Session 3 Flashcards
What is the primary intestinal loop?
The midgut elongates enormously.
Runs out of space – Makes a loop that:
- has the SMA as its axis
- is connected to the yolk sac by the vitelline duct
- has cranial and caudal limbs
Why in the 6th of foetal growth week do we see herniation?
- During 6th week, growth of the primary intestinal loop is very rapid – elongation
- Liver is also growing rapidly – abdominal cavity is too small to accommodate both so intestines herniate into the umbilical cord.
Describe midgut rotation and descent of the caecal bud.
As midgut loop herniates through umbilicus there is a single 90 degree counter-clockwise turn around the superior mesenteric artery.
There’s then a further 90 degree rotation which inverts the loop so cranial portion is now caudal in position.
As herniation is resolved there is a final 90 degree rotation. 270 degrees anti clockwise in total.
This ensures the cranial limb is positioned so that it returns to the abdominal cavity first so it and its derivates are pushed to the left hand side. The caudal limb then returns second.
As the loop is herniated, it’s continuing to elongate and develop. After folding there should be complete resolution of the herniation.
The caecel bud descends down along the right hand side of the abdomen to give us the ascending colon.
This process of rotation and descent ensures normal disposition of the abdominal viscera. (Transverse colon should be in front of the duodenum). It also minimises movement of duodenum as its plastered to posterior wall.
How might malrotation occur?
- Incomplete rotation – Midgut loop makes only one 90° rotation resulting in left-sided colon
- Reversed rotation – Midgut loop makes one 90° rotation clockwise resulting in transverse colon passing posterior to the duodenum
What are the risks associated with midgut defects?
- Most complications present in the neonatal period
- Risk of volvulus (an obstruction caused by twisting of the stomach or intestine) which can lead to strangulation and then ischaemia
What happens to the remnants of the yolk stalk after herniation from primary intestinal loop rotation?
• The vitelline duct can persist resulting a number of different abnormalities e.g vitelline cyst
Meckel’s diverticulum - ileal diverticulum, the most common GI anomaly
Vitelline cyst - Vitelline duct forms fibrous strands
Vitelline fistula - Direct communication between the umbilicus and intestinal tract
What is Meckel’s diverticulum?
Ileal diverticulum left after closing of the vitelline duct.
- 2% population affected
- 2 feet from ileocaecal valve
- Usually detected in under 2s
- 2:1 ratio male:female Sometimes can contain ectopic pancreatic or gastric tissue
What is recanalisation in gut development?
- The primitive gut is a simple tube
- In some gut structures, cell growth becomes so rapid that the lumen is partially or completely occluded – affected regions: oesophagus, bile duct, small intestine
- Recanalisation occurs to restore the lumen. This is a normal process (not pathological)
- If recanalisation is wholly or partially unsuccessful, atresia or stenosis of the structure can occur
Describe possible atresias and stenoses of the GI tract
- Atresia – Lumen obliterated
- Stenosis – Lumen narrowed
- Most occur in duodenum – Most likely cause is incomplete canalisation – But “vascular accidents” may also contribute
- Pyloric stenosis – hypertrophy of the circular muscle in the region of the pyloric sphincter – NOT a recanalisation failure. Its a common abnormality of the stomach in infants – narrowing of the exit from the stomach causes characteristic projectile vomiting
Describe development of anal canal and the implications that this has
Proctodeum - A small depression in the caudal most region of the embryo.
• At the cloacal membrane in the proctodeum there’s a junction between two embryonic germ layers (ectoderm and endoderm). This cloacal membrane ruptures which gives an opening for the bladder and an opening for the hindgut. The outer covering of ectoderm at the proctodeum is pushed up into embryo so the exiting distal portion of each tract is lined with ectoderm This explains the differences between above and below the pectinate line. As superior structure is derived from hindgut (endoderm) and inferior lining is derived from ectoderm.
Above pectinate line
- Blood supply from IMA
• Innervation
– pelvic parasympathetics (S2,3 and 4)
– Sympathetic
• Inferior mesenteric ganglion and plexus
- Columnar epithelium
- Lymph. drainage = internal iliac nodes
• Below pectinate line
– Blood supply from Pudendal artery
– somatic innervation - S2,3 and 4 pudendal nerve
– Stratified epithelium
– Lymph. drainage = superficial inguinal nodes
So above the pectinate line the only sensation possible is stretch as it only has visceral sensory afferents, while below the pectinate line the tissue is temperature, touch and pain sensitive as it has somatic innervation
Describe visceral pain of GI
- Visceral pain poorly localised
- The pain pattern reflects the development of the structure
- Foregut and its derivatives – epigastrium pain
- Midgut – periumbilical pain
- Hindgut – suprapubic pain
But remember that the parietal peritoneum receives somatic innervation
Describe some important hindgut abnormalities
- Imperforate anus – Failure of anal membrane to rupture
- Anal / anorectal agenesis
- Hindgut fistulae
Which midgut and hindgut structures have retained/fused mesenteries?
- Mesenteries are retained by: – Jejunum – Ileum – Appendix – Transverse colon – Sigmoid colon
- Structures of midgut/hindgut with fused mesenteries: – Duodenum – Ascending colon – Descending colon – Rectum (no peritoneal covering in distal 1/3)
Give an overall timeline for gut development in the foetus
Briefly summarise the mesenteries?
Ventral mesentery
Lesser omentum - foregut to liver
Falciform ligament attaches liver to ventral body wall
Dorsal mesentery
– Greater omentum
– Gastrolienal ligament attaches stomach to spleen
– Lienorenal ligament attaches spleen to kidney
– Mesocolon supplies ascending and proximal 2/3 transverse colon
– Mesentery proper supplies Jejunal and ileal loops
Describe the neural control of swallowing and the gag reflex
Reflex arc for swallowing and gag reflex: Mechanoreceptors in pharyngeal wall detect presence of bolus, when these are activated, impulse is sent along glossopharyngeal nerve to medulla which sends an efferent response down the vagus nerve to pharynx which stimulates pharyngeal constrictors. After 6 months hyper active gag reflex moves back in the mouth to allow eating of solid food. Before this baby would just gag it up.
What is xerostomia?
Dry mouth
Describe the basic topography of the salivary glands
3 pairs of salivary glands: parotid glands, submandibular glands and sublingual glands
Parotid are largest and they have a duct that passes over the masseter muscle and pierces through the buccinator opening into the mouth lateral to the second top molar via the parotid duct.
Submandibular gland has a duct which opens on the medial side of the floor of the mouth whereas the sublingual salivary glands have ducts that open on the lateral side of the floor of the mouth.
Sublingual glands sit above mylohyoid muscle. To examine sublingual glands try to compress gland using find and thumb in the mouth and under the chin. Others just palpate externally.
Describe the neural control of the salivary gland
Complex regulation of salivary secretion:
- Primarily neural (autonomic) as hormonal control would be too slow.
- Parasympathetic is main driver (increases production)
- Sympathetic also stimulates secretion of small amounts of saliva, but high levels also causes vasoconstriction to cut off salivary production. Parotids receive innervation from cranial nerve IX (Glossopharyngeal nerve) and the submandibular and sublingual are innervated by cranial nerve VII (facial nerve)
Draw the Pectinate line
