Embryology Flashcards
What structures does the embryological midgut form? How is the midgut supplied by blood?
MIDGUT (continuous with yolk sac) - superior mesenteric artery & vein, vagus nerve (PS), superior mesenteric ganglion & lesser splanchnic plexus (S)
- duodenum (distal to entrance of bile duct)
- jejunum
- ileum
- caecum
- ascending colon
- proximal 2/3 transverse colon
What structures does the embryological foregut form? How is the foregut supplied?
FOREGUT (blind diverticula) - coeliac trunk, greater splanchnic nerve plexuses
- oesophagus
- stomach
- pancreas, liver, gallbladder
- duodenum (proximal to entrance of bile duct)
What structures does the embryological hindgut form? How is the hindgut supplied by blood?
HINDGUT (blind diverticula) - inferior mesenteric artery & vein, pelvic S2-S4 (PS), inferior mesenteric ganglion & least splanchnic plexus (S)
- distal 1/3 transverse colon
- descending colon
- sigmoid colon
- rectum
- upper anal canal
- internal lining of bladder & urethra
Describe the structure of the primitive gut tube within the embryo. Outline the development of the primitive gut tube.
3rd wk - primitive gut tube pinches off from yolk sac
Stomatodeum (future mouth) moves rostrally to becomes the proctodeum (future anus) + opening at umbilicus (?????? check)
Internal lining: endoderm (epithelium)
External lining: splanchnic mesoderm (muscle & peritoneum)
Suspended in intraembryonic coelem by double layer of splanchnic mesoderm (surrounds the new gut -> mesentery formed)
What is the definition of a mesentery?
Double layer of peritoneum attaching abdominal organs to the posterior abdominal wall, as well as supplying them with blood and lymph
Organs with a mesentery:
- jejunum
- ileum
- appendix
- transverse colon
- sigmoid colon
Organs with fused mesentery:
- duodenum
- ascending colon
- descending colon
- rectum
note: no peritoneal covering in distal 1/3 of large intestine
How does the mesentery differ along the gut tube? What do the different portions of mesentery form?
DORSAL MESENTERY suspends the entire gut tube from the dorsal body wall
- greater omentum
- gastrolienal ligament (stomach -> spleen)
- lienorenal ligament (spleen -> kidney)
- mesocolon
- mesentery proper (jejunal & ileal loops)
VENTRAL MESENTERY only present in the foregut
- lesser omentum (foregut -> liver)
- falciform ligament (liver -> ventral body wall)
note: liver develops in ventral mesentery
What is the definition of omenta?
OMENTUM = specialised double layer of peritoneum attached to the stomach, formed by rotation, which connects the stomach to other abdominal organs (liver, spleen, intestine, ++?)
Greater omentum (formed from dorsal mesentery):
- covers intestines like an apron (first structure seen when the abdominal cavity is opened)
- heat insulator & prevents friction between abdominal organs
- left sac -> greater sac -> greater omentum
Lesser omentum (formed by ventral mesentery):
- links the stomach to the liver
- free edge conducts the portal triad
- right sac -> lesser sac -> lesser omentum
Describe how the stomach rotates during development. What is the purpose of this?
Primitive stomach rotates longitudinally (wraps ventral mesentery) and around the A-P axis (cardia & pylorus move horizontally, pushing the greater curvature inferiorly, as the dorsal border grows faster than the ventral border)
- puts vagus nerves anterior and posterior instead of left and right
- shifts cardia and pylorus from the midline (so stomach lies obliquely)
- contributes to moving the lesser sac behind the stomach
- creates the greater omentum
- ventral border rotates to lie on the right, dorsal border rotates to lie on the left
- loop of duodenum rotates to the right
What does the term retroperitoneal mean? What structures are retroperitoneal?
Structures not suspended within the abdominal cavity are retroperitoneal
Primarily retroperitoneal (develop & remain outside parietal peritoneum):
- thoracic oesophagus (abodminal is intraperitoneal)
- rectum (excluding lower 1/3 = extraperitoneal)
- kidneys
- aorta & IVC
- ureters
- adrenal glands
Secondarily retroperitoneal: initially intraperitoneal, but due to expansion of the GI tract the posterior mesentery fuses with the posterior abdominal wall
-> covered anteriorly by peritoneum -> therefore can be mobilised (put hands around)
- asc. & desc. colon
- 2nd & 3rd parts of duodenum (proximal segment is intraperitoneal)
- pancreas (excluding tail)
SADPUCKER
= suprarenal gland, aorta/IVC, duodenum (2nd & 3rd part), pancreas (excluding tail), ureters, colon (asc. & desc.), kidneys, (o)esophagus, rectum
How do the trachea and oesophagus separate during development? What anomalies can result from this?
Foregut extends from lung bud to liver bud
4th wk: respiratory diverticulum forms in ventral wall of foregut at junction with the pharyngeal gut, which then develops into the tracheoesophageal seputm
5h & 6th wk: lumen of oesophagus is obliterated (recanalised by the end of the embryonic period)
Ventral: respiratory primordium
Dorsal: oesophagus
Anomalies: blind-ended (oesophageal atresia), fistulae, congenital hiatus hernia (stomach pulled up through hiatus in diaphragm as oesophagus is too short)
note: anomalies can be due to posterior displacement (spontaneous) or anterior displacement (mechanical pressure on foregut dorsal wall)
Outline the steps involved in physiological herniation and just subsequently.
Primary intestinal loop elongates and herniates into the proximal part of the umbilical cord (6th week) (divided into cranial and caudal limbs by vitelline duct & superior mesenteric artery)
90 degrees: limbs horizontal to superior mesenteric artery
180 degrees: cranial limb moves to left side
270 degrees: caudal limb (transverse colon) superior to cranial limb
360 degrees: caudal limb (transverse colon) now anterior to cranial limb (small intestine) & both have returned to the abdominal cavity (cranial first)
Caecal bud descends, creating the ascending limb of the colon
(starts in RUQ under right lobe of liver, moves to RIF)
note: jejunum returns to cavity first (to the left)
What can occur as a result of malrotation during physiological herniation?
Incomplete rotation: midgut only rotates by 90 degrees once
-> left-sided colon
Reversed rotation: midgut rotates by 90 degrees once clockwise
-> transverse colon posterior to the duodenum
+ failure of caecal bud to descend = subhepatic caecum
What can occur as a result of the vitelline duct persisting?
Vitelline cyst = vitelline duct forms fibrous strands
Vitelline fistula = direct communication between the umbilicus and the intestinal tract
Meckel’s diverticulum = ileal diverticulum
- most common GI abnormality
- 2% of pop., 2ft from ileocaecal valve, 2 inches long, usually detected in <2yrs, 2:1 male:female (true of most embryological defects)
- can contain ectopic gastric/pancreatic tissue (therefore inflammation can cause symptoms similar to appendicitis)
What is recanalisation? What results if this does not occur?
During development, organs with a lumen become partially/completely obliterated due to rapid cell growth -> these need to be recanalised during development
Failure:
- atresia (lumen completely obliterated as recanalisation does not occur)
- stenosis (lumen narrowed)
Most commonly fails in the duodenum:
Upper duodenum = failure of recanalisation
Lower duodenum = vascular accident (impaired blood supply -> loss of lumen e.g. malrotation, volvulus, body wall defect)
note: pyloric stenosis (children) is NOT due to a recanalisation failure (due to hypertrophy of circular muscle around pyloric sphincter which causes projectile vomiting)
What are some abdominal wall defects, and what are the differences between them?
GASTROSCHISIS = failure of closure of abdominal wall during folding of embryo -> gut tube & derivatives outside of body cavity -> necrosis
EXOMPHALOS/OMPHALOCOELE = persistence of physiological herniation -> gut tube & derivatives outside of body cavity (covered in amnion only) (often associated with other structural anomalies)
note: exomphalos differs from umbilical hernia as a hernia is covered with skin & subcutaneous tissue
