Inclusions consist largely of secretory vesicles, pigment granules, neutral fat, other lipid droplets, and glycogen

Inclusions are cytoplasmic or nuclear structures with characteristic staining properties. They are considered "nonliving" components of the cell. Some of them, such as secretory vesicles and pigment granules, are surrounded by a plasma membrane; others, e.g., lipid droplets and glycogen, are not and reside within the cytoplasmic or nuclear matrix.

Secretoiy vesicles and neutral fat often constitute most of the cytoplasmic volume, compressing the other formed organelles into a thin rim at the margin of the cell. This structure is typical of the intestinal goblet cell (see Fig. 16.23, page 497) and the adipose cell of connective tissue (see Fig. 6.2, page 159). Cells with regulated secretoiy pathivays store proteins within large vesicles in their cytoplasm for hours or even days. These vesicles do not fuse with the plasma membrane until the cell is activated for ex-ocytosis by an external signaling mechanism (see page 28).

Glycogen may be seen in the light microscope only after special fixation and staining procedures. It is usually lost during routine processing of tissue for light microscopy. Liver and striated muscle cells, which usually contain large amounts of glycogen, may display empty regions where the glycogen was localized. Glycogen appears in electron micrographs as granules 25 to 30 nm in diameter or as clusters of such granules that often occupy significant portions of the cytoplasm (Fig. 2.53).

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