CELL INJURY

Ø  Normal cell has fairly narrow range of function and structure by its genetic program, by neighboring cells and availability of metabolites but handle normal physiologic demands (homeostasis.) If excessive physiologic stress or pathologic stimuli bring a physiologic and morphologic cellular adaptation, a new but altered steady state is achieved which preserve viability and functional change. When the limit of adaptive response is exceeded or adaptation is not possible a sequence of events follow known as cell injury. 

Ø  The cell injury is reversible up to certain limit but if the pathological stimuli persist or severe enough at the beginning the cell reaches the point of no return: irreversible cell injury and cell death.

Causes of cell injury

1.     Oxygen deprivation- due to hypoxia (lack of oxygen) or ischemia (lack of blood supply).

2.    Physical agents.

3.    Chemical agents or drugs.

4.    Infectious agents.

5.    Immunologic reactions.

6.    Genetic derangements

7    Nutritional imbalance.

Ø  Other factors responsible to cell injury are the duration and the severity of the injurious agent, the vulnerability and state of adaptivity of the injured cell.

Biochemical sites of action of pathological stimuli:

   There are 4 intracellular systems which are particularly vulnerable:

A.    Maintenance of the integrity of cell membrane

B.    Aerobic respiration- mitochondrial oxidative phosphorilation and ATP production.

C.    Protein synthesis

D.    Genetic apparatus of the cell.

Ø  The structural and biochemical elements of the cell are integrated so that the injury of one part brings secondary effects to other parts of the cell.

Ø  The morphologic changes of the cell injury become apparent only after some critical biochemical system has been deranged.

General biochemical mechanism

Ø  Some injurious agents attack well defined site e.g. an enzyme. Particularly vulnerable are glycolysis, oxidative phosphorilation.

Ø  Anyhow there are common biochemical systems important in the mediation of cell injury and cell death whatever the causative agent is and they are:

A.ATP depletion

Ø  It is important in many synthesis-degradation processes e.g. protein synthesis. Lipogenesis, phospholipids turnover. ATP is produced in 2 ways a) oxidative phosphorilation b) glycolitic pathway. ATP depletion is common consequence of ischemia and toxic injury.

B.  Oxygen and oxygen derived radicals

Ø  Cell generates energy by reducing oxygen to water. During this process small amount of reactive oxygen forms are produced as unavoidable byproduct in mitochondrial respiration. Some of these free radicals can damage lipid, proteins and nucleic acids.

Ø  Cells have defense system to prevent injury from the radicals; but imbalance between the production of free radicals and radical scavenging system results in oxidative stress associated by cell injury.

C.  Intracellular Ca and loss of Ca homeostasis

Ø  In the cyt, Ca concentration is low compared to extra cellular, 0.1umole/l and 1.3mmole/l respectively. In addition, Ca is sequestered in the mitochondria and ER.

Ø  Such gradient is maintained by membrane associated energy dependent Ca—Mg ATPase. Ischemia and certain toxins cause early incease of the cyt, Ca owing to net influx across the membrane and from the Mitch and ER.

Ø  Sustained Ca concentration results in non specific membrane permeability and activation of many enzymes: phospholipase (membrane damage) protease, ATPASE (hastening ATP depletion) endonuclase (fragmentation of chromatin)

D.  Defects of membrane permeability

Ø  Early loss of selective permeability is common in all forms of cell injury. Such defects can result from ATP depletion, Ca influx, bacterial toxins, virus and lyses from complements.

E.  Irreversible Mitch damage

Ø  All mammalian cells are obligate dependent of the oxidative phosphorilation regardless of their glycolitic ability. Irreparable damage of the Mitch, will ultimately kill the cell.

Ø  All types of injurious agents target mitch,

Ø  The damage is commonly expressed in mitch, permeability transition in the inner membrane and sometimes, leakage of cytochrome C, which is the integral component of the electron transport, into the cytosol.

Reversible cell injury

v  Ischemia is one of the most studied causes of cell injury.

Sequence of events in cell injury:

1) Reducing ATP results in diminishing energy dependent Na pump, Na influx and out diffusion of potassium. This results in isotonic gain of water, cell and ER swelling. A second mechanism of cell swelling is due to increased intracellular osmotic load due to catabolic accumulation of lactates and phosphates.

2) Reduction in oxygen induces to the cell to rely on anaerobic respiration glucolysis. The ATP reduction and increase of ADP stimulate phosphofructokinase and phosphorilase to increase the anaerobic respiration using the glucose from stored glycogen and hence its depletion. The glucolysis results also in accumulation of lactic acid and phosphates from hydrolysis of phosphate esters. This reduces the intracellular PH.

3) Detachment of ribosome from the rER and dissociation of poly-monoribosomes con consequent reduction in protein synthesis

Functional of Cell Injury

Functional consequence can occur in the cell injury (heart muscle stop contraction after 60 seconds of ischemia)

v Morphology of cell injury

Cell, ER, and mitochondria swell, chromatin clumping, detachment of ribosome from rER

Irreversible cell injury (IRCI)

·      If the ischemia persists irreversible cell injury ensues.

 Morphologic characteristics of IRCI:

1)     Marked mitochondrial swelling.

2)    Lysosomal swelling and their rupture.

3)    Large amorphous densities in mitochondrial matrix.

4)    Extensive damage of plasma membrane and membrane defects

5)    Nuclear picnosis

¨   After the cell death, its components is degraded with a wide leakage of enzymes in the extra cellular space and then to the blood. Extra cellular macromolecules enter in the cell. The cell become replaced by a large mass of phospholipids known as myelin figures that are phagocytosed by other cells or further degraded into fatty acids. When these fatty acids combine with calcium, soaps are formed.

¨   Leakage of enzymes in the blood provides important clinical parameters e.g. transaminase, lactate dehydrogenase, creatine kinase.

Reperfusion injury

   Several mechanisms are proposed:

1)     The reperfusions bring large amount of oxygen which the damaged mitochondria cannot assimilate causing formation of oxygen free radicals. The oxidases of WBC also contribute this process.

2)    Reactive oxygen promotes mitochondrial permeability transition and reduces its ability to form ATP.

3)    Injured tissue (endothelial cells) produces cytokines and adhesive molecules which attract neutrophiles to the site of injury.

FREE RADICALS

CELL INJURY INDUCED BY FREE RADIACLS

¨    They are chemicals with single unpaired electron in their outer orbit. They unstable and react adjacent molecules whether organic or inorganic, like proteins, lipids, carbohydrates particularly key molecules of the membrane and nucleic acid.

¨    They initiate autocatalytic reactions that are the molecule they react with is converted into free radicals propagating the chain of damage.

Sources of free radicals

¨   Absorption of radiant energy like U.V, X rays and gamma rays. They can hydrolysis water into hydrogen and hydroxyl ions as free radicals.

¨   Chemical or drugs like carbon tetrachloride.

¨   In oxidation- reduction reactions the reduction of oxygen into water small amount of toxic intermediates forms such as super oxide, hydrogen peroxide and hydroxyl ions.

¨   Transitional metals like (H2 O2+ Fe and Cu donate or accept electrons and catalyze free radical formation as in Fenton reaction (H2 O2+ Fe2+ =Fe3+ + HO- + HO-).

¨   Nitric oxide

Effects of free radical relevant to cell injury are:

1)     Lipid peroxidation of the membrane

¨    The damage begins when the double bonds of the unsaturated fatty acids of the membrane lipids is attacked by free radicals especially the hydroxyl ions resulting in the production of peroxides which are unstable and reactive con consequent autocatalytic process chain and propagating damage. This causes extensive membrane damage.

¨    When the radicals are blocked by scavenging system like vitamin E the outcome is favorable. 

2)   Oxidative changes of the proteins

   Oxidation of amino acids produces side chain formation and fragmentation of the protein backbone. This brings the destruction of enzymes and multicatalytic proteasome complex.

3)   DNA LESION

   The radicals react with thymine producing single stranded breaks in the DNA.

FREE RADICAL REMOVAL

1.     Antioxidants: vitamins E, A, C and glutathione.

2.    Iron and copper bind the free radicals to transport and storage proteins.

3.    Many enzymes which act as scavenging system e.g. catalase, GSH found at the site of free radical production.

4.    Are inherently unstable and decay spontaneously.  

 

Morphology of cell injury

Reversible cell injury

Two patterns can be recognized in light M: cell swelling and fatty change.

•      Macroscopically– pallor, turgor and increase in weight.

•      Microscopically– small clear vacuoles (distended ER.)

TEM–

1.     Plasma membrane changes e.g. blebing, distortion of microvillus, myelin figures and losing intercellular attachment                                                  

2.    Mitochondria swelling and amorphous densities.

3.    Dilatation of rER with detachment of the polyribosome

4.    Disaggregation of granular and fibrillar chromatin.