In addition, some cells (e.g., osteoclasts involved in bone resorption and neutrophils involved in acute infammation) may release lysosomal enzymes directly into the extracellular space to digest components of the extracellular matrix.
Lysosomes in some cells are recognizable in the light microscope because of their number, size, or contents.
The numerous azurophilic granules of neutrophils (white blood cells) represent lysosomes and are recognized in aggregate by their specific staining.  Lysosomes that contain phagocytized bacteria and fragments of damaged cells are often recognized in macrophages.
Hydrolytic breakdown of the contents of lysosomes often produces a debris-filled vacuole called a residual body that may remain for the entire life of the cell. For example, in neurons, residual bodies are called age pig-
FIGURE 2.21 • Pathways of delivery of materials for digestion in lysosomes. Most of the small extracellular particles are internalized by both receptor-mediated endocytosis and pinocytosis. These two endocytic patways are labeled with red arrows. Large extracellular particles such as bacteria and cellular debris are delivered for cellular digestion via the phagocytotic pathway (blue arrows). The cell also uses lysosomes to digest its own organelles and other intracellular proteins via the autophagic pathway (green arrows). Intracellular particles are isolated from the cytoplasmic matrix by the isolation membrane of the sER, transported to lysosomes, and subsequently degraded.
ment or lipofuscin granules. Residual bodies are a normal feature of cell aging. The absence of certain lysosomal enzymes can cause the pathologic accumulation of undigested substrate in residual bodies. This can lead to several disorders collectively termed lysosomal storage diseases (see Folder 2.1).
Autophagy represents the major cellular pathway in which a number of cytoplasmic proteins, organelles, and other cellular structures are degraded in the lysosomal compartment (Fig. 2.22). This important process maintains a  well-controlled balance between anabolic and catabolic cell functions and permits the cell to eliminate unwanted or unnecessary organelles. Digested components of organelles are recycled and reused for normal cell growth and development.