APR - WS: BLMW Supporting Digestion

Question 1

List some components of saliva.

Answer 1

• mucus
• enzymes (mainly alpha-amylase)
• anti-bacterial enzymes (such as lysozyme to prevent dental decay)
• proteins (e.g. lactoferrin)
• antibodies (e.g. IgA secreted by plasma cells in supporting connective tissue)
• inorganic ions (teeth mineralisation/repair)
• proteins to coat teeth for protection

Question 2

How much saliva does an average man produce a day?

Answer 2

600 - 1500 ml/day

Question 3

List some functions of saliva.

Answer 3

• lubricate and bind masticated food
• moisten and dissolve food (e.g. for taste)
• begin digestion (alpha-amylase, lipase)
• antiseptic (IgA & lysozyme)
• coat epithelium of oral cavity
• teeth maintenance and repair

Question 4

How are the three major salivary glands stimulated?

Answer 4

They secrete in response to physical, chemical and psychological stimuli (via parasympathetic activity).

Question 5

List the three main pairs of salivary glands.

note: there is a fourth pair that were just discovered

Answer 5

Parotid: mainly serous, producing thin watery secretion rich in enzymes and antibodies

Sublingual: predominately mucous cells producing a viscid secretion

Submandibular: Contains both serous and mucous secretory cells

*Tubarial: Just discovered

Question 6

List some minor salivary glands.

Answer 6

• buccal
• labial
• lingual
• palatine

Question 7

How do exocrine glands get their secretions to the epithelial surface?

Answer 7

They secrete substances onto the epithelial surface via a duct system.

The substance is secreted from the acini system, and these secretions are then further modified and conducted through the ductal system to be sent to the epithelial surface.

Question 8

What are the two types of acini in salivary glands?

Answer 8

Serous acini: more watery secretions, enzymes, etc.

Mucous acini: more mucus secretions

Question 9

What are the cell types in salivary acini?

Answer 9

• Epithelial Mucous acinar cells: with mucous droplets (merocrine)
• Epithelial Serous acinar cells: with serous granules (merocrine)
• Myoepithelial cells: wrap around an acinus and contract to expel its contents

Question 10

Describe the journey that salivary secretions make to get from the acini to the epithelial surface (and the cell types in each section).

Answer 10

acinus 
(basement membrane)
      ↓
intercalated duct
(squamous epithelium)
      ↓
striated duct
(cuboidal to columnar epithelium)
      ↓
intralobular duct
(cuboidal to columnar epithelium)
      ↓
interlobular duct
(pseudostratified columnar epithelium)
      ↓
lobar duct
(columnar stratified epithelium)
      ↓
main duct

Question 11

What would serous acini look like under histological fixing?

Answer 11

• well-stained zymogen granules

- rounded nuclei more centrally located

Question 12

What would mucous acini look like under histological fixing?

Answer 12

• poorly stained mucigen granules

- nuclei flattened against basement membrane

Question 13

What would serous demilune look like under histological fixing?

Answer 13

Serous demilune is an artifact of the fixation process.

It can be identified by mixed acinus with serous and mucus cells.

Question 14

What would intercalated ducts look like under histological fixing?

Answer 14

• lined by simple cuboidal epithelium

Question 15

Since mucous acini predominate in the sublingual gland, how do the ducts change?

Answer 15

They have short ducts as it is needed to avoid blockage by viscous secretions.

Question 16

Describe the general location and structure of the pancreas.

Answer 16

• it is a compound lobulated gland lying in the concavity of the duodenum
• it is a combined exocrine (bulk of the tissue) and endocrine gland
• it is a collagenous capsule which invaginates to form septa between lobules

Question 17

What would you expect to see on a histological slice of the pancreas?

Answer 17

• you would see that the exocrine portion of acini forms the bulk of the tissue
• the endocrine gland is in the form of Pancreatic Islets/Islets of Langerhan (not intensely stained), and embedded in exocrine tissue
• the collagenous capsule invaginates to form septa between lobules through which run the larger ducts

Question 18

Describe the Islets of Langerhans.

Answer 18

Cells in Islets are arranged in irregular cords around capillaries (because they are producing hormones that need to be delivered to the blood system).

Cells in Islets are smaller and more-lightly stained than the exocrine cells.

Question 19

What are the two main types of Islet cells, what hormone do they produce, and what does that hormone do?

Answer 19

There are two main types of cells in the Islets:

Beta cells (~70%) secrete insulin: a peptide hormone promoting glucose absorption from blood into liver, adipocytes and skeletal muscle to convert into glycogen via glycogenesis (muscle/liver) or triglycerides via lipogenesis (adipocytes/liver)

Alpha cells (~20%) secrete glucagon: peptide hormone binds glucagon receptor and causes liver to convert stored glycogen into glucose (glycogenolysis) which is released into blood to raise glucose concentration. Also increases fatty acids in blood from liver and adipose tissue.

Question 20

What are the minor types of cells of the Islets, and what do they secrete?

Answer 20

Delta cells (5-10%) secrete somatostatin.

F (or PP) cells (1-2%) secrete pancreatic polypeptide

Other, minor cell types secrete vasoactive-intestinal peptide (VIP), substance P, motilin, serotonin, etc.

Question 21

What is pancreatic juice released in response to?

Answer 21

Released in response to:

• cholecystokinin (CCK) from duodenal mucosa
• secretin from duodenal mucosa
• gastrin from stomach

Question 22

How much pancreatic juice is secreted per day?

Answer 22

Up to 1500 ml/day.

Question 23

What does pancreatic juice contain?

Answer 23

It contains many enzymes (e.g. lipase, amylase, deoxyribonuclease, and ribonuclease) and proenzymes - to prevent autodigestion - (e.g. trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase, phospholipase A2) to digest constituents of food.

It is also rich in bicarbonate ions to help neutralise acidic chyme and so optimise pH for enzyme activity.

Question 24

How is the trypsinogen proenzyme activated?

Answer 24

Duodenal enterocytes synthesises enteropeptidase (aka enterokinase) that activates trypsinogen to trypsin, a trigger enzyme that then activates other proenzymes.

Question 25

What are the two types of cells in the acini in the pancreas?

Answer 25

• acinar cells

- centroacinar cells

Question 26

Describe the acinar cells in the pancreas (i.e. what type they are, what they release, what stimuli they respond to).

Answer 26

• they are pyramidal epithelial cells with round, basally located nuclei, basophilic cytoplasm and well developed rough ER and Golgi
• they have apices with eosinophilic secretory zymogen granules containing enzymes/proenzymes - these are released into acinar lumen by exocytosis (merocrine)
• they secrete in response to cholecystokinin (CCK) from enteroendocrine cells of duodenal mucosa and gastrin from endocrine cells of stomach
• CCK acts directly on acinar cells, but also via activating vagus motor neurones for parasympathetic control of acinar cells

Question 27

Describe the centroacinar cells in the pancreas (i.e. what type they are, what they release, what stimuli they respond to).

Answer 27

• they are epithelial cells with pale nuclei
  and sparse pale cytoplasm
• they are located in the centre of acini
• they form terminal lining of
  intercalated ducts
• they secrete water and electrolytes (HCO3−, Cl−, Na+, K+) with bicarbonate ions particularly important for neutralisation of acidic chyme
• they secrete in response to secretin from the enteroendocrine cells of the duodenal muscosa

Question 28

Describe the journey that pancreatic secretions make to get from the acini to the epithelial surface.

Answer 28

acini
(simple cuboidal epithelium)
       ↓
intercalated ducts
       ↓
intralobular ducts
       ↓
interlobular ducts
(stratified cuboidal)
       ↓
pancreatic duct
       ↓
common bile duct  OR sphincter of Odi
       ↓
duodenal lumen

Question 29

List some microanatomical differences between the pancreas and salivary glands (especially the parotid).

Answer 29

• there are striated ducts in the parotid but not pancreas
• there are centroacinar cells in the pancreas but not the parotid
• there are Islets of Langerhan in the pancreas and not the parotid

Question 30

Briefly summarise the pancreatic function from when chyme enters the stomach to digestion.

Answer 30

Chyme enters the duodenum, which stimulated the mucosal cells to release CKK and secretin.

These stimulate acinar cells to release their zymogen granules, and the centroacinar cells to release electrolytes (esp. bicarbonate) into the duct system. This conveys the secretions to the duodenum where digestion occurs.

CKK also activates the parasympathetic nervous system to again stimulate the acinar cells to release their content.

Question 31

List some ofthe principal liver functions.

Answer 31

Protein metabolism:

• synthesis of non-essential amino acids.
• production of plasma proteins, e.g. albumin, clotting factors, proteins which transport lipids, iron, etc.

Carbohydrate metabolism:

• converts carbohydrates and proteins into fatty acids and triglyceride.
• regulates blood glucose concentration

Fat metabolism:
- synthesis of cholesterol, phospholipids and plasma lipoproteins.

Storage:

• iron (e.g. haemoglobin processing)
• vitamins
• glycogen

Detoxification:

• metabolic waste products - deamination of amino acids to ammonia which liver detoxifies to urea
• detoxification of steroid hormones and lipid soluble drugs, metabolism of alcohol.
• clearance of bilirubin, also from red blood cells.
• substances excrete via bile

Secretion:
- synthesis + secretion of bile (containing many products above)

Resisting infections:
- making immune factors/removing bacteria from blood

Question 32

Describe the location (and general physical structure) of the liver.

Answer 32

• it is located in the upper right of abdominal cavity, beneath diaphragm/ over the stomach, pancreas, intestines, right kidney
• it is divided into left and right lobes of 8 segments containing lobules
• it is covered by fibrous capsule (Glisson’s capsule) that invaginates as fibroelastic connective tissue to support lobules
• the lobules connect to ducts that ultimately drain into common hepatic duct

Question 33

What is the functional unit of the liver?

Answer 33

The liver lobule.

Question 34

Describe the blood supply of the liver (digestion function-wise), and how the blood travels through the liver.

Answer 34

The liver is supplied by the hepatic artery and hepatic portal vein.

The hepatic portal vein brings substances absorbed by GI tract, which acts as metabolic regulator of body. The hepatic artery brings oxygenated blood.

Terminal branches of the hepatic artery and hepatic portal vein of portal tracts/triads supply the lobules.

The blood percolates via sinusoids through liver lobules and is exposed to hepatocytes over a large surface area.

The blood is collected by a terminal hepatic (centrilobular) venule (aka central vein), which coalesce to form right and left hepatic veins that leave liver to join inferior vena cava.

Question 35

Describe what can be found in the portal tract/triad.

Answer 35

• terminal portal venule of hepatic portal vein, with a thin wall
• terminal branches of the hepatic artery (arterioles with thick walls)
• lymphatics - thin walls often collapse so hard to see
• bile ductules (lined by simple cuboidal
  epithelium)
• hepatocytes arranged in sheets around sinusoids
• sinusoids receiving blood from both the hepatic portal vein and hepatic artery

Question 36

Describe the liver sinusoids.

Answer 36

The blood flows through the liver in sinusoids.

The sinusoid walls are made of discontinuous, fenestrated epithelium: endothelial cells with flat nuclei and thin fenestrated cytoplasm.

There is a narrow space (space of Disse) between endothelial lining cells of sinusoids and hepatocytes.

Question 37

What types of cells can be found can be found in liver sinusoids?

Answer 37

Hepatocytes (epithelial) form sheets usually one cell thick accessing the space of Disse with microvilli.

Phagocytic macrophages/Kupffer cells are located in inner sinusoidal walls for removal of worn erythrocytes/debris.

Stellate cells (aka Ito or hepatic lipocytes) produce ECM/meshwork of reticulin fibres (collagen type III) that support hepatocytes and sinusoids (also store vitamin A).

Question 38

Describe the synthesis of bile.

Answer 38

Bile is an emulsifying agent in the gut to facilitate breakdown of lipids for absorption (and of fat- soluble vitamins A, D, E, K).

Bile salts are also bactericides.

Bile is synthesised from cholesterol and breakdown products of blood cells (bilirubin) destroyed by macrophages of spleen or Kupffer cells.

Question 39

Describe the delivery of bile from the bile canaliculi.

Answer 39

Bile is collected in bile canaliculi between hepatocytes that drain into canal of Hering, then bile ductules of triads, and eventually stored in gallbladder via the cystic duct.

Bile is stored and concentrated in the gallbladder.

Ultimately, the common bile duct empties into duodenum.

Bile salts are reabsorbed in the terminal part of ileum and returned to the liver by the portal system and so can be recycled.

Question 40

Describe the structure of the gallbladder.

Answer 40

It is a muscular sac situated just under liver, which stores and concentrates bile.

It has a reservoir with 100ml capacity.

It is lined by simple tall columnar epithelium with basally-located nuclei and irregular microvilli. It has a loose submucosa which is rich in elastin, blood vessels, lymphatics.

The smooth muscle is in transverse, longitudinal and oblique orientations

Question 41

Describe the function of the gallbladder.

Answer 41

It actively concentrates bile 5-10x by pumping NaCl from basolateral border to create an osmotic gradient (this displaces basolateral surface causing splits between cells).

The water goes into the lymphatics in lamina propria.

Question 42

How is the release of contents from the gallbladder controlled?

Answer 42

CCK (Cholecystokinin) is released from neuroendocrine cells of duodenum mucosa in response to lipids.

CCK causes the smooth muscle of gall bladder to contract.

The bile discharges into the duodenum via the cystic duct and common bile duct, controlled by sphincter of Oddi.

Bile acts as an emulsifying agent to facilitate hydrolysis of lipids by pancreatic lipases.

Question 43

Briefly summarise the microanatomy of the liver.

Answer 43

The nutrient rich blood arrives at the portal vein from the GI tract, and the oxygen rich blood from the hepatic artery, both of which percolate through the sinusoids, having maximum exposure to the hepatocyte, and then exits via the centrilobular vein.