Lecture 9 - Embryology 2 Flashcards
Formation of the Midgut. when does it happen?
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
Physiological Herniation and Rotation of the Midgut. when does it happen?
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
When does the midgut return to the abdomen and what happens then?
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
what does the caecum develop
develops a wormlike diverticulum – vermiform appendix
what happens to the vitelline duct during rotation
obliterated during this process.
what is the total rotation undergone by the midgut by the time the midgut has completely returned to the abdomen?
anticlockwise 270 degrees
Descent of Caecum
the caecum descends from below the liver to the right iliac fossa after midgut returns to abdomen
What happens to the ascending and transverse colon as a result of the descent of the caecum
gets pulled down
What happens to the dorsal mesentery of the ascending and descending colons
shortens and degenerates
pulls them against posterior abdominal wall - secondarily retroperitoneal
what causes the variable position of the appendix?
descent of caecum
majority - retrocaecal position
how is the appendix suspended?
by a mesentery
it is relatively mobile
can project inferiorly towards pelvic brim - affect symptoms and site of appendicitis
meckel’s/ileal diverticulum
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
rule of 2’s for meckel’s diverticulum
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
Omphalocele
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
Non-rotation of the midgut
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
Reversed Rotation of the Midgut
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
Abnormal Rotation of the Midgut and Volvulus
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
formation of the hindgut
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)
When and how does the urorectal septum form?
weeks 4-6
layer of mesoderm extends caudally to separate the urogenital sinus and anorectal canal
how does the perineal body form?
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
what is the cloacal membrane derived from
ectoderm
formation of the anal canal
Become continuous when cloacal (anal) membrane degenerates
what is the upper 2/3rd of the anal canal derived from
The upper 2/3 is derived from hindgut – endoderm
what is the lower 1/3rd of the anal canal derived from
proctodeum - ectoderm
what does the pectinate line mark?
Junction between endoderm and ectoderm derivatives
Congenital Rectourethral and Rectovaginal Fistulas
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
Imperforate Anus
Failure of anal membrane to degenerate
Usually requires immediate surgery to allow evacuation of faeces
Good long term prognosis in the majority of cases
Innervation of the GIT
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
What is the enteric nervous system derived from?
derived from neural crest cells (ectoderm origin) that migrate from neural tube to GIT
Hirschsprung Disease/Congenital Aganglionic Megacolon
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
What is Hirschsprung Disease/Congenital Aganglionic Megacolon usually associated with
Genetic condition most commonly associated with trisomy 21
Treatment for Hirschsprung Disease/Congenital Aganglionic Megacolon
Only effective treatment is to remove the affected bowel and anastomose the remaining healthy bowel with the anus
