S3: salivation, swallowing & more embryology

Question 1

Describe the role of rotation of the mid-gut loop

Answer 1

Midgut grows faster than the abdominal cavity & by week 6 it protrudes through the abdominal wall & into the umbilical cord
Herniated midgut forms a loop with the superior mesenteric artery within the umbilical cord – distal part of the loop develops a caecal bulge & proximal part becomes convoluted
Midgut loop rotates while in the umbilical cord & then returns in the abdomen around week 10 (undergoes three 90 degrees rotations – small intestine on left and large intestine on right)

Question 2

Describe malrotation and reversal rotation

Answer 2

Malrotation = only one 90-degree rotation -> left sided colon
Reversal rotation = transverse colon behind small intestine – one 90-degree rotation clockwise

Question 3

Describe omphalocoele

Answer 3

Persistence of physiological herniation of midgut
Midgut structures have peritoneal coverings -> not exposed to amniotic fluid
Mortality is high due to other developmental abnormalities

Question 4

Describe gastroschisis

Answer 4

Failure of closure of the abdominal wall following folding of the embryo which results in gut tube & derivates outside the body cavity
Contents not covered in peritoneum -> exposed to amniotic fluid -> negatively affects gut development
Mortality is lower than omphalocoele

Question 5

List the different vitelline duct abnormalities

Answer 5

Vitelline cyst - patent middle section of the vitelline duct
Vitelline fistula - connection remains fully intact (discharge of faecal material out of umbilicus)
Meckel’s diverticulum

Question 6

Describe Meckel’s diverticulum

Answer 6

Persistent yolk sac remnant in the midgut
Rule of two’s – 2% of the population, located 2 feet proximal to ileo-caecal valve
Detected in under 2’s
2:1 ratio M:F

Question 7

What happens if recanalization of intestines fails?

Answer 7

Initially developing intestines have no lumen, canalisation, occurs in weeks 6-8
If this fails -> partial or full obstruction
Recanalisation occurs in the oesophagus, bile duct & small intestine
Partial = stenosis, unsuccessful = atresia

Question 8

Describe pyloric stenosis

Answer 8

NOT a recanalisation failure but the hypertrophy of the pyloric sphincter
Causes projectile vomiting in infants

Question 9

Describe the importance of rupturing the cloacal membrane

Answer 9

At first the hindgut ends blindly at the cloacal membrane, which separates it from the proctodaeum (ectoderm which covers the external anal opening)
When the membrane ruptures (around 7th week), the hindgut is connected to the exterior; therefore, the anal canal has a dual origin
Cloaca = region at the end of the hindgut that divides into an anterior urogenital sinus & a posterior anorectal canal

Question 10

Describe the division of the anal canal

Answer 10

Divided into superior and inferior parts by the pectinate line – regions differ in vasculature, nerve supply, lymphatic drainage & histological features
Above = vague pain, below = localised pain

Question 11

Describe abnormalities of the hindgut

Answer 11

Imperforate anus - failure to rupture anal membrane
Anorectal agenesis – problems with blood supply causing part of hindgut to not form
Fistulae – abnormal connection between two epithelial lined hollow cavities (eg. bladder and vagina)

Question 12

Describe the functions of saliva

Answer 12

1) Lubrication – mucus content which allows swallowing & speech
2) Protection – buffers acid, washed away debris stuck in teeth, antibacterial (lysosomes: break down bacterial cell walls & lactoferrin: reduces iron availability)
3) Digestion – salivary amylase & lingual lipase

Question 13

What are the different salivary glands and the components of saliva they secrete?

Answer 13

Exocrine glands

1) Parotid gland (25%) – serous & lots of enzymes
2) Sublingual (5%) – mucous & enzymes (but less than parotid)
3) Submandibular (70%) – serous & mucous

Question 14

Explain the mechanisms of secretion of serous saliva

Answer 14

Saliva produced in the acinus of salivary gland
Myoepithelial cells on the acinus contract to push saliva out
Ductal modification = movement of sodium & chloride ions in > than movement of potassium & bicarbonate ions out -> creates a hypotonic solution as ductal cells are relatively impermeable to water
Saliva is secreted

Question 15

What is the difference of saliva composition at rest and active?

Answer 15

At rest – low flow rate, increased contact time with ductal cells -> most hypotonic solution
Active – high flow rate, decreased contact time with ductal cells (more secretion of bicarbonate ions) & least modification -> less hypotonic solution

Question 16

Describe the control of salivary secretion

Answer 16

Autonomic control

1) Sympathetic – less watery
2) Parasympathetic – parotid gland (glossopharyngeal), submandibular & sublingual (facial nerve)

Question 17

Describe pathology of salivary secretion

Answer 17

Xerostomia – not enough saliva
-causes: medications, autoimmune, dementia, radiotherapy & dehydration
-can cause infections, tooth decay and halitosis
MUMPs – inflammation of the parotid gland
Sjogrens (autoimmune) – dry mouth & swollen/painful salivary glands
Salivary stones – becomes calcified causing pain when swallowing
-most common location is Whartons duct

Question 18

List the three phases of swallowing

Answer 18

Oral
Pharyngeal
Oesophageal

Question 19

Describe the oral phase

Answer 19

Voluntary
Preparing the bolus
Move bolus to oropharynx
Tongue movements used to move the bolus posteriorly in the oral cavity

Question 20

Describe the pharyngeal phase

Answer 20

Involuntary
Soft palate elevates -> protects nasopharynx
Pharyngeal constrictors move bolus downwards
Larynx elevates -> closes the epiglottis
Vocal cords adduct
Relaxation of upper oesophageal sphincter

Question 21

Describe the oesophageal phase

Answer 21

Involuntary
Upper oesophageal sphincter closes
Rapid peristalsis in oesophagus

Question 22

What is dysphagia? How do you differentiate between the different causes?

Answer 22

Dysphagia = difficulty swallowing
Problems coordinating swallowing – material entering respiratory tract & swallowing FLUIDS is hard
-causes: cerebrovascular accident, Parkinson’s disease, multiple sclerosis etc
Physical blockage – swallowing FOOD is hard
-causes: fibrous rings, oesophageal cancer & achalasia (failure of LOS to relax)

Question 23

Describe the importance of the urorectal septum

Answer 23

During 4th to 7th week, urorectal septum descends orderly and separates the cloaca into the anterior and posterior anorectal spaces
Mesoderm

Question 24

What is the proctodeum?

Answer 24

A layer of ectoderm overlying a depression where the anus will form

Question 25

Describe the sensory innervation and blood supply above and below the pectinate line

Answer 25

Above - S2, S3, S4 pelvic parasympathetics & inferior mesenteric artery
Below - S2, S3, S4 pudendal nerve & pudendal artery