IA MVST HISTOLOGY
TEXT FOR CAL MODULE ON THE
LIVER, PANCREAS AND SALIVARY GLANDS
This program will focus on the structure of the liver, the pancreas and the salivary glands
in mammals.
This CAL module is designed to run alongside the activities scheduled for this class.
Supplementary information may be obtained from your Histology handbook, Demonstration
material, and from your Homeostasis lecture notes.
1. LIVER
LM of pig liver (very low power)
The liver is a gland arranged into several hundred lobules (L), the structural units.
In cross-section, the liver lobules are polygonal in shape. Their boundaries are demarcated by a
thin layer of collagenous supporting tissue (S), as shown in this LM of pig liver.
In humans and most other mammals, the supporting tissue in the lobules boundaries is minimal
or absent. Therefore, the boundaries are not well defined (see next LM of rabbit liver).
LM of rabbit liver (low power)
At the corners of the lobules boundaries, in areas known as portal tracts (PT), terminal
branches of the portal vein, hepatic artery and bile duct run together.
Portal tracts are the entry sites of blood from both the portal vein and hepatic artery systems
into sinusoid capillaries.
Blood flows from the portal tracts, along the sinusoids, to the central vein.
Blood from the sinusoids drains to the terminal hepatic venule, or central vein, (V) located at the
centre of each lobule. Terminal hepatic venules drain to the hepatic vein and inferior vena cava.
LM of rabbit liver (low power) – (Continues)
The hepatocyte is the main cell of the liver parenchyma.
In each lobule, the hepatocytes (H) are arranged in sheets, usually one cell thick, which
branch and link with each other.
The hepatocyte sheets are separated by sinusoid capillaries (Sc) (shown as empty spaces in this
LM).
The hepatocyte sheets (H) and the sinusoids (Sc) radiate out from the central vein or terminal
hepatic venule (V) to the periphery of the lobule.
LM of rabbit liver (low power) – (Continues)
This LM shows a typical portal tract, containing terminal branches of the portal vein (PV),
hepatic artery (A), bile duct (B), and lymphatic vessels (L), supported by connective tissue.
Because branches of the portal vein, hepatic artery and bile duct are always identified in
tissue preparations, portal tracts are often referred to as portal triads. Lymphatics, on the other
hand, are often collapsed and therefore not easily identified.
The bile ducts (B) of the portal tracts merge into larger ducts, which ultimately drain bile from
the liver into the duodenum, via the common bile duct.
LM of rabbit liver (low power) – (Cont.)
Hepatocytes are polyhedral in shape, have round central nuclei and prominent nucleoli. Some
hepatocytes are binucleated.
Bile is synthesized by the hepatocyte and secreted into a network of bile canaliculi (Bc),
which drains into small bile ducts (not seen). These, in turn, drain into the ducts of the portal
triads.
Bile canaliculi are small channels formed by the plasma membranes of apposing hepatocytes.
In this LM, bile canaliculi (Bc) are stained brown (a fortuitous result from using ironhaematoxylin stain).
LM of rabbit liver (low power) – (Cont.)
The liver sinusoids (Sc) have a wide lumen, and gaps between the endothelial cells (not seen at
this magnification). These gaps allow direct contact between the plasma and the hepatocyte
surface.
Large macrophages (Kupffer cells, K) are anchored to the endothelium. Kupffer cells
participate in the removal of damaged red cells, bacteria and other particulate debris from the
circulation.
At least two of the hepatocyte surfaces are bathed directly in blood plasma.
This mediates a very important function of the liver - to monitor and maintain the normal level
of most plasma proteins.
Electron micrograph of liver
The ultrastructure of the liver reflects its many complex functions (amongst them, synthesis
of bile and plasma proteins, nutrient and vitamin metabolism, inactivation of toxins and other
substances).
Under EM, the cytoplasm of hepatocytes appears crowded with a variety of organelles: rER,
sER, free ribosomes, Golgi apparatus (G), mitochondria (M), lysosomes (Ly) and peroxisomes.
Depending on the nutritional status, lipid (L) droplets and glycogen (g) granules are present in
variable amounts.
Note the bile canaliculus (Bc), formed by the plasma membrane of adjacent hepatocytes, tightly
bound by junctional complexes. Microvilli (Mv) project into the canaliculus.
Electron micrograph of liver (Cont.)
This EM illustrates the close association between the sinusoids and the hepatocytes.
The hepatocytes are separated from the sinusoids by the space of Disse (D), which is continuous
with the sinusoid lumen via gaps (g) between the sinusoid lining cells.
Microvilli (Mv) protrude from the hepatocyte surface to the space of Disse, increasing the
surface area for metabolic exchange.
This arrangement enables the hepatocyte surfaces adjacent to the sinusoids to be bathed in
plasma.
2. PANCREAS
The pancreas is a mixed exocrine and endocrine gland:
1. The exocrine units are the pancreatic acini or end-pieces.
The pancreatic acini secrete pancreatic juice, an alkaline fluid rich in digestive enzymes and
electrolytes. The pancreatic juice enters into the duodenum.
2. The endocrine units, known as the islets of Langerhans, are dispersed throughout the
exocrine pancreas.
The main hormones secreted by the endocrine pancreas are insulin and glucagon, with smaller
quantities of somatostatin and pancreatic polypeptide.
LM of Pancreas
The pancreas is surrounded by a capsule of loose supporting tissue rich in collagen.
Extensions of supporting tissue from the capsule into the pancreas divide it into lobules.
Excretory ducts (D), nerves and blood vessels are found in the areas of supporting tissue.
About 80–85% of the volume of the pancreas is occupied by the exocrine component.
The endocrine component makes up about 1–2% of the volume.
Pancreatic acini are composed of clusters of epithelial cells, pyramidal in shape. Their apices
surround a small lumen that is the site where the intercalated duct begins.
The intercalated duct is the smallest branch of a merging duct system. Centroacinar cells (Cc)
are sometimes found marking the start of the intercalated duct.
The cytoplasm of acinar cells is basophilic (stained blue in this LM) at the base due to the
abundant rER.
The apices of the cells (stained pink) contain numerous eosinophilic granules filled with inactive
enzymes.
Note the intercalated ducts lined by squamous or low cuboidal cells. Intercalated ducts
drain into intralobular ducts.
Intralobular ducts drain into interlobular ducts.
As the ducts increase in size the epithelium changes to stratified cuboidal.
EM of acinar cells
Acinar cells are pyramidal in shape with a basally located nucleus.
The cytoplasm is packed with rough ER (rER), reflecting abundant protein synthesis (mostly
pancreatic enzymes).
The enzymes are stored in zymogen granules (Z) of various electron-densities. Mature granules
are much more electron-dense than newly formed granules and tend to aggregate in the
cytoplasm near the acinar lumen.
A prominent Golgi apparatus (G) and scattered mitochondria (m) are present.
The endocrine portion of the pancreas is composed of clusters of hormone-secreting cells,
called islets of Langerhans, scattered throughout the exocrine tissue.
The islets make up about 1-2% of the volume of the pancreas.
The islets are variable in size and tend to have an oval shape.
Islet cells are smaller than acinar cells and have a pale-staining cytoplasm.
In humans, there are over 1 million islets.
Resin section – Pancreas
The pancreatic islets have an abundant blood supply with arterioles feeding a network of
fenestrated capillaries that run between the secretory cells.
Blood from the islets drains into the hepatic portal vein.
3. SALIVARY GLANDS
There are three pairs of major salivary glands in mammals (the parotid, submandibular and
sublingual glands), and numerous minor salivary glands scattered throughout the oral cavity.
Salivary glands are divided into lobules (L) by supporting tissue extensions (septa, s) from a
surrounding capsule (C).
Each lobule contains many salivary secretory units.
LM of cat submandibular salivary gland
The secretory units are arranged into terminal branched acino-tubular structures called
salivary end-pieces or acini (see your Handbook for types of secretory end-pieces).
Salivary acini (A) may contain clusters of mucus-secreting cells, serous-secreting cells or a
mixture of both, depending on the gland.
This LM of a submandibular gland shows mostly mixed acini, with predominance of mucous
cells.
The acinar cells merge to form intercalated ducts (not seen), the smallest of a branching duct
system, which drain into larger 'striated' ducts (their name due to their striated appearance
under light microscopy).
The composition of saliva, secreted by the acinar cells, is modified by specialised cells of the
intercalated and striated ducts.
This LM shows examples of both serous (S) and mucous (M) acini, or end-pieces.
Mucous cells have flat, basally located nuclei and poorly stained cytoplasm.
Serous cells have round, centrally located nuclei and strongly stained cytoplasm.
In mixed acini, serous cells often form crescent-shaped caps, called demilunes (d), around the
mucous end-piece.
LM - resin section of rat parotid salivary gland
The parotid salivary glands are composed almost exclusively of serous (S) secretory acini, or
end-pieces, as shown in this LM.
Serous cells are pyramidal or cuboidal in shape and secrete a watery solution containing
enzymes, antibodies and inorganic ions. Their cytoplasm is strongly stained.
Note the excretory ducts (D) of various sizes and the blood vessels (v).
The histological structure of the parotid gland is reminiscent of that of the pancreas. Both
glands are composed of serous (protein-secreting) acini. However, parotid salivary glands have
a more prominent duct system.
Salivary glands are richly innervated and contain a profuse blood supply.
Many blood vessels, nerves, ganglions and excretory ducts are present, supported by connective
tissue (s).
--------------------------------------------------------------------------------------------------------------------This LM shows an area of supporting tissue containing an artery (Ar), a large excretory duct
(D) with stratified cuboidal epithelium, a small parasympathetic ganglion (g) and nerve fibres
(n).
Intralobular excretory ducts (not seen) drain into larger interlobular ducts (D).
-----------------------------------------------------------------------------------------------------------------This micrograph shows an excellent example of a parasympathetic ganglion in the salivary
gland parenchyma, surrounded by secretory acini with mucous (M) and serous (S) cells.
Note the numerous large nerve cell bodies (b) and axons (n).
The parasympathetic nervous system is the main regulator of both salivary gland secretion
and growth.
Stimulation of the parasympathetic nervous system increases salivary gland secretion and blood
flow.
Salivary glands also contain lymphatic vessels (L) in areas of supporting tissue, as shown in
this micrograph of a parotid gland.