1 Cell Injury and Cell Death 11

Ischaemia

Decreased mitochondrial oxidative phosphorylation

Decreased ATP

Increased glycolysis Decreased Na+ K + ATPase Detachment of

(anaerobic respiration) pump activity ribosomes

↓ pH ↓ Glycogen stores • Inf lux of Ca2+, H2O, Na+ Decreased protein

• Eff lux of K+ synthesis

Clumping of nuclear chromatin Lipid deposition/fatty change

Cellular swelling Loss of microvilli Blebs ER swelling Myelin figures

FLOWCHART 1.4. Sequence of events in reversible injury.

• Irreversible injury (Flowchart 1.5)

↓ pH Membrane injury Ischaemia

Intracellular release • Loss of membrane phospholipids due to

of lysosomal enzymes phospholipases
• Cytoskeletal alterations due to proteases
• Lipid peroxidation and
↓ Ribonucleic DNA damage due to free radicals
protein, nuclear changes
and loss of cell shape
FLOWCHART 1.5. Sequence of events in irreversible injury.

Q. Write briefly on free radical-mediated cell injury.

Ans. Free radicals are chemical species with an unpaired electron in their outer orbit. They
react with inorganic and organic molecules (proteins, lipids and carbohydrates), which are
mainly present in membranes and nucleic acids.
Free radical production is induced by
• Absorption of radiant energy: UV rays, X-rays.
• Enzymatic metabolism of exogenous chemicals/drugs: CCl4 to CCl3.
• Reduction–oxidation reaction processes that occur during normal metabolism: Formation of
superoxide anion (O2–), hydrogen peroxide (H2O2), hydroxyl ion (.OH).
• Reactions involving transition metals: iron (Fenton reaction), copper, etc.
• Reactions involving nitric oxide (NO): acts as a free radical and can be converted to highly
reactive peroxynitrite anion (ONOO–) as well as NO2 and NO–3 .
Effects of free radicals:
• Lipid peroxidation: Lipid and free radical interactions produce peroxides (initiation).
Peroxides are reactive and unstable species, which start a chain reaction of lipid per-
oxidation (propagation). In some cases, chain reaction may be terminated by antioxidants.
• Modification of proteins by oxidation: Oxidation of amino acid residue side chain leads to
formation of protein–protein cross-linkage and disruption of the protein backbone
resulting in protein fragmentation.
• DNA lesions: Attack thymine and other nucleotides of nuclear and mitochondrial DNA to
produce single- or double-stranded breaks in DNA as well as cross-linking of DNA strands.

mebooksfree.com
12 SECTION I General Pathology

Inactivation of free radicals is brought about by

• Antioxidants: vitamins A, C, E and b-carotene.
• Iron- and copper-binding proteins: transferrin, ferritin, lactoferrin, ceruloplasmin (decrease
available free metal by binding to it).
• Enzymes: catalase, superoxide dismutase, glutathione peroxidase (catalyse free radical
breakdown).

Q. Define necrosis and describe its various morphological patterns.

Ans. Disturbances of the external environment beyond the limits of homeostasis lead to
premature cell death, which is called necrosis. Necrosis may be caused by ischaemia, infec-
tion, poisoning, etc., and is invariably pathological. It usually precipitates an inflammatory
response and is accompanied by cell swelling, lysis and lysosomal leakage (Flowchart 1.6).
Self-digestion of cells by enzymes liberated from its own lysosomes on the other hand is
labelled autolysis (Table 1.3).

Severe membrane damage

Lysosomal enzymes enter the cytoplasm

Progressive degradation of the lethally injured cells (necrosis)

Leaking of cellular contents

Acute inflammation (due to leaked contents)

FLOWCHART 1.6. Sequence of events in cellular necrosis.

The morphological features of necrosis vary with its type. Changes common to most
types include
1. Cytoplasmic changes
• Increased eosinophilia of the cytoplasm, which is due to
• loss of normal cytoplasmic basophilia caused by the loss of RNA and
• denaturation of cytoplasmic proteins which then bind strongly to the dye eosin:
• Glassy homogenous cytoplasm due to loss of glycogen.
• Swelling and vacuolation of the cytoplasm (occurs after enzymatic digestion has
started).
• Cellular and organelle swelling may eventually lead to discontinuities in cell and
organelle membranes and ultimately rupture.
• Formation of myelin figures (phospholipid masses derived from damaged cell
membranes).
2. Nuclear changes
The changes in nucleus appear in one of the following three patterns:
• Nuclear shrinkage and increased basophilia (pyknosis)
• Nuclear fragmentation (karyorrhexis)
• Loss or fading of basophilia due to DNase activity (karyolysis)
Morphological patterns of necrosis include
1. Coagulative necrosis
• It is the most common pattern of necrosis and is caused by ischaemic injury resulting
in hypoxic death of cells in all tissues except the brain.
• There is preservation of the basic architectural outlines and type of tissue can be
recognized but cellular details are lost.

mebooksfree.com
 1 Cell Injury and Cell Death 13

Viable cardiac
myocytes

Infarcted
myocardium

FIGURE 1.7. Infarcted myocardium surrounded by viable cardiac myocytes (H&E; 1003).

• Cell injury leads to increasing intracellular acidosis, which denatures not only struc-
tural proteins but also enzymatic proteins, and so blocks the proteolysis of the cell,
thereby preventing loss of architecture of the tissue.
• On gross examination, the affected tissue is pale in colour and firm in texture.
• Microscopically, increased eosinophilia of the cytoplasm and decreased basophilia of
the nucleus are observed. Myocardial infarction is an excellent example in which
acidophilic, coagulated anucleate cells are seen (Fig. 1.7).
Mechanism of evolution of coagulative necrosis is shown in Flowchart 1.7.

Decreased pH

Denaturation of structural as well as enzymatic proteins

Lack of enzymatic proteins blocks proteolysis

Preservation of basic architecture of cell/tissue

FLOWCHART 1.7. Mechanism of evolution of coagulative necrosis.

2. Liquefactive necrosis (colliquative necrosis)

• This occurs in situations in which enzymatic breakdown is more prominent than
protein denaturation unlike coagulative necrosis (Table 1.4).
• It is usually associated with bacterial or fungal infections because microbes stimulate
the accumulation of leukocytes and liberation of enzymes from these cells.
• The organ–cellular architecture is lost, and the tissue is digested and converted into
a liquefied mass, which appears creamy yellow in colour and is called ‘pus’.
• Liquefactive necrosis is most commonly seen in organs that have a high-fat and low-
protein content (eg, the brain), or those with a high-enzymatic content (eg, the
pancreas), and typically causes gangrene of intestine (Fig. 1.8) and limbs and hy-
poxic death in brain.
• Lack of a proper collagenous connective tissue framework in an organ also aids to
this type of necrosis.

mebooksfree.com
14 SECTION I General Pathology

Full thickness
liquefactive
necrosis of the
bowel

Disintegrating
neutrophils/debris

FIGURE 1.8. Liquefactive necrosis/gangrene of intestine (H&E; 1003).

Mechanism of evolution of liquefactive necrosis is shown in Flowchart 1.8.

Bacterial infection and accumulation of inflammatory cells

Release of enzymes
Autolysis and heterolysis
FLOWCHART 1.8. Mechanism of evolution of liquefactive necrosis.

3. Gangrenous necrosis
This is a clinical term, not a specific pattern of necrosis. It is usually used in context
of the lower limbs, which have lost their blood supply and have undergone necrosis,
initially coagulative (dry gangrene), and later liquefactive due to secondary bacterial
infection and immigrating leukocytes (wet gangrene) (Table 1.6).
Mechanism of evolution of gangrenous necrosis is shown in Flowchart 1.9.

Bacterial infections and accumulation of inflammatory cells

Release of enzymes
Autolysis and heterolysis
FLOWCHART 1.9. Mechanism of evolution of gangrenous necrosis.

4. Caseous necrosis
• This type of necrosis is typically associated with tuberculous infection.
• On gross examination, the necrotic areas appear cheesy white (caseous). Micro-
scopically, the debris appears amorphous, eosinophilic and granular (Fig. 1.9), and
is surrounded by a distinct inflammatory reaction called granulomatous reaction.
• Tissue architecture is completely obliterated unlike coagulative necrosis (Table 1.5).
Dystrophic calcification may be seen.
5. Enzymatic fat necrosis
• It refers to a focal area of fat destruction that converts adipocytes to necrotic cells
with shadowy outlines and basophilic calcium deposits, surrounded by an inflam-
matory reaction (Fig. 1.10).
• It is typically seen in acute pancreatitis and traumatic fat necrosis of breast.
Mechanism of evolution of enzymatic fat necrosis in acute pancreatitis is shown in Flowchart 1.10.

mebooksfree.com
 1 Cell Injury and Cell Death 15

Caseous necrosis

Dystrophic calcification

FIGURE 1.9. Section from a lymph node showing amorphous, eosinophilic and granular de-
bris (caseous necrosis) surrounded by a granulomatous reaction composed of Langhans giant
cells and chronic inflammatory cells (H&E; 1003).

FIGURE 1.10. Fat necrosis in the breast showing disruption of normal adipocytes and accumula-
tion of lipid-laden foamy histiocytes and a multinucleate giant cell (H&E; 2003).

Release of activated pancreatic lipases into pancreas and peritoneal cavity

Focal areas of destruction of fat and release of fatty acids

Released fatty acids combine with calcium (saponification)

Chalky white areas

FLOWCHART 1.10. Mechanism of evolution of enzymatic fat necrosis.

mebooksfree.com