XVI. LIVER, GALL BLADDER, PANCREAS, AND SALIVARY GLANDS

A. Liver

Slides 38, 39 (liver)

The human liver receives 2/3 to 3/4 of its blood supply from the hepatic portal vein and the remainder from the hepatic artery. The hepatic portal vein carries nearly all of the nutrients absorbed from the gut except fats and cholesterol. Oxygen supplied by the hepatic artery is essential, especially after a meal. The hepatic vein carried blood away from the liver. Bile which is produced in the liver by hepatocytes is secreted into bile canaliculi, collected by the hepatic ducts, and transported along the common bile duct.

The lobes of the liver (4 in humans) are covered by a layer of fibrous connective tissue. In most places a layer of mesothelium lies over the connective tissue capsule. The slides of pig liver (slide 39) it is easy to see the "classic lobules" within the lobes. These are areas of liver cells partitioned off by connective tissue called interlobular septa that are continuous with the capsule. Blood vessels and ducts can be found around the lobules in portal spaces (canals). In the center of each lobule is the central vein. These will not be seen in every section of each lobule. These small veins carry blood away from the lobule.

Within the lobules the hepatocytes are arranged in anastomosing sheets that are perforated by tunnels. Blood from the margins of the lobules flows slowly from the fine branches of the hepatic portal vein and hepatic artery into the sinusoids. The sinusoids are composed mainly of lining cells and Kupffer cells. After passing through the parenchyma, the blood enters the perforated walls of the central vein. The bile canaliculi are channels between the hepatocytes. They have no special walls of their own.

The canaliculi make connections with the bile ducts at the margins of the lobule via small vessels called the ducts of Hering. The sinusoids of most mammalian livers are discontinuous. They are separated from the hepatocytes by a narrow fluid filled gap called the space of Disse (perisinusoidal space). This is difficult to see in sections of normal liver with the light microscope, but is obvious in EM's. Exchanges between the blood and hepatocytes occur through this space.

In addition to the hepatocytes, Kupffer cells and lining cells (endothelial cells) there is a population of fat storing cells. These are not commonly seen in sections but they can be stained by gold impregnations for light microscopy. Before studying the human liver, observe a slide of pig liver. The interlobular septa make it easier to visualize the lobules. In human livers it is more difficult to see where one lobule begins and the other ends. Identify the following:

a. hepatocytes

b. lobules

c. Kupffer cells (EM's)

d. portal spaces and vessels in them

f. central vein

g. bile ducts (cuboidal epithelium)

h. liver (Glisson's) capsule

i. sinusoids, space of Disse (EM's)

B. Gall Bladder

Slide X (gall bladder)

The gall bladder stores bile and concentrates it by active reabsorption of water. The mechanism involves a sodium pump. A hormone (cholecystokinin) liberated from the duodenum caused the gall bladder to contract forcing the bile into the duodenum. Examine slide X of the gall bladder. When it is not distended the mucosa is extremely folded and made up of tall epithelial cells no goblet cells are present. Identify the:

a. columnar epithelium of the mucosa

b. lamina propria

c. muscularis mucosa (fibromuscular layer with many elastic fibers)

d. serosa (covers part of the organ)

C. Pancreas

Slide 41 (pancreas)

The pancreas is a compound tubulo-alveolar or acinous gland. It is both an exocrine and endocrine gland. The islets of Langerhans which are the endocrine component will be examined at a later lab. The acini or alveoli contain secretory cells of the general type called serous cells. They contain the zymogen granules. There are no mucus cells except in the larger ducts. In the center of the acini are the centroacinar cells. They are continuous with the cells of the intercalated duct. The next largest ducts are the interlobular ducts. The lobules are separated by delicate loose connective tissue fibers.

The interlobular ducts carry secretory products to the main pancreatic ducts. The entire gland is surrounded by loose connective tissue. Compared with the ducts of the salivary glands, the ducts of the pancreas are obscure. This is a good diagnostic feature to keep in mind. Examine the slide of the pancreas first under low power and note the division of the gland into lobes and lobules by connective tissue strands. Study the alveoli or acini under high power and note their morphology.

D. Salivary Glands

Slides 68, 97, 98 (parotid, submaxillary and sublingual)

There is a considerable amount of variation in the salivary glands of various mammals. The terminal cells in these compound glands are secretory and are either mucus or serous or mixed (seromucous). In some cases the ducts are also secretory. the three major salivary glands are the parotid, submaxillary (submandibular) and sublingual. The lessor glands of the oral cavity include the labial, buccal, glossopalatine and lingual glands.

The parotid gland (slide 68) of most primates is a purely serous gland. Note the connective tissue capsule, septa, and lobules. The serous secretory cells are basophilic and have round nuclei. They look similar to pancreatic serous cells. The ducts of the salivary glands are designated as intercalated and striated.

Are there any centroacinar cells in the parotid gland? The submaxillary gland is a mixed gland since it contains both mucus and serous cells in the human and monkey. Examine slide 97 and note the character of the mucus cells. They have clear cytoplasm and the nuclei are flattened in the basal part of the cell. Notice how obvious the ducts are on this slide).