Notes: Morphology of Necrosis and Apoptosis

Morphology of Necrosis and Programmed Cell Death

• Mechanisms of cell injury (overview):

  • 2 main categories: compromised membrane integrity; diminished energy production; they interact and amplify each other.

  • Energy failure → failure of Na⁺/K⁺ pumps, water and ions influx (Na⁺, Ca²⁺), mitochondrial and endoplasmic reticulum dysfunction; free radicals can damage membranes.

• Irreversible injury and cell death: general concepts

  • Irreversible injury leads to cell death with characteristic morphological changes.

  • Reversible injury: membranes largely intact; mitochondria may swell; ER swells; ribosomes detach but remain functional enough to allow recovery.

  • Irreversible injury: nucleus fragments; mitochondria may calcify or swell; ER disintegrates; ribosomes are lost; lysosomes rupture releasing lytic enzymes.

• Necrosis vs apoptosis (basic distinction)

  • Necrosis (uncontrolled injury): cell swelling, membrane rupture, inflammation; tissue damage often in groups.

  • Apoptosis (programmed cell death): energy-dependent, controlled; membranes intact during most of the process; cell fragments into apoptotic bodies; no surrounding inflammation.

• Necrosis: gross and micro features

  • Gross features: necrotic tissue tends to be lighter in color, may be swollen; inflammation around the area.

  • Histologic features of necrosis: swollen cytoplasm; clumped chromatin; nucleus may be pale (karyolysis) or fragmented (karyorrhexis); cytoplasm more eosinophilic; vacuolation; possible calcium deposition in mitochondria.

  • Nuclear changes:

  • Karyolysis: pale, washed-out nucleus.

  • Karyorrhexis: fragmentation of the nucleus.

  • Nuclei may become shrunken/condensed in some contexts.

• Types of necrosis

  • Coagulative necrosis: death is rapid but tissue architecture is preserved; cells look eosinophilic and ghost-like nuclei; common in infarcts; white/pale infarcts usually; hemorrhagic rims possible with dual blood supply.

  • Liquefactive necrosis: tissue becomes soft and liquefies due to enzymatic digestion; common in pancreas; histology shows loss of cell boundaries and nuclei debris; tissue appears mushy.

  • Caseous (cheesy) necrosis: cheese-like appearance; typical with certain bacterial infections (e.g., cheesy gland in sheep); dead tissue with a granulomatous inflammatory response.

• Necrosis as part of inflammation and repair

  • Necrosis commonly triggers inflammation: vascular engorgement and inflammatory cell infiltration.

  • Repair after necrosis can occur by fibrosis (scar formation) or by regeneration/replacement through proliferation of surrounding cells.

• Postmortem change vs ante-mortem necrosis

  • Ante-mortem necrosis: would elicit an inflammatory host response.

  • Postmortem decomposition: lack of inflammatory response; changes occur after death, influenced by temperature and environment.

• Postmortem factors affecting tissue death

  • Higher temperatures and prolonged time increase decomposition and bacterial proliferation; helps distinguish antemortem necrosis from postmortem changes.

• Controlled cell death: apoptosis vs necrosis

  • Apoptosis: programmed, energy-dependent process; active self-destruction by the cell; membrane integrity is maintained until the later stages; cell fragments into apoptotic bodies that are phagocytosed; does not provoke inflammation.

  • Necrosis/oncosis (cell injury): loss of membrane integrity; uncontrolled cell death; inflammatory response is elicited; tends to affect groups of cells rather than isolated cells.

  • Terminology notes: apoptosis is programmed cell death; oncosis is often used for injury-driven necrosis; in common usage here, apoptosis = programmed death; necrosis = injury-driven degeneration.

• Morphology of apoptotic cells

  • Cells shrink, show membrane blebbing, and form apoptotic bodies.

  • Nucleus is typically dense and may show chromatin condensation.

  • Plasma membrane remains intact during early stages; no leakage of intracellular contents; minimal inflammation.

• Biological roles and triggers of apoptosis

  • Physiological: tissue remodeling (e.g., mammary gland involution after lactation), limb/tan development, organ sculpting during development.

  • Pathological: infection, malignant transformation, immune cell-induced killing (cytotoxic T cells induce apoptosis in infected or abnormal cells).

  • Some pathogens inhibit apoptosis to prolong host cell survival; reverse is also observed in immune responses.

• Practical contrasts

  • Apoptosis vs necrosis: apoptosis = single cells or small clusters; necrosis = groups of cells affected.

  • Inflammation: absent in apoptosis; present in necrosis.

  • Energy dependence: apoptosis requires energy and intact mitochondria; necrosis results from acute energy failure and membrane damage.

• Quick takeaways for exam review

  • Necrosis is an uncontrolled form of cell death with inflammation; includes coagulative, liquefactive, and caseous patterns.

  • Coagulative necrosis preserves tissue architecture; infarcts are classic examples.

  • Liquefactive necrosis involves enzymatic digestion of tissue; pancreas and brain are common sites.

  • Caseous necrosis yields a cheese-like appearance in certain bacterial infections.

  • Apoptosis is a controlled, energy-dependent process with intact membranes and no inflammation; important in development, remodeling, immune defense, and cancer biology.