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Organs associated with the digestive tract
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Include the salivary glands, pancreas, liver and gallbladder
Facilitate transport and digestion of food in the GIT
Salivary glands moisten and lubricate food and initiate digestion of
carbohydrates and lipids; also secrete protective bacteriostatic
substances such as IgA, lysozyme, lactoferrin
Pancreas produces digestive enzymes that act in the small intestine
and hormones required for nutrient metabolism
Liver has many roles
o Produces bile for emulsification of fat
o Protein and carbohydrate metabolism
o Metabolism of toxic drugs and substances
o Synthesis of blood plasma proteins and clotting factors
Gallbladder absorbs water from bile to concentrate it

Salivary glands
Structure
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Exocrine glands produce saliva
Three pairs of large salivary glands (parotid, submandibular, and
sublingual) also minor ones in mucosa and submucosa of oral cavity
A capsule of connective tissue surrounds each major salivary gland
the parenchyma has secretory end pieces and a branching duct
system arranged in lobules, separated by connective tissue septa
originating from the capsule
In the intralobular duct system, secretory end pieces open into
intercalated ducts lined by cuboidal epithelial cells
Intercalated ducts join to form striated ducts, which contain numerous
ion transporters; membrane of lining cells is infolded to increase surface
area
In the large salivary glands, the connective tissue contains lymphocytes
and plasma cells; the latter secrete IgA which forms a complex with
secretory component from the epithelia cells and enters into the saliva
Striated ducts converge into interlobular/excretory ducts, with
pseudostratified/stratified columnar epithelium
Main duct empties into the oral cavity, lined with non-keratinised
stratified squamous epithelium
Vessels and nerves enter the gland at a hilum and branch into the
lobules

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Capillaries surrounding the secretory end pieces are important for ANS
stimulation of the glands
PNS stimulation results in copious watery secretion (little protein
content)
SNS inhibits secretion

Different salivary glands
Parotid gland
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Branched acinar gland
Exclusively serous cells, containing secretory granules of amylase and
proline-rich proteins
Amylase hydrolyses ingested carbohydrates in the mouth
Proline-rich proteins have antimicrobial properties and bind Ca2+
which may help maintain surface enamel

Submandibular gland
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Branched tubloacinar gland, secretory portions have both serous and
mucous cells
Serous cells are the main component, few mucous cells
Some mucous tubules are capped with serous cells, known as serous
demilunes
Serous cells have lateral and basal membrane infoldings to increase
surface area
In addition to amylase and proline-rich proteins, serous cells also
secrete lysozyme which hydrolyses bacterial cell walls

Sublingual gland
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Branched tubuloacinar gland containing serous and mucous cells
Mucous cells predominate; serous cells only present in demilunes on
mucous tubules
Major salivary product is mucous, but serous demilunes also secrete
amylase and lysozyme

Minor salivary glands
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Usually mucous
B-lymphocytes secreting IgA are common within these glands

Salivary secretions
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Saliva has digestive, lubrication and protective functions; pH is normally
6
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9, acts as a buffer
Secretions can be serous, mucous or seromucous
Parotid secretion is serious, and submandibular and sublingual
secretions are seromucous; minor glands secrete mostly mucous
saliva is modified by cells of the duct system draining from the secretory
units; Na+ and Cl- is resorbed and growth factors and enzymes added

Types of cells
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The two types of secretory cells are serous cells and mucous cells
Also myoepithelial cells present

Serous cells
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Polarised, protein secreting cells, usually pyramidal shape
Joined to adjacent cells to form an acinus with a small lumen
Mostly produce digestive enzymes and other proteins

Mucous cells
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More cuboidal/columnar
Organised as tubules
Produce hydrophilic glycoprotein mucins important for moistening and
lubrication functions of saliva

Myoepithelial cells
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Located in the basal lamina of secretory units and ducts
More developed in the secretory units
Prevent distension when the lumen fills with saliva and contraction aids
secretion of the product

Taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Pancreas
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Mixed exocrine-endocrine gland producing digestive enzymes and
hormones
Thin capsule of connective tissue surrounds the pancreas, sending
septa inside the organ to divide pancreatic lobules
Digestive enzymes produced by the larger exocrine portion of the
gland, which are acinar glands
Hormones are synthesised in clusters of endocrine epithelial cells known
as pancreatic islets

Exocrine pancreas
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Exocrine acini are composed of serous cells surrounding a small lumen
Varied number of zymogen granules per cell (max after fasting)
Exocrine pancreas secretes 1
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Clinical note
Acute necrotising pancreatitis
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Proenzymes are activated and digest pancreatic tissue
Causes include infection, gallstones, alcoholism, drugs, trauma

Extreme malnutrition
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E
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kwashiorkor
Pancreatic acinar cells and other active protein secreting cells atrophy
and lose much of their RER, hindering production of digestive enzymes

Liver
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Second biggest organ after the skin
Large right lobe and smaller left lobe
Interface between digestive system and the blood; nutrients absorbed
in the digestive tract are processed for use by other parts of the body
Most blood in the liver (70-80%) comes from the portal vein, with
remaining from the hepatic artery
All the material absorbed via the intestines reach the liver by the portal
vein, apart from chylomicrons in the plasma
Liver gathers, transforms and accumulates metabolites from the blood
and for neutralising and eliminating toxic substances
Elimination occurs in the bile (exocrine secretion needed for lipid
absorption)
Liver also produces plasma proteins e
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albumin, fibrinogen and
various carrier proteins

Stroma
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Covered by a fibrous capsule of connective tissue
o Thicker at the hilum (portal vein and hepatic artery enter,
hepatic ducts and lymphatics exit)
o Vessels and ducts covered by connective tissue until
termination/origin in the portal spaces in the lobules

Hepatic lobules
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Hepatocytes are epithelial cells grouped into interconnected plates
Arranges into hepatic lobules, the structural and functional unit of the
liver
Each lobule has six portal areas at the periphery and a central vein
Portal zones at the corners of the lobules comprise connective tissue
encompassing the portal triad
o Branch of the portal vein
o Branch of the hepatic artery
o Branch of the bile duct
Portal areas also have nerve fibres and lymphatics
Hepatocytes are arranged radially around the ventral vein
Hepatic sinusoids run between the plates of cells, and drain to the
central vein
Sinusoids formed of fenestrated endothelial cells
Perisinusoidal space separates the hepatocytes from the endothelial
cells; microvilli of the hepatocytes project into the space for exchange

As well as endothelial cells, Kuppfer cells and Ito cells (fat storing) are
associated with liver sinusoids
Kuppfer cells are located between sinusoidal endothelial cells and on
the luminal surface within the sinusoids
o Break down old erythrocytes and free heme, remove bacteria
and debris, acts as APCs
Ito cells have small lipid droplets containing vitamin A; store vitamin A,
produce ECM components and regulate local immunity

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Taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Hepatocyte is an extremely versatile cell
o Protein synthesis
o Inactivation and detoxification (oxidation, methylation,
conjugation)
o Glycogen store (granules in cytosol), and glucose release
o Triglyceride store (lipid droplets)
o Gluconeogenesis (conversion of amino acids and lipids into
glucose)
o Amino acid deamination (producing urea)
o Bile secretion
Hepatocytes don’t store proteins, but continuously release them into
the bloodstream

Bile secretion and function
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Bile secretion by hepatocytes is an exocrine function; hepatocytes
take up, transform and secrete blood components into bile
Bile components include water, electrolytes, bile acids, phospholipids,
cholesterol, heme pigments e
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bilirubin
Needed for lipid emulsification, thus lipase digestion and absorption

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Most bilirubin is derived from degradation of haemoglobin from
senescent erythrocytes, which occurs mainly in macrophages of the
spleen but also those in liver sinusoids
Bilirubin is released from macrophages and binds to albumin, then
taken up by hepatocytes
Bilirubin is conjugated to glucoronate, forming bilirubin glucuronide,
which is secreted into bile canaliculi
Bilirubin glucuronide is released into the gut with bile
o Some metabolised to other pigments and excreted, giving
faeces its colour; other is resorbed and filtered from blood in the
kidneys, giving urine its yellow colour

Drug and xenobiotic detoxification
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Drugs and toxic substances can be inactivated by oxidation,
methylation and/or conjugation
Enzymes mostly in SER of hepatocytes
Glucoronyltransferase conjugates several compounds to glucoronate
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steroids, barbiturates, antihistamines, anticonvulsants

Taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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e
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Liver regeneration
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Strong capacity for regeneration, despite slow cell renewal rate
Loss of hepatocyte tissue from toxins etc
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Clinical note
Hepatic cancer
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Most malignant liver tumours from hepatocytes or
choliangiocytes
Usually associated with acquired disorders e
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Hep B/C
infection or cirrhosis
In exocrine pancreas, tumours also arise from duct epithelia

Reference
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Junqueira’s Basic Histology: Text and Atlas
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