Figure 222

Electron micrograph of autophagosomes in an hepatocyte. This electron micrograph shows several autophagosomes containing degenerating mitochondria. Note the surrounding lysosomes that had been stained with acid phosphatase, x 12,600. (Courtesy of Dr. William A. Dunn, Jr.)

and assists in its transport through the lysosomal membrane into the lumen, where it is finally degraded.

Lysosomes in some cells are recognizable in the light microscope because of their number, size, or contents

The numerous azurophilic granules of neutrophils (polymorphonuclear neutrophilic leukocytes) are 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, which may remain for the entire life of the cell. For example, in neurons, residual bodies are called "age pigment" 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. In Tay-Sachs disease, for example, which was first described in 1881, the absence of a lysosomal galactosidase (/^-hexosaminidase) in neurons produces concentric lamel-lated structures in residual bodies that accumulate in the cell and interfere with normal functions.

Rough-Surfaced Endoplasmic Reticulum

The protein synthetic system of the cell consists of the rough endoplasmic reticulum and ribosomes

The cytoplasm of a variety of cells engaged chiefly in protein synthesis stains intensely with basic dyes. The basophilic staining is due to the presence of RNA. That portion of the cytoplasm that stains with the basic dye is called ergastoplasm. The ergastoplasm in secretory cells, e.g., pancreatic acinar cells, is the light microscopic image of the organelle called the rough endoplasmic reticulum (rER).

With the TEM, the rER appears as a series of interconnected, membrane-limited flattened sacs called cisternae, with particles studding the exterior surface of the membrane (Fig. 2.23). These particles, called ribosomes, are at-

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