Lysosomes: These are membrane-bound bodies that are morphologically heterogeneous. They are mostly spherical, electron-dense structures varying in size from 0.2 to 0.5 microns in diameter. They contain a variety of lytic enzymes. Most of these enzymes are active only at unphysiologic (acidic) hydrogen ion concentrations. Acid phosphatase serves as a histochemical marker for lysosomes. Their main role is the intracellular digestion of senescent orgenelles and phagocytosed material and in the metabolism of certain substances such as glycogen, cerebrosides, gangliosides, sphingomyelin and others.