: Liver, Salivaries, and Pancreas
Typical liver lobule, with cords of parenchymal cells radiating from central vein. (H&E stain).
Liver lobule outlined by blue connective tissue septa. Central vein is in the center. portal canals lie out at the "corners", in the connective tissue. (Mallory stain)
Portal canal (triad) as seen in liver of most mammals, including humans. The largest vessel is a branch of the portal at upper left and right of portal area are two branches of bile duct with simple cuboidal lining; between them lies a branch of the hepatic artery with pink tunica media (smooth muscle). The irregular-shaped empty spaces with endothelial linings are lymphatics.
Another portal area with branches of the portal vein (largest), the hepatic artery (to right), and bile duct (above, center - with cuboidal epithelium)
Diagram of arrangement of lobule. A portal triad is at the left. Blood in the branches of the hepatic artery and portal vein enters the sinusoids, between the cords of liver cells, and courses toward the central vein, which is a tributary of the hepatic vein. Bile flows in the opposite direction, from the center out, toward the tributaries of the bile duct. Bile canaliculi are tiny channels which exist between the cell surfaces of neighboring hepatic parenchymal cells. "Parenchyma" means the working cells of an organ - in this case, the hepatic cord cells (hepatocytes). They are supported by a delicate connective tissue stroma.
Detail of hepatic cords with sinusoids coursing between them. The lining endothelial cells have pulled away from the cord cells somewhat, leaving an enlarged space of Disse, which ordinarily could be seen only at the EM level. The nuclei associated with the sinusoids here may belong either to endothelial cells or to macrophages (the Kupffer cells). The blue at the top of the field is connective tissue of Glisson's capsule; this is out at the edge of the liver.
Diagram of EM of liver cell and its contacts. Notice that some surfaces lie next to a space of Disse and a sinusoid (as at the top), while others are next to another hepatocyte and include a small bile channel (see left and right sides). The bile canaliculi have no other wall than the cell surfaces of the of the hepatocytes.
EM of bile canaliculus -- the empty spot between adjacent hepatocytes. Essentially, it is a widening of the intercellular space. Notice the typical organellar composition of the adjoining hepatocytes cytoplasm.
Detail of silvered bile canaliculi. Notice the little hollow channels lying between adjacent hepatocytes. Where the channels are cut in cross- section you'll see tiny circles. Clearly, these canaliculi form an all- pervasive network throughout the liver. Between the double rows of hepatocytes lie sinusoids with black red blood cells in them.
Detail of hepatic cords stained in ordinary H&E, showing a cross-cut bile canaliculus between two cells near the upper right corner of the field. (Look for a tiny oval opening in the horizontal line between cells near the top edge of the field; a dark, tight junction seals the oval at each end.) Large intercellular spaces elsewhere in the field are sinusoids.
Kupffer cells lying in sinusoids and loaded with phagocytized carbon particles.
EM of liver cell (hepatocyte), showing aggregates of glycogen (1). Compare their size with the ribosomes lining the endoplasmic reticulum (2).
Wall of gall bladder, showing high, branching mucosal folds. These are not villi. The rest of the wall contains connective tissue and thin strands of smooth muscle.
Detail of mucosal fold with very clear simple columnar epithelium - no goblet cells! While the epithelial cells do tend to have some microvilli on their apical border, these do not form the concentrated brush (or striated) border that is typical of cells lining the intestinal villi.
Low power view to show the typical structure of a compound tubulo- alveolar gland. The pink, solid-looking masses are lobules composed of secretory alveoli (acini). Separating the lobules from each other are pale connective tissue septa. Running within the septa (as best seen in the upper left corner of this picture) are darkly staining interlobular ducts, which can be seen here and there invading the lobules as intralobular ducts. As these smaller ducts continue to branch within the substance of the lobule, they ultimately end in secretory units, the alveoli (or acini).
Interlobular area of gland, showing several structures lying in the connective tissue. Both a longitudinal and cross cut of an interlobular duct are toward the bottom of the field, with their typical, two-layered stratified cuboidal epithelial lining. Above are several blood vessels filled with dark pink-staining red blood cells. In the upper center is quite a large parasympathetic ganglion packed with neurons. Lobules of glandular tissue are at the periphery of the field.
Mixed salivary gland, showing branching intralobular ducts.
Diagram of secretory units at the end of a small duct of salivary gland. Mucous and serous units are the things to notice here plus a serous demilune capping the mucous tubule at the right. ("Demilune" means "half moon.") Compare the position of the nuclei within serous vs. mucous cells.
Secretory acini of mixed salivary gland (such as the submandibular gland). (The parotid gland is wholly serous.) In this stain the serous units are dark, the mucous ones light. The bright pink line to the right is a thin connective tissue interlobular septum.
In the middle is a dark, serous demilune capping a mucous acinus.
Mixed salivary gland stained with Mallory to show the bright blue areolar connective tissue stroma that supports the acini and carries capillaries and nerves. The finest fibers would be reticular. The acinar cells, both mucous and serous, constitute the parenchyma of the gland.
Special fixation to show zymogen granules in the serous acini; these cells are producing protein secretions (enzymes, etc.). Mucous acini are white. A pale pink duct (intralobular) is at the left.
High magnification of mucous acinus with small lumen in the center. In the serous acini occupying most of the rest of the field, red zymogen granules are distinct.
Panoramic view of pancreas showing the same kind of lobular structure seen in the salivary gland. Note interlobular vessels in the connective tissue septa.
Another view of pancreas, with thin connective tissue septa dividing the parenchyma into lobules of secretory acini. Intralobular ducts are present but are not as clearly defined as in the salivary glands. The pancreas is essentially a serous gland. It is distinguishable from the parotid gland by the presence of scattered, pale islets of Langerhans, which constitute the endocrine portion of this organ.
Higher magnification of a pale islet of Langerhans surrounded by secretory acini. Most of the islet cells are beta cells, which secrete insulin directly into the blood stream.
Very high power view of pancreatic acinar cells with some pale centroacinar cells in the middle of the acinus. The centroacinar cells represent the very end of the duct system, projecting into the lumen of the acinus. Only the pancreas has such an arrangement. Notice that the zymogen granules of the acinar cells lie at the apical ends; nuclei are toward the base. The granules are precursors of pancreatic digestive enzymes. These acinar cells, like the serous acinar cells of the salivary gland, are structurally like Paneth cells of the small intestines and chief cells of the stomach; all of them secrete enzymes.
EM detail of cytoplasm of pancreatic acinar cell loaded with lamellae of rough endoplasmic reticulum, typical of a protein-secreting cell. The cell nucleus is at upper left.