Examination of hepatocytes with the TEM shows the Golgi apparatus to be much more elaborate than those seen in routine histologic specimens. Heavy-metal preparations (Golgi stains) of thick sections of liver give an indication of the extent of the Golgi network. As many as 50 Golgi units, each consisting of three to five closely stacked cisternae, plus many large and small vesicles, are found in hepatocytes. These “units” are  actually branches of the tortuous Golgi apparatus seen in heavy-metal preparations. Elements of the Golgi apparatus concentrated near the bile canaliculus are believed to be associated with the exocrine secretion of bile. Golgi cisternae and vesicles near the sinusoidal surfaces of the cell, however, contain electron-dense granules 25 to 80 nm in diameter that are believed to be VLDL and other lipoprotein precursors. These substances are subsequently released into the circulation as part of the endocrine secretory function of the hepatocytes. Similar dense globules are seen in dilated portions of the sER and, occasionally, in the dilated ends of rER cisternae, where they are synthesized.
Lysosomes concentrated near the bile canaliculus correspond to the peribiliary dense bodies seen in histologic sections.
Hepatocyte lysosomes are so heterogeneous that they can only be positively identified, even at the TEM level, by histochemical means. In addition to normal lysosomal enzymes, TEM reveals other components:
Hepatocyte lysosomes may also be a normal storage site for iron (as a ferritin complex) and a site of iron accumulation in certain storage diseases.
FIGURE 18.13 • Scanning electron micrograph of the luminal surface of the bile duct. Bile duct is lined by epithelial lining cells called cholangiocytes. Their apical surfaces exhibit numerous short microvili projecting into the lumen of the bile duct. Each cholangiocyte possesses a long primary cilium which sense changes in luminal flow of the bile. Note that all cilia are bent in the same direction of a bile flow. 3,600. (Courtesy of Dr. Tetyana V. Masyuk.)
The number of lysosomes increases in a variety of pathologic conditions, ranging from simple obstructive bile stasis to viral hepatitis and anemia. However, although the range of normal liver function—particularly the rate of bile secretion—is quite wide, no statistically significant morphologic changes take place in the Golgi apparatus or lysosomes of the peribiliary cytoplasm to correlate with the rate of bile secretion.
The biliary tree is the three-dimensional system of channels of increasing diameter that bile flows through from the hepatocytes to the gallbladder and then to the intestine. In the adult human liver, there are more than 2 kilometers of interconnecting bile ductules and ducts of different sizes and shapes. These structures are not only passive conduits, but they are also capable of modifying bile flow and changing its composition in response to hormonal and neural stimulation.
The biliary tree is lined by cholangiocytes which monitor bile flow and regulate its content.
Cholangiocytes are epithelial cells that line the biliary tree. When examined in TEM, cholangiocytes are identified by their organelle-scant cytoplasm, presence of tight junctions between adjacent cells, and presence of complete basal lamina. An apical domain of cholangiocytes appears similar to hepatocytes with microvilli projecting into the lumen. In addition, each cholangiocyte contains primary cilium that sense changes in lumenal flow resulting in alterations of cholangiocytes secretion (Fig. 18.13). Small bile ductules are lined by small cholangiocytes, mainly cuboidal in shape, but as the diameter of the bile duct increases, they become progressively larger and more columnar in shape.