CELL INJURY AND DEATH By Dr.K.V.Bharathi

Cell injury Results when cells are stressed so severely that they are no longer able to adapt or when cells are exposed to damaging agents. Cell injury can be reversible or irreversible.

Reversible cell injury Functional and morphologic changes are reversible if the damaging stimulus is removed. The features are:decreased oxidative phosphorylation,ATP depletion and cellular swelling.

Irreversible injury and cell death. With continuing damage,injury becomes irreversible. Cells undergo morphologic changes recognisable as cell death. Cell death is of 2 types-necrosis and apoptosis.

Reversible Irreversible L M Changes Cell death E M Changes Effect Cell Function Gross Changes Duration of injury

Reversible and irreversible injury

Necrosis With severe membrane damage,lysosomal enzymes enter cytoplasm & digest cell wall. Cellular contents leak out. Necrosis is always pathologic.

Apoptosis Programmed cell death. Noxious stimuli that damage DNA result in nuclear dissolution without complete loss of cell membrane integrity. Can be physiologic or pathologic.

Causes of cell injury Oxygen deprivation. Physical agents eg:mechanical trauma,burns,deep cold,barotrauma,electric shock. Chemical agents & drugs eg:poisons,environmental pollutants,CO,asbestos,alcohol,narcotic drugs etc. Infectious agents-viruses,rickettsiae,bacteria,fungi,protozoa and helminths. Immunologic reactions-anaphylaxis,autoimmune disorders. Genetic derangements. Nutritional imbalances-PEM,obesity,specific vitamin deficiencies etc.

Mechanisms of cell injury Depletion of ATP-affects activity of Na,K-ATPase pump.This results in anaerobic glycolysis. Mitochondrial damage-leakage of cytochrome-C into cytosol,resulting in apoptosis. Influx of Ca & loss of Ca homeostasis,leading to activation of ATPases,phospholipases,proteases & endonucleases.

ATPases Hasten ATP depletion. Phospholipases cause membrane damage. Proteases breakdown membrane & cytoskeletal proteins. Endonucleases cause DNA & protein fragmentation.

Free radical injury Oxygen derived. Free radicals are chemical species that have a single unpaired electron in an outer orbit. Energy created by this unstable configuration is released through reactions with adjacent molecules. They initiate autocatalytic reactions.

Free radicals are initiated by: Absorption of radiant energy(u-v or ionising):Water is hydrolysed to(OH)&(H) free radicals. Enzymatic metabolism of exogenous chemicals or drugs ,eg:CCl4 converted to CCl3. Redox reactions in the cell: (O2),(H2O2) &(OH). Transition metals like iron and copper. Nitrous oxide.

Effects of free radicals: Lipid peroxidation of membranes. Oxidative modification of proteins. Lesions in DNA.

Inactivation of free radical reactions. Can be enzymatic or non-enzymatic. Enzymatic:catalase,superoxide dismutases,glutathione peroxide. Non-enzymatic:-Antioxidants eg: Vitamins A,E,C & glutathione . -Binding of iron & calcium with storage & transport proteins eg:transferrin,lactoferrin,ceruloplasmin. Defects in membrane permeability can affect mitochondria,plasma membrane,other cellular membranes.

Mechanisms Mitochondrial dysfunction. Loss of membrane phospholipids. Cytoskeletal abnormalities. Reactive O2 species. Lipid breakdown products.

Earliest changes in reversible cell injury are: Decreased generation of ATP. Loss of cell membrane integrity. Defects of protein synthesis. Cytoskeletal damage. DNA damage. Within limits,the cell can compensate for these derangements.Persistent or excessive injury leads to irreversible injury.

Increase of free radicals Ischemia Cell oxygen tension Membrane injury Loss of phospholipids, Increase of free radicals Lipid breakdown Reduced ATP Incr. Glycolysis Decr. Protein systhesis Decr glycogen Lipid deposition Leakage of cell enzymes Calcium influx Intracellular lysosomal enzyme release Reversible injury: cell swelling microvlli loss blebs ER swelling myelin figures chromatin clumping Irreversible injury Reduced basophilia Nuclear changes Protein digestion

Characteristic phenomena of irreversibility: Inability to reverse mitochondrial dysfunction. Development of profound disturbances in membrane function. Therefore,in cardiac muscle death there is leakage of CKMB & troponin. In injury to bile duct epithelium & liver,serum alkaline phosphatase is raised. In hepatocyte injury,transaminases are raised.

Light microscopic patterns of reversible cell injury: Cellular swelling & fatty change. Morphology in cellular swelling-Gross:Pallor,increased turgor & increase in organ weight. Micro:small clear vacuoles seen within cytoplasm. Fatty change-seen in injured myocardial cells and hepatocytes.There is appearance of small or large lipid vacuoles in the cytoplasm.

Necrosis A spectrum of morphologic changes that follow cell death in living tissue,due to progressive degradative action of enzymes on the lethally injured cells. Leaked out contents of necrotic cells may elicit inflammation in the surrounding tissue. The morphologic appearance is due to denaturation of proteins and enzymatic digestion. The enzymes are derived from lysosomes of the dead cells themselves-AUTOLYSIS.

Pathology of necrotic cells Increased eosinophilia with a glassy homogeneous appearance as a result of glycogen particles. Moth-eaten appearance in cells with vacuolated cytoplasm due to enzymatic digestion of cytoplasmic organelles. Calcification of dead cells. Replacement of dead cells by whorled phospholipid masses called myelin figures(ultrastructurally).

Nuclear changes in necrotic cells: Karyolysis-basophilia of chromatin may fade. Pyknosis-nuclear shrinkage and increased basophilia. Karyorrhexis-nuclear fragmentation. Disappearance of nucleus.

Coagulative necrosis Denaturation of protein is the primary pattern. Basic cell outline is preserved for some days eg:acute MI.