Overview
This topic covers the non-organelle cytoplasmic inclusions and the vesicular transport of material into and out of the cell.
Cellular inclusions are accumulations of substances in the cytoplasm that are not organelles: stored nutrients, secretory products, pigments and crystals. This topic covers their definition, classification and appearance in sections, the methods used to show them, and clinically important examples.
Definition and classification of inclusions
Inclusions are products of the cell's own metabolism or material taken up from outside. Older texts call them paraplasm or metaplasm. Unlike organelles, they are not metabolically active structures, and the cell can survive without them.
- They are temporary: their amount changes with the functional state of the cell. Liver glycogen, for example, increases after a meal and falls during fasting.
- Many have no surrounding membrane (glycogen, lipid droplets, most exogenous pigments); others lie in membrane-bound compartments (secretory granules, lipofuscin, hemosiderin).
- Several are dissolved or poorly seen in routine paraffin sections stained with hematoxylin and eosin, so special stains or frozen sections are needed.
Textbooks draw the boundary differently: some treat secretory granules as part of the secretory pathway rather than as inclusions, and some list the hemoglobin of erythrocytes among the pigments.
Some texts also name excretory inclusions: metabolic end products waiting to be removed from the cell.