Dynamics and Stages of the Necrotic Process
Tissue death goes through several consecutive stages. First, paranecrosis develops—reversible changes followed by necrobiosis, dominated by catabolic and hydrolytic reactions. Following cell death, autolysis occurs—the breakdown of the dead substrate by lysosomal enzymes. When the pH is excessively high, lysosomal membranes become labile, leading to protein denaturation and the development of coagulative necrosis.
In the early stages, changes are typically indistinguishable both visually and under light microscopy. Reliable ultrastructural signs of irreversibility can only be detected using electron microscopy.
Macroscopic Features: Consistency and Color
Late stages of necrosis present various macroscopic features, including changes in color, tissue density, and specific odors:
- Liquefactive (wet) necrosis: Dead tissue is soft, fluid-rich, and undergoes myomalacia.
- Coagulative (dry) necrosis: The necrotic zone is dense, dry, and the tissue may become mummified.
The color of necrotic masses depends on the presence of blood, pigments, and the demarcation inflammation zone, which creates a characteristic red-brown rim. Typical dead tissue has a white or yellowish hue. When infiltrated by blood, colors range from red and brown to green due to the transformation of hemoglobin-derived pigments; bile staining is also possible. Putrefactive breakdown produces a foul odor.
Infarcts and Their Variants
An infarct is a specific type of necrosis that develops due to the cessation or reduction of blood flow. Depending on the mechanism and location, three main forms are distinguished:
- White infarct: Characteristic of the spleen and brain.
- White infarct with a hemorrhagic rim: Found in the heart and kidneys, where the rim forms due to demarcation inflammation.
- Red (hemorrhagic) infarct: Develops in the lungs (often in the setting of chronic venous congestion) as a result of blood soaking into the necrotic tissues.