Lecture 9 - Embryology 2

Question 1

Formation of the Midgut. when does it happen?

Answer 1

week 5 .
rapid elongation of dorsal mesentery - primary intestine loop which communicates with the yolk sac through the vitelline duct
has cranial caudal limbs
cranial - distal duodenum, jejunum and proximal ilieum
caudal - distal ileum, caecum, appendix, ascending colon and proximal 2/3 transverse colon

Question 2

Physiological Herniation and Rotation of the Midgut. when does it happen?

Answer 2

week 6
rapid elongation of midgut - growth of liver
not enough room in the abdomen, therefore, the primary intestinal loop herniates into the umbilical cord

As herniation occurs, the midgut also rotates 90° anti-clockwise - cranial limb- right
caudal limb to the left

jejunoileal loops form

Question 3

When does the midgut return to the abdomen and what happens then?

Answer 3

week 10
midgut returns to the abdomen and rotates a further 180° anti-clockwise
brings the proximal jejunal loops to the left side and the caecum lies inferior to the liver

Question 4

what does the caecum develop

Answer 4

develops a wormlike diverticulum – vermiform appendix

Question 5

what happens to the vitelline duct during rotation

Answer 5

obliterated during this process.

Question 6

what is the total rotation undergone by the midgut by the time the midgut has completely returned to the abdomen?

Answer 6

anticlockwise 270 degrees

Question 7

Descent of Caecum

Answer 7

the caecum descends from below the liver to the right iliac fossa after midgut returns to abdomen

Question 8

What happens to the ascending and transverse colon as a result of the descent of the caecum

Answer 8

gets pulled down

Question 9

What happens to the dorsal mesentery of the ascending and descending colons

Answer 9

shortens and degenerates

pulls them against posterior abdominal wall - secondarily retroperitoneal

Question 10

what causes the variable position of the appendix?

Answer 10

descent of caecum

majority - retrocaecal position

Question 11

how is the appendix suspended?

Answer 11

by a mesentery
it is relatively mobile
can project inferiorly towards pelvic brim - affect symptoms and site of appendicitis

Question 12

meckel’s/ileal diverticulum

Answer 12

A remnant of the vitilline duct that creates an outpocketing of the ileal wall
most common GI malformation - affects 2%
Usually asymptomatic but may contain ectopic pancreatic or gastric tissue causing inflammation, ulceration and bleeding

Question 13

rule of 2’s for meckel’s diverticulum

Answer 13

Affects 2% population
2 times more common in males 
2 feet (50 cm) from ileocaecal junction
2 inches (3-6 cm) long
Symptomatic in 2% cases

Question 14

Omphalocele

Answer 14

Failure of midgut to return to abdomen in week 10
increased mortality - cardiac and neural tube defects
prenatally diagnosed
varies in size
different to gastroschisis

Question 15

Non-rotation of the midgut

Answer 15

undergoes initial 90° anti-clockwise rotation but fails to rotate a further 180°
small intestine on the right side and large intestine on the left
Usually asymptomatic

Question 16

Reversed Rotation of the Midgut

Answer 16

Initial 90° anticlockwise rotation however, gut the rotates 180° clockwise
Total rotation - 90° clockwise instead of 270° anti-clockwise
Gut enters abdomen in correct order except duodenum lies ventral to transverse colon

Question 17

Abnormal Rotation of the Midgut and Volvulus

Answer 17

cause ports that would normally be retroperitoneal (e.g. duodenum) to remain suspended by dorsal mesentery
can lead to volvulus - twisting
acute obstruction of the bowel and bilious vomiting
constrict arterial supply to the gut causing ischaemia and infarction

Question 18

formation of the hindgut

Answer 18

the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and cranial 2/3 anal canal
distal end of hindgut - anorectal canal
ventral part - urogenital hiatus (bladder, pelvic urethra etc)

Question 19

When and how does the urorectal septum form?

Answer 19

weeks 4-6

layer of mesoderm extends caudally to separate the urogenital sinus and anorectal canal

Question 20

how does the perineal body form?

Answer 20

week 7, the cloacal membrane ruptures creating the anal opening and a ventral opening for the urogenital sinus
The tip of the urorectal septum lies between them and forms the perineal body

Question 21

what is the cloacal membrane derived from

Answer 21

ectoderm

Question 22

formation of the anal canal

Answer 22

Become continuous when cloacal (anal) membrane degenerates

Question 23

what is the upper 2/3rd of the anal canal derived from

Answer 23

The upper 2/3 is derived from hindgut – endoderm

Question 24

what is the lower 1/3rd of the anal canal derived from

Answer 24

proctodeum - ectoderm

Question 25

what does the pectinate line mark?

Answer 25

Junction between endoderm and ectoderm derivatives

Question 26

Congenital Rectourethral and Rectovaginal Fistulas

Answer 26

Uncommon, affects 1 in 5,000 births
Caused by abnormal cloaca e.g. too small or failure of urorectal septum to extend caudally
Opening of hindgut is shifted ventrally to the urethra in males and the vagina in females

Question 27

Imperforate Anus

Answer 27

Failure of anal membrane to degenerate

Usually requires immediate surgery to allow evacuation of faeces

Good long term prognosis in the majority of cases

Question 28

Innervation of the GIT

Answer 28

innervated by the enteric nervous system (division of autonomic nervous system):

Myenteric (Auerbach’s) plexus between the circular and longitudinal muscle layers co-ordinates muscle contraction

Submucosal (Meissner’s) plexus between the circular muscle and mucosa and regulates secretion

Question 29

What is the enteric nervous system derived from?

Answer 29

derived from neural crest cells (ectoderm origin) that migrate from neural tube to GIT

Question 30

Hirschsprung Disease/Congenital Aganglionic Megacolon

Answer 30

Failure of neural crest cells to migrate to bowel

Absence of enteric ganglia leads to bowel obstruction due to lack of peristalsis

This causes dilation of the aganglionic part of the bowel – usually rectum and sigmoid colon

Question 31

What is Hirschsprung Disease/Congenital Aganglionic Megacolon usually associated with

Answer 31

Genetic condition most commonly associated with trisomy 21

Question 32

Treatment for Hirschsprung Disease/Congenital Aganglionic Megacolon

Answer 32

Only effective treatment is to remove the affected bowel and anastomose the remaining healthy bowel with the anus