S2: introduction to embryology of the GI tract & abdominal wall hernias Flashcards
Describe the anatomy of the inguinal canal in the male
Anterior wall – aponeurosis of the external oblique, reinforced by the internal oblique muscle laterally
Posterior wall – transversalis fascia
Roof – transversalis fascia, internal oblique & transversus abdominis
Floor – inguinal ligament, thickened medially by the lacunar ligament
Two openings – superficial (exit) and deep (entrance) rings
What is a hernia?
The protrusion of an organ or fascia through the wall of cavity that normally contains it
Describe the development of the inguinal canal and how inguinal hernias develop embryologically
Gubernaculum attaches the inferior portion of the gonad to the future scrotum or labia and guides them during the descent
Inguinal canal = pathway by which the testes leave the abdominal cavity and enter the scrotum
Processus vaginalis normally degenerates, but if it fails to do so – indirect inguinal hernia, a hydrocele or interferes with the descent of the testes
Gubernaculum becomes a small scrotal ligament, tethering the testes to the scrotum and limiting their movement
Explain how indirect inguinal hernias develop
Indirect inguinal hernias – caused by the failure of the processus vaginalis to regress
-peritoneal sac enters the inguinal canal via the deep inguinal ring -> degree to which the sac herniates depends on the amount of processus vaginalis still present
Describe the clinical presentation of an inguinal hernia
Fullness/swelling Increase in size Discomfort Nausea/vomiting Necrotic bowel -> peritonitis
Compare direct and indirect inguinal hernias
Indirect are more common & males are more commonly affected
Indirect hernias – exit the abdomen through the deep inguinal ring & enter the inguinal canal to a variable distance, some pass through to the scrotum
->pass laterally to the inferior epigastric vessels
Direct hernias – pass directly through the abdominal wall in an area of potential weakness called Hesselbach’s triangle
->pass medially to the inferior epigastric vessels
Describe the relationship between the femoral canal and the inguinal ligament
Inguinal ligament forms the anterior border of the femoral canal
Explain how femoral hernias develop and the clinical presentation
Part of the small intestine protrudes through the femoral ring
Presents as a lump situated inferolateral to the pubic tubercle -> more common in women, due to wider bony pelvis
NB: borders of femoral canal are tough -> can compress the hernia, interfering with its blood supply = strangulated hernia
Describe the anatomy and location of umbilical hernias
Commonly found in infants
Hernia at the site of the umbilicus – not usually painful
-defect in the linea alba -> umbilical cord passes through the umbilical ring
80-90% close by age 3 (no treatment unless it is painful or very large)
Describe the common incisional sites used for abdominal surgery
1) Midline incision -> through linea alba, extendable, particularly uncomfortable post-operatively
2) Paramedian -> poor cosmetics, can damage nerves/structures
3) Grid iron -> appendicectomy
4) Pfannenstiel -> obstetric & urology
5) Kocher -> open cholecystectomy (removing gallbladder)
Explain how the primitive gut, the abdominal wall and the coelomic cavity are formed
Folding of the embryo creates a primitive gut tube, lined with endoderm -> divisible into three regions: foregut, midgut and hindgut (week 3)
Mesoderm surrounding the gut splits into layers – somatic (develops into the abdominal wall) & splanchnic (smooth muscles of the gut wall)
Space created by the split = coelomic cavity
Describe the development of the coelomic cavity and peritoneal cavity
Coelomic cavity begins as one large cavity – the forerunner of the pleural cavity and peritoneal cavity
Parietal peritoneum is formed by the somatic portion of lateral plate mesoderm
Visceral peritoneum is formed by the splanchnic portion of lateral place mesoderm
The greater and lesser sacs of the peritoneal cavity are formed by the rotation of the stomach
Describe the fate of the embryonic dorsal and ventral mesenteries
The dorsal and ventral mesenteries become the various peritoneal folds and reflects and give passage to vessels and nerves in the adult
Greater omentum is derived from the dorsal mesentery
Lesser omentum is derived from the ventral mesentery
What are mesenteries?
Mesentery = double fold of peritoneum that suspends the gut tube from the abdominal wall
Dorsal mesentery – developing gut is attached to the roof of the abdominal cavity
Ventral mesentery – foregut is attached to the floor
Explain the basic development of the stomach
When the stomach develops the dorsal border develops faster giving the stomach its characteristic shape (greater curve)
Explain the basic development of the liver
The liver grows into the ventral mesentery dividing it into two parts – falciform ligament and the lesser omentum
Develops from an anterior bud off the foregut
-cranial portion: liver
-caudal portion: gallbladder
Liver’s connection to foregut is via the common bile duct
Explain the basic development of the pancreas
Foregut structure which develops from two portions
1) Dorsal portion (larger) forming most of the gland
2) Ventral portion (smaller) forming most of the duct system
Rotation of the foregut causes the ventral bud to be forced round the back & inferior to the dorsal bud -> eventually fuse and ductal systems fuse from both
Nestles in the developing C-shape of duodenum
Explain the basic development of the duodenum
Duodenum’s lumen is obliterated in the 5th and 6th weeks of its development and only recanalized by the end of the embryonic period
Describe the borders of Hesselbach’s triangle
Medial – lateral border of rectus abdominis
Lateral – inferior epigastric vessels
Inferior – inguinal ligament
Describe the borders of the femoral ring
Lateral – femoral vein
Medial – lacunar ligament
What are para-umbilical hernias?
Affects adults – hernia through the linea alba
Risk factors – obesity, greater incidence in females
Risk of strangulation
What is an incisional hernia?
Herniates through previous incision
Majority remain asymptomatic
Risk factors: previous surgery (especially emergency), obesity, midline, wound infection
What are the three sections of mesoderm and their derivatives?
Paraxial - forms somites -> skeletal muscles, vertebrae and cartilage Intermediate - kidneys & gonads Lateral plate -splanchnic: viscera -somatic: body walls
What is the vitelline duct?
Only connection from the midgut to another structure
Connects the midgut to the yolk sac
What does the splanchnopleuric & somatopleuric mesoderm form? What does the intraembryonic coelom form?
Splanchnopleuric mesoderm - forms the viscera
Somatopleuric mesoderm - forms the body walls and dermis
Intraembryonic mesoderm - thoracic and abdominal cavities
Describe the blood supply to the three sections of the gut
Foregut - coeliac artery
Midgut - superior mesenteric artery
Hindgut - inferior mesenteric artery
How does the trachea and oesophagus separate?
Lung bud off the foregut on the anterior surface
Tracheoesophageal septum separates the respiratory bud and oesophagus
Problems: blind-end oesophagus and tracheoesophageal fistula
What is the foramen of Winslow? Describe what the left and right vagal fibres form
Small space which connects the greater and lesser sacs
Left vagal fibres - anterior vagal trunk
Right vagal fibres - posterior vagal trunk
Describe the peritoneal ligaments (anterior to posterior)
1) Falciform ligament - anterior abdominal wall to liver
2) Lesser omentum - liver to stomach
3) Gastro-splenic ligament - stomach to spleen
4) Spleno-renal ligament (lieno-renal) - spleen to kidney
Explain how direct inguinal hernias develop
Direct inguinal hernias – acquired, usually in adulthood, due to weakening in the abdominal musculature
- peritoneal sac bulges into the inguinal canal via the posterior wall medial to the epigastric vessels & can enter the superficial inguinal ring
- > sac is NOT covered with the coverings of the contents of the canal
