APOPTOSIS AND NECROSIS
APOPTOSIS
All the cells in our body are highly regulated and not only control the rate of cell division,
but also by the rate of cell death. When cells are no longer needed and they become a
threat to the organism, they undergo a suicidal programmed cell death or APOPTOSIS. This
process involves a specific proteolytic cascade that causes the cell to shrink and condense,
to disassemble its cytoskeleton, and to alter its cell surface so that a neighbouring
phagocytic cell (e.g.macrophage) can attach to the cell membrane and digest it.
Apoptosis is an orderly cell death that results in disassembly and phagocytosis of the cell
body before any leakage of its contents occurs and neighboring cells usually remain healthy.
Apoptosis is initiated by activation of a family of proteases called CAPASES. These are
enzymes that are synthesized and stored in the cell as inactive PROCAPASES. The
mechanisms of activation of capases are complex, but once activated the enzymes cleave
and activate other procapases, triggering a cascade that rapidly breaks down proteins within
the cell. The cell thus dismantles itself and, and its remains are rapidly digested by
neighboring phagocytic cells.
NECROSIS
In contrast to programmed cell death, cells that die as a result of an acute injury usually
swell and burst due to a loss of cell membrane integrity – called NECROSIS. Necrotic cells
may spill their contents causing inflammation and injury to neighboring cells
1. Identify causes of cell injury and death
 Oxygen deprevation (Hypoxia) – Ischaemia, respiratory disease, anaemia
 Physical agents - mechanical trauma, extremes of temperature, sudden atmospheric
pressure changes, radiation, electric shock
 Chemical agents and drugs – High O2 concentration, poisons (arsenic, cyanide),
insecticides, carbon monoxide, drugs and alcohol
 Infectious agents – Viruses, bacteria, parasites
 Immunological agents – anaphylaxis, autoimmune responses, both endogenous and
exogenous antigens
 Genetic derangements – errors of metabolism, chromosomal abnormalitites
 Nutritional imbalances (metabolic derangements – inability to utilise nutrients
correctly e.g. Diabetes Mellitus)
 Nutritional imbalances – protein-calorie deficiencies, vitamin deficiencies,
nutritional excess especially lipids
Cell injury is either reversible or irreversible
Reversible cell injury – Characterized by generalised swelling of the cell and its organelles,
blebbing of the plasma membrane, detachment of the ribosomes from the ER and the
clumping together of the nuclear chromatin
Irreversible cell injury – characterized by increasing swelling of the cell, swelling and
disruption of the lysosomes, presence of large amorphous densities in swollen
mitochondria, disruption of cellular membranes and profound nuclear changes.
Mechanisms of cell injury:
The cellular response to cell injury depends on the type of injury, its duration and its severity
The consequences of cell injury depend on the type, state and adaptability of the injured cell
Cell injury results from functional and biochemical abnormalities in one or more of several
essential cellular components:
 Aerobic respiration – involving mitochondrial oxidative phosphorylation and ATP
production
 Cell membrane integrity – on which the ionic and osmotic homeostasis of the cell
and its organelles depends
 Protein synthesis
 The cytoskeleton
 The integrity of the genetic apparatus of the cell
The depletion of ATP has a series of flow on effects including:
 ↓ activity of the plasma-membrane Na+ pump – this causes Na+ to accumulate inside
the cell and K+ to diffuse out thus, causing the cell to swell and the ER to dilate
 Altered cellular energy metabolism - ↓ ATP →↑anaerobic glycolysis (to maintain
the cells energy source) → ↑ Lactic acid and inorganic phosphate formation →
↓ intracellular pH → ↓ activity of cellular enzymes
 Ca2+ pump failure – causes and influx of Ca2+ into the cell
 Structural disruption of the protein synthetic apparatus occurs (with prolonged ATP
depletion) – causing detachment of the ribosomes from the RER and dissociation of
polymers into monomers → ↓ protein synthesis and irreversible damage to
mitochondria and lysosomal membranes
Mitochondrial damage
Mitochondria can be damaged by:
 ↑ cytosolic Ca2+
 Oxidative stress
 Breakdown of the phospholipids
 Lipid breakdown products
Influx of intracellular calcium and loss of calcium homeostasis
Ca2+ ions are important mediators of cell injury. Cytosolic free calcium is maintained at
relatively low levels compared to the extracellular levels. This gradient is maintained by the
Ca2+ - Mg2+-ATPase. Damage to the cell causes an increase in the cytosolic calcium
concentrations due to an influx of Ca2+ across the plasma membrane as well a release of
Ca2+ from the mitochondria and ER. This ↑ in Ca2+ in turn activates a number of enzymes
with deleterious effects. These enzymes include:
 ATPases (which ↑ ATP depletion)
 Phospholipases (cause membrane damage)
 Proteases (break down membrane and cytoskeleton proteins)
 Endonucleases (DNA and chromatin fragmentation)
2. Identify the gross and light microscopic appearances of various forms of necrosis
and outcomes (inflammation, regeneration, repair)
Necrotic cells show
 ↑ oesinophilia
 A more glassy homogenous appearance (due to the loss of glycogen particles)
 Cytoplasm appears vacuolated and moth-eaten (because the enzymes have digested
the cytoplasmic organelles
 Overt discontinuities in plasma and organelle membranes
 Dilated mitochondria with large amorphous densities
 Intracytoplasmic myelin figures
 Amorphous osmiophilic debris
 Aggregates of fluffy material (denatured protein)
NB: See diagrams on p20/21 Robins and Cotran – Pathological Basis of Disease
Coagulative Necrosis (see p22 - Robins and Cotran – Pathological Basis of Disease)
 Preservation of the general tissue architecture
 Characteristic of hypoxic death of cells in all tissues except the brain
Liquefactive Necrosis (see p22 - Robins and Cotran – Pathological Basis of Disease)
 Characteristic of cell death due to infarction and cell death in the CNS
 Accumulation of inflammatory cells
 Dead cells are completely digested
 Tissue is transformed into a liquid viscous mass
 Can be creamy yellow in colour due to pus
Gangrenous Necrosis
 Not really a distinctive pattern of cell death
 When bacterial infection is superimposed, coagulative necrosis is modified by the
liquefactive action of the bacteria and associated inflammation (“wet gangrene”)
Caseous Necrosis (see p22 - Robins and Cotran – Pathological Basis of Disease)
 Distinctive form of coagulative necrosis
 Characteristic of TB infection
 Caseous – derived from its cheesy white appearance of the necrosed area
 Necrotic focus appears as amorphous granular debris with a distinctive
inflammatory boarder
 Unlike regular coagulative necrosis, the tissue architecture is completely obliterated
Fat Necrosis (see p23 - Robins and Cotran – Pathological Basis of Disease)
 Not really a distinctive pattern of cell death
 Descriptive term for focal areas of fat destruction
 Activated enzymes liquefy fat cell membranes, released fatty acids combine with
calcium to produce grossly chalky white areas (fat saponification)
3. Outline the sequence of morphological changes that occur in cells undergoing
apoptosis
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Cell Shrinkage
o Cell is smaller in size, cytoplasm is dense and the organelles are more tightly
packed together
Chromatin condensation
o The most characteristic feature of apoptosis
o Chromatin aggregates peripherally (under the nuclear membrane) into dense
masses of various shapes and sizes
o Nucleus may break up – producing 2 or more fragments
Formation of cytoplasmic blebs and apoptotic bodies
o Extensive surface blebbing then undergoes fragmentation into membranebound apoptotic bodies composed of cytoplasm and tightly packed
organelles – with or without nuclear fragments
Phagocytosis of apoptotic cells or cell bodies usually by macrophages
o Apoptotic bodies are rapidly degraded within lysosomes
o Adjacent healthy cells migrate or proliferate to replace the spaces occupied
by apoptotic cells
4. List the circumstances in which cell death by apoptosis might be expected
Apoptosis in physiological situations
 The programmed destruction of cells during embryogenesis
 Hormone-dependent involution in the adult
 Cell deletion in proliferating cell populations
 Death of host cells that have served their useful purpose (e.g. neutrophils
following acute inflammatory response)
 Elimination of potentially harmful self-reactive lymphocytes (autoimmune)
 Cell death by induced cytotoxic T cells (defense mechanism against viruses and
tumors, eliminates virus-infected and neoplastic cells)
Apoptosis in pathological conditions
 Cell death produced by a variety of injurious stimuli
 Cell injury in certain viral diseases
 Pathological atrophy in parenchymal organs after duct obstruction
 Cell death in tumors
 In some situations where cell death is mainly due to necrosis, the pathway of
apoptosis may also contribute
5. Outline the biochemistry and molecular biology of apoptosis with particular
reference to DNA cleavage, proteases and expression of oncogenes and tumour
suppressor genes
Protein Cleavage
Protein hydrolysis is a specific feature of apoptosis and involves the activation of several
members of the cystine protease family called CAPASES. Capases are present in many cells
as inactive pro-enzymes, which when activated, induce apoptosis. These capases cleave
cellular proteins (lamins) breaking up the nuclear scaffold and cytoskeleton. Capases also
activate DNAses which degrade nuclear DNA.
DNA breakdown
Characteristic breakdown of DNA into large 50-300 kilobase pieces. The fragments may be
visualized by agarose gel electrophoresis as DNA ladders. Endonuclease activity also forms
the basis for detecting cell death by cytochemical techniques that recognize double
stranded breaks in DNA – although internucleosomal DNA cleavage is not specific to
apoptosis
Phagocytic Recognition
Express phosphatidylserine in some outer layers of plasma membranes, in some types of
apoptosis, proteins sectreted by phagocytes may bind to apoptotic cells and opsonoize the
cells for phagocytosis and early recognition of dead cells by macrophages results in
phagocytosis without the release of proinflammatory cellular components.