What is reperfusion injury? Discuss the mechanism of reperfusion injury.

Reperfusion injury:

Restoration of blood flow to ischemic tissues can promote recovery of cells if they are reversibly injured, but  can also paradoxically exacerbate the injury and cause cell death.

Mechanism:

The likely answer is that new damaging processes are set in motion during reperfusion, causing the death of cells that might have recovered otherwise. Several mechanisms have been proposed.

• Oxidative stress. New damage may be initiated during reoxygenation by increased generation of reactive oxygen and nitrogen species.
• Intracellular calcium overload. Intracellular and mitochondrial calcium overload  begins during acute ischemia; it is exacerbated during reperfusion due to influx of calcium resulting from cell membrane damage and ROS mediated injury to sarcoplasmic reticulum.
• Inflammation. Ischemic injury is associated with inflammation as a result of “dangers signals” released from dead cells, cytokines secreted by resident immune cells such as macrophages, and increased expression of adhesion molecules by hypoxic parenchymal and endothelial cells, all of which act to recruit circulating neutrophils to reperfused tissue.
• Activation of the complement system may contribute to ischemia-reperfusion injury.

Write down the mechanism of irreversible cell injury.

Mechanisms of cell injury are as follows:

• Depletion of ATP: Reduction in ATP levels is fundamental cause of necrotic cell death. ATP depletion and decreased ATP synthesis are frequently associated with both hypoxic and chemical injury. The  major causes of ATP depletion are reduced supply of oxygen and nutrients, mitochondrial damage, and the actions of some toxins.
• Mitochondrial Damage: Mitochondria are critical players in cell injury and cell death by all pathways. Mitochondria can be damaged by increases of cytosolic Ca2+, reactive oxygen species, oxygen deprivation and mutations in mitochondrial genes in some inherited diseases.
• Influx of Calcium and Loss of Calcium Homeostasis: Calcium ions are important mediators of cell injury.
• Accumulation of Oxygen-Derived Free Radicals: Cell injury induced by free radicals, particularly reactive oxygen species, is an important mechanism of cell damage in many pathologic conditions, such as chemical and radiation injury, ischemia-reperfusion injury, cellular aging, and microbial killing by phagocytes.
• Defects in Membrane Permeability: Early loss of selective membrane permeability, leading ultimately to overt membrane damage, is a consistent feature of most forms of cell injury. Membrane damage may affect the functions and integrity of all cellular membranes.
• Damage to DNA and Proteins.

What are the morphologic changes in cell injury.

Morphologic changes in reversible cell injury:

Light microscopy:

Cellular swelling is the first manifestation of almost all forms of injury to cells. It causes some pallor, increased turgor, and increase in weight of the organ. On microscopic examination, small clear vacuoles may be seen within the cytoplasm; these represent distended and pinched-off segments of the ER. This pattern of nonlethal injury is sometimes called hydropic change or vacuolar degeneration. Cells may also show increased eosinophilic staining, which becomes much more pronounced with progression to necrosis. 

The ultrastructural changes of reversible cell injury include: 

1.Plasma membrane alterations, such as blebbing, blunting, and loss of microvilli 

2.Mitochondrial changes, including swelling and the appearance of small amorphous densities 

3.Dilation of the ER, with detachment of polysomes; intracytoplasmic myelin  figures may be present 

4.Nuclear alterations, with disaggregation of granular and fibrillar elements.

Consequences of Hypoxic injury

Consequences of hypoxic injury:

• As the oxygen tension within the cell falls, there is loss of oxidative phosphorylation and decreased generation of ATP. The depletion of ATP results in failure of the sodium pump, leading to efflux of potassium, influx of sodium and water, and cell swelling. 
• There is also influx of Ca2+, with its many deleterious effects. 
• There is progressive loss of glycogen and decreased protein synthesis. The functional consequences may be severe at this stage.
• The cytoskeleton disperses, resulting in the loss of ultrastructural features such as microvilli and the formation of blebs at the cell surface. 
• Myelin figures derived from degenerating cellular membranes, may be seen within the cytoplasm or extracellularly. They are thought to result from unmasking of phosphatide groups, promoting the uptake and intercalation of water between the lamellar stacks of membranes. 
• At this time the mitochondria are usually swollen, as a result of loss of volume control in these organelles; the ER remains dilated; and the entire cell is markedly swollen, with increased concentrations of water, sodium, and chloride and a decreased concentration of potassium. 
• If ischemia persists, irreversible injury and necrosis ensue.

Describe briefly the sources and possible consequences of increased cytosolic calcium in cell injury.

Sources of calcium:

1. Intracellular source: Mitochondria, Endoplasmic reticulum.

2. Extracellular source.

Consequences of increased cytosolic calcium:

• The accumulation of Ca2+ in mitochondria results in opening of the mitochondrial permeability transition pore and failure of ATP generation. 
• Increased cytosolic Ca2+ activates a number of enzymes with potentially deleterious effects on cells. These enzymes include phospholipases (which cause membrane damage), proteases (which break down both  membrane and cytoskeletal proteins), endonucleases (which are responsible for DNA and chromatin  fragmentation), and ATPases (thereby hastening ATP depletion).
• Increased intracellular Ca2+ levels also result in the induction of apoptosis, by direct activation of caspases and by increasing mitochondrial permeability.