Figure 253

Electron micrographs of a liver cell with glycogen inclusions, a. Low-magnification electron micrograph showing a portion of an hepato-cyte with part of the nucleus (N, upper left). Glycogen (C) appears as irregular electron-dense masses. Profiles of rough endoplasmic reticulum (rER) and mitochondria (M) are also evident, x 10,000.

b. This higher-magnification electron micrograph reveals glycogen (G) as aggregates of small particles. Even the smallest aggregates (arrows) appear to be composed of several smaller glycogen particles. The density of the glycogen is considerably greater than that of the ribosomes (lowerleft). x52,000.

Lipid inclusions (fat droplets) are also usually extracted by the organic solvents used to prepare tissues for both light and electron microscopy. What is seen as a "fat droplet" in light microscopy is actually a hole in the cytoplasm that represents the site from which the lipid was extracted.

Crystalline inclusions contained in certain cells are recognized in the light microscope. In humans, such inclusions are found in the Sertoli (sustentacula!") and Leydig (interstitial) cells of the testis. With the TEM, crystalline inclusions have been found in many cell types and in virtually all parts of the cell, including the nucleus and most cytoplasmic organelles. Although some of these inclusions contain storage material or remnants of cellular structures, the significance of others is not clear.

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