When the limits of adaptive capacity are exceeded or when no adaptive response is possible then it is termed as cell injury.
A. Reversible cell injury:
It denotes pathologic changes that can be reversed when the stimulus J(or stress) is removed or if the cause of injury is mild.
B. Irreversible cell injury:
It denotes pathologic changes that are permanent and cause cell death.
Causes of cell injury:
2. Chemical agents
Low oxygen to the tissue, which is due to:
Ø Ischemia (loss of blood supply) due to arterial occlusion
Ø Problem in respiratory system
Ø Problem in circulatory system
Ø Abnormality of hemoglobin
Ø Decreased tissue perfusion as in hypotension, shock and cardiac failure
All leads to hypoxic cell injury, which may be reversible or irreversible.
If the nucleus is intact then it is termed as reversible cell injury. Hypoxia affects mitochondria which results in decreased synthesis of ATP. The reversible changes include:
i. Cellular swelling
ii. Desegregation of ribosomes and failure of protein synthesis
iii. Reduced intracellular pH
iv. Appearance of myelin figures and cell blebs
i. Cellular swelling:
Ø Failure of Na, K ATPase pump due to deficiency of ATP results in accumulation of Na ions inside and K ions outside the cell.
Ø Excess Na ions inside the cell exert osmotic pressure and pulls water inside, resulting in cellular swelling.
ii. Ribosomal desegregation:
Ribosomes become detached from the rough endoplasmic reticulum due its swelling and therefore protein synthesis is reduced.
iii. Reduced intracellular pH:
Ø Lack of ATP synthesis increases rate of anaerobic respiration (anaerobic glycolysis) which leads to production of pyruvic acid and subsequently (at last) lactic acid.
Ø Accumulation of lactic acid decreases intracellular pH which causes clumping (aggregation) of nuclear chromatin and further disruption (breakdown) of cytoplasmic organelles.
iv. Appearance of Myelin Figures and Cell Blebs:
Intracellular whorl likes structures originating from damaged membrane.
An outward cell membrane deformity
If the nucleus is not intact then it is termed as irreversible injury. The irreversible changes are:
i. Nuclear changes
ii. Cytoplasmic changes
Nucleus may show one of the following three patterns of changes.
The nucleus becomes shrunken due to clumping of chromatin called pyknosis.
The pyknotic nucleus may break (disintegration) into numerous small particles, the process called karyorrhexis.
The nucleus undergoes lysis (disintegration) without pyknosis called karyolysis.
§ Mitochondrial Vacuolization
§ Loss of plasma membrane (cell membrane, nuclear membrane etc)
§ Excess influx of calcium into cell
§ Release of lysosomal enzymes causing Autolysis (self breaking)
2. Chemical injury:
Chemical causes cell injury by the following mechanisms:
a) Direct attack:
Some chemicals attack directly on critical molecular component or cellular organelle (cell membrane, ribosome, mitochondria etc).
b) Attack through free radicals:
Mostly chemicals cause injury by the formation of free radicals.