23 [10-1]:Introduction

The liver is the largest gland in the body and has both endocrine and exocrine functions. The structural plan of the liver is a reflection of its vascular supply, and it is important to understand the transitional relationships between the incoming hepatic artery and portal vein, and the hepatic vein which drains blood from the liver into the inferior vena cava.

In addition to recognizing the landmarks of the classical lobule, the student should also be aware of the boundaries and axial structures of the portal lobule and hepatic acinus.

The liver has three primary functions- 

1. Secretion of bile for the emulsification of ingested fats in the small intestine.
2. Processing intermediary metabolism products such as-
        a. Storing sugars in the form of glycogen (glycogensis).
        b. Breaking down of glycogen to sugars for utilization by 
           the body (glycogenolysis).
        c. Converting fats and proteins to carbohydrate (gluconeogenesis).
        d. Deamination of amino acids with the production of urea as a by-product.
        e. Metabolism and storage of fats.
        f. Production of plasma proteins.
        g. Detoxification. 
3. The filtration of blood.

 
[10-2]:LIVER
(Pig, H&E) [#82]- Each liver lobe is surrounded by a connective tissue capsule (Glisson's capsule), and each lobe is further subdivided into (classical) lobules by loose connective tissue. While in the pig each lobule is clearly demarcated by connective tissue, in the human liver, demarkation between adjacent lobules is not clear and connective tissue can be seen mainly in the sites that contain branches of the hepactic artery, hepatic portal vein and bile duct.
  [10-3]:(Pig, H&E) [#82]- The axis of the classical or anatomical lobule is formed by the central vein (which forms the beginning of the hepatic vein). Notice that the parenchymal cells (hepatocytes) form plates that appear as cords of cells that are flanked on either side by anastomosing blood sinusoids. The sinusoids are lined by endothelial cells and phagocytic Kupffer cells. The perisinusoidal space (of Disse) is difficult to visualize but is present between the parenchymal cells and the sinusoids. The blood sinusoids receive blood from the distributing branches of the hepatic portal vein and to a lesser extent, from branches of the hepatic artery at the outer margins of the lobule. Blood moves centrally through the sinusoids toward the central vein. The central vein drains venous blood away from the lobule, into a sublobular vein that courses along at the base of the lobule. Sublobular veins unite to form larger veins which can be seen in solitary as tributaries of the main hepatic vein.
  [10-4]:(Pig, H&E) [#82]- The axis of the classical or anatomical lobule is formed by the central vein (which forms the beginning of the hepatic vein). Notice that the parenchymal cells (hepatocytes) form plates that appear as cords of cells that are flanked on either side by anastomosing blood sinusoids. The sinusoids are lined by endothelial cells and phagocytic Kupffer cells. The perisinusoidal space (of Disse) is difficult to visualize but is present between the parenchymal cells and the sinusoids. The blood sinusoids receive blood from the distributing branches of the hepatic portal vein and to a lesser extent, from branches of the hepatic artery at the outer margins of the lobule. Blood moves centrally through the sinusoids toward the central vein. The central vein drains venous blood away from the lobule, into a sublobular vein that courses along at the base of the lobule. Sublobular veins unite to form larger veins which can be seen in solitary as tributaries of the main hepatic vein.
 
  [10-5]:(Pig, H&E) [#82]- Locate a portal canal or portal triad at the marginal angles between adjacent lobules. This contains branches or tributaries of four structures which are surrounded by loose connective tissue. These four structures are- interlobular branches of hepatic portal vein, hepatic artery, bile duct, and a lymph vessel.
  [10-6]:Bile canaliculi (0.5-1.5 um wide) may be seen at higher magnifications as intercellular channels that form a "chicken wire" pattern along the surfaces of adjacent parenchymal cells. Bile passes through the canaliculi towards the periphery of the lobules where it is conducted into branches of the bile duct called bile ductules. These in turn open into interlobular bile ducts located in the portal triads.
  [10-7]:Liver (trypan blue injection) [#39]- Kupffer cells (see arrow) lining the hepatic sinusoids have been demonstrated by allowing them to phagocytose particles of the dye (trypan blue).
  [10-8]:Liver (Best's carmine stain for glycogen) [#86]- Note the small amounts of glycogen in the hepatocytes using Best's carmine stain.
  [10-9]:TEM #15-1 (Liver-Guinea Pig; Low magnification X12,800)- View showing hepatocytes. Note the parts of three adjoining bile canaliculi seen in cross-section and the typical organelles found in these cells (RER, lysosomes, peroxisomes).
  [10-10]:TEM #15-6 (Liver-Dog; Low magnification X10,500)- Cross-section view of an interlobular bile duct. The micrograph shows the lumen of the bile duct. Note the desmosomal junctions between cells, the microvilli, the basal lamina, and the surrounding connective tissue matrix. Find a Golgi complex.
  [10-11]:TEM #15-5 (Liver-Rat; Intermediate magnification X20,000)- Portions of two adjacent hepatocytes. Note the bile canaliculus.
  [10-12]:TEM #15-4 (Liver-Rat; Low power magnification)- View showing stacks of RER and the bile canaliculus. Note the appearance of lipid droplets, glycogen granules, lysosomes, mitochondria, and the space of Disse.
  [10-13]:SEM #15-7 (Liver, X6,000)- Shows bile canaliculi, microvilli, reticular fibers, and part of a sinusoidal network. The surface view shows the space of Disse between adjacent hepatocytes. Note that this scanning image (in contrast to the transmission micrographs) reveals the extracellular surface topography.
  [10-14]:Gall Bladder (H&E) [#87]- The common hepatic bile duct empties into the gall bladder just below the liver. Identify the following different tissues and cell types comprising the gall bladder.
  1. High mucosal folds (rugae) that branch or fuse with each other.
  2. Epithelial cells that are very tall and pale with a weakly specialized microvillous border (no goblet cells) line the mucosal folds.
  3. No glands (except in the neck portion of the gall bladder).
  4. Muscularis organized into interlacing bundles of smooth muscle; some are longitudinal bundles but mostly they are circular.
[10-15]:Gall Bladder (H&E) [#87]- The common hepatic bile duct empties into the gall bladder just below the liver. Identify the following different tissues and cell types comprising the gall bladder.
  1. High mucosal folds (rugae) that branch or fuse with each other.
  2. Epithelial cells that are very tall and pale with a weakly specialized microvillous border (no goblet cells) line the mucosal folds.
  3. No glands (except in the neck portion of the gall bladder).
  4. Muscularis organized into interlacing bundles of smooth muscle; some are longitudinal bundles but mostly they are circular.
[10-16]:TEM #15-2 (Gall Bladder, Cat, Low magnification view X6,000)- Shows layers of cells (microvillous border, epithelium, lamina propria, and muscle layer) comprising the gall bladder. Observe the large mucin droplets in the epithelium, and the glycogen deposits in the smooth muscle. Compare this TEM view with the previous light microscope image.
  [10-17]:PANCREAS
The pancreas is the second largest gland connected with the alimentary tract. It is also both an exocrine and endocrine gland. The main portion of the gland is comprised of pancreatic acini which elaborate exocrine secretions (digestive enzymes), while the islets of Langerhans synthesize and secrete several hormones (e.g. insulin, glucagon, somatostatin).
 
Pancreas (H&E) [#80]- The connective tissue septa, extending from a thin capsule surrounding the gland, divide the parenchyma into a number of lobules. Note interlobular ducts coursing within the dense connective tissue septa between lobules. Also identify intralobular ducts within the lobules and note that they are lined with cuboidal to low columnar epithelium. The intralobular ducts are connected with smaller ducts called intercalated ducts lined with flattened cells. These intercalated ducts take their origin from cells in the central portion of the acini called centroacinar cells.
  [10-18]:Exocrine portion of pancreas. The acinar cells are pyramidal in shape and are characterized by large zymogen granules generally located in the acidpphilic apical portion of the cells. The basal basophilic portion of the acinar cell is richly endowed with rough endoplasmic reticulum, indicative of a protein synthesizing cell.
  [10-19]:The exocrine portion of the pancreas secretes about 15 enzymes for digesting food in the small intestine (carbohydrates, lipids, phospholipids, proteins, and nucleic acids.) The pancreas secretes an alkaline fluid that raises the pH of the duodenal lumen. Occasional sections show pale-staining centroacinar cells in the hilus of the acinus in continuity with the epithelium. These centrally located cells along with cells of the intralobular ducts are thought to contribute to the electrolytes component (bicarbonate-rich fluid) of the pancreatic juice.
  [10-20]:Pancreatic duct is clearly visible.
  [10-21]:Endocrine portion. The islets of Langerhans which characterize the pancreas are randomly dispersed throughout the gland. Islets appear as aggregates of light staining cells. The islets are not heavily encapsulated and are separated from the acini by a very thin connective tissue layer. In an H&E preparation, the various cell types cannot be distinguished.
  [10-22]:TEM #15-3 (Pancreas, Rat low magnification X12,000)- Image showing an acinar cell and acinus lumen. An elaborate Golgi apparatus, and numerous zymogen granules are the principal organelles in these cells. Identify the apical and basal ends of the cell.
  [10-23]:

HISTOLOGIC LOOK ALIKES-

Gallbladder vs Small Intestine

Similarity- Both have prominent mucosal folds and smooth-muscle muscularis externas.



Gallbladder-
1. No true villi- mucosal folds resemble villi
2. No goblet cells (except in neck region)
3. Diverticula (sinuses) present
4. No muscularis mucosae
5. Smooth-muscle layers thin and interlacing
 
Small Intestine-
1. Many villi
2. Abundant goblet cells
3. No diverticula
4. Muscularis mucosae present
5. Two distinct smooth-muscle layers
 
Pancreas vs Partoid
 
Similarity- Both have serous acini with pyramidal cells.
 
Pancreas-
1. All serous acini with pyramidal cells
2. Minute acinar lumina associated with centroacinar cells
3. Few fat cells
4. Islet tissue prominent
 
Partoid-
 
1. All serous acini with pyramidal cells
2. Small acinar lumina - no centroacinar cells
3. Many fat cells