twisting of intestine, or compression by other structures if things aren't where they're supposed to be.
57
Describe the rule of 2s for meckels diverticulum
```
2% of the population
2 feet from ileocecal valve
2 inches long
2:1 male:female
Usually detected in under 2s
```
58
What is atresia, with regards to gut embryological development?
Luminal is obliterated
59
What is stenosis, with regards to gut embryological development?
Lumen is narrowed
60
Where is atresia/stenosis most likely to occur in the gut tube?
The duodenum
If upper, cause is usually recanalisation failure, if lower then cause is usually vascular accident (caused by malrotation, volvulus, body wall defect)
61
What is pyloric stenosis?
Hypertrophy of the circular muscle in the region of the pyloric sphincter, not a recanalisation failure. A common abnormality of the stomach in infants (characteristic projectile vomiting)
62
What is gastroschisis?
Failure of closing of the abdominal wall during folding of the embryo. Leaves gut tube and derivatives outside the body cavity. Not covered in pleura, difficult to fix, poor mortality rate.
63
What is omphalocoele (AKA examphalos)?
Persistence of physiological herniation. Differs from umbilical hernia because hernias have covering of skin and subcutaneous tissue (gut had completed physiological herniation sequence)
64
What does the hindgut give rise to?
Distal 1/3 of the transverse colon, descending colon, rectum, superior part of anal canal, epithelium of urinary bladder.
65
What does the pectinate line separate in the anal canal?
Histiologically distinct superior and inferior parts. Indicates differences in arterial supply, venous and lymphatic drainage and innervation.
66
Describe the progression of the the cloaca?
At 6wks, the hindgut ends in the cloaca, separated from the outside by the cloacal membrane. The cloaca is then subjected to anteroposterior subdivision - a wedge of mesoderm grows down into the cloaca, dividing it into the urogenital sinus anteriorly and the anorectal canal posteriorly.
67
At 6wks, what separates the cloaca from the outside?
The cloacal membrane
68
What separates the cloaca into the the urogenital sinus and the anorectal canal?
The cloaca is then subjected to anteroposterior subdivision - a wedge of mesoderm grows down into the cloaca, dividing it into the urogenital sinus anteriorly and the anorectal canal posteriorly
69
What supplies the blood, nervous, and lymphatic drainage for the anal canal above the pectinate line?
```
IMA
Pelvic parasympathetic (S2,3,4)
Lymphatic drainage by internal iliac nodes
```
70
What type of cell is present above the pectinate line of the anal canal?
Columnar epithelium
71
What supplies the blood, nervous, and lymphatic drainage for the anal canal below the pectinate line?
Pedendal artery
S2,3,4 pudendal nerves
Lymphatic drainage by superficial inguinal nodes
72
What type of cell is present below the pectinate line of the anal canal?
Stratified epithelium
73
What is a consequence of having 2 embryonic tissues contributing towards the anal canal?
The only possible sensation is stretch above the pectinate line, whilst below the tissue is sensitive to temperature, touch and pain.
74
Describe how visceral referred pain might show for foregut and its derivatives
Epigastric pain
75
Describe how visceral referred pain might show for midgut and its derivatives
Periumbilical
76
Describe how visceral referred pain might show for hindgut and its derivatives
Suprapubic
77
Name some possible hindgut abnormalities
Imperfoate anus
Anal/anorectal agensis
Hindgut fistulae
78
List some structures that retain their mesenteries
Jejunum, ileum, appendix, transverse colon, sigmoid colon
79
List some structures with fused mesenteries
Duodenum, ascending colon, descending colon, rectum (no peritoneal covering in distal 1/3)
80
What does the dorsal mesentery become?
The greater omentum, gastrolienal ligament (stomach to spleen), lienorenal ligament (spleen to kidney), mesocolon, mesentery proper (jejunal and ileal loops)
81
What does the ventral mesentery become?
The lesser omentum (foregut to liver), falciform ligament (liver to ventral body wall)
82
What nerves innovate the midgut?
Parasympathetic - vagus
| Sympathetic - superior mesenteric ganglion and plexus
83
What nerves innovate the hindgut?
Parasympathetic - pelvic (S2,3,4)
| Sympathetic - inferior mesenteric ganglion and plexus
84
What are the urinary and reproductive systems comprised of embryologically?
Intermediate mesoderm
85
What are the 3 kidney systems that form sequentially in the development of the kidney?
Pronephros
Mesonephros
Metanephros
86
What does a nephrotome consist of?
Glomerulus
Bowmans capsule
Proximal and distal convoluted tubule
Collecting duct
87
Does the pronephros ever function in humans?
No
88
What is the pronephros?
The first kidney system (never functions in humans). It is a duct that extends from the cervical region to the cloaca and drives the development of the next developmental stage.
89
What is the urogenital ridge?
Region of intermediate mesoderm giving rise to both the embryonic kidney and the gonad.
90
When is the mesonephros present?
~weeks 4-9
91
Where do mesonephric tubules develop?
Caudal to the pronephric region
92
What is the 'embryonic kidney'?
Mesonephric tubules + mesonephric duct
93
What does the mesonephric duct do, that induces the development of the definitive kidney?
Sprouts the ureteric bud
94
Which reproductive system does the mesonephric duct play a big part in?
Male
95
What does the ureteric bud induce the development of?
The definitive kidney within intermediate mesoderm of the caudal region of the embryo
96
What is the collecting system derived of embryologically?
The ureteric bud itself
97
What is the excretory component of the kidney derived from?
Intermediate mesoderm under the influence of the ureteric bud
98
What occurs, with regards to blood vessels, during he 'ascent' of the kidney?
As the kidney moves upwards it sprouts more vessels from the abdominal aorta, the original ones regress.
99
List some possible congenital kidney defects?
Ureteric bud may fail to interact with intermediate mesoderm - renal agenesis
Ectopic ureter
Splitting of ureteric bud (can be partial or complete). Symptomatic consequence is ectopic ureteral opening; multiple uretas
Multicystic kidney disease - atresia of ureter
Polycystic kidney disease - recessive (early presentation, poor prognosis)
Fistulae
Exstrophy of bladder (bladder opening onto abdominal wall)
Ectopic urethral orifices
100
What is the cloaca?
Early on in development, the GI, urinary, and reproductive tracts end in a single structure, called the cloaca.
101
What is the urogenital sinus created from?
Hindgut by urorectal septum
102
What is the urogenital sinus continuous with?
Umbilicus (urichus, median umbilical ligament)
103
What are the 3 parts of the urogenital sinus?
Upper part - future bladder
Pelvic - part of future urethra
Phallic - part of future urethra
104
What are the 4 parts of the male urethra?
Pre-prostatic
Prostatic
Membranous
Songs (phallic part of urogenital sinus)
105
What is 'hypospadias'?
Defect in the fusion of urethral folds. Urethra opens onto the ventral surface rather than at the end of the glans.
