CELL INJURY, CELLULAR ADAPTATIONS, MECHANISM OF CELL I NJURY

1. MITHUN VENUGOPAL

2.  Cell injury
 Types of cell injury
 Pathogenesis of cell injury
 Cellular adaptation- hypertrophy, hyperplasia, metaplasia, dysplasia, hypertrophy
 Reversible cell injury
 Irreversible cell injury/cell death-necrosis, apoptosis.

3.  Cells actively interact with their environment, constantly adjusting their structure and
function to accommodate changing demands and extracellular stresses.
 The intracellular environment of cells is normally tightly regulated such that it remains fairly
constant, a state referred to as homeostasis.
 As cells encounter physiologic stresses (such as increased workload in the heart) or
potentially injurious conditions (such as nutrient deprivation), they can undergo adaptation,
achieving a new steady state and preserving viability and function.
 If the adaptive capability is exceeded or if the external stress is inherently harmful or
excessive, cell injury develops.

4.  Within certain limits, injury is reversible, and cells return to their stable baseline; however, if
the stress is severe, persistent, or rapid in onset, it results in irreversible injury and death of
the affected cells.
 Cell death is one of the most crucial events in the evolution of disease in any tissue or organ.
It results from diverse causes, including ischemia (lack of blood flow), infections, toxins, and
immune reactions.
 Cell death also is a normal and essential process in embryogenesis, the development of
organs, and the maintenance of tissue homeostasis.

6.  The causes of cell injury span a range from gross physical trauma, such as after a motor vehicle
accident, to a single gene defect that results in a nonfunctional enzyme in a specific metabolic
disease. Most injurious stimuli can be grouped into the following categories.
1. Hypoxia & ischemia
2. Toxins
3. Infectious agents
4. Immunologic reactions
5. Genetic abnormalities
6. Nutritional imbalance
7. Physical agents
8. Aging

7. HYPOXIAAND ISCHEMIA:
 Hypoxia, which refers to oxygen deficiency, and ischemia, which means reduced blood
supply, are among the most common causes of cell injury.
 Both deprive tissues of oxygen, and ischemia, in addition, results in a deficiency of essential
nutrients and a build up of toxic metabolites.
 The most common cause of hypoxia is ischemia resulting from an arterial obstruction, but
oxygen deficiency also can result from inadequate oxygenation of the blood, as in a variety of
diseases affecting the lung, or from reduction in the oxygen-carrying capacity of the blood, as
with anemia of any cause, and carbon monoxide (CO) poisoning.

8. TOXINS:
 Potentially toxic agents are encountered daily in the environment; these include air pollutants,
insecticides, CO, asbestos, cigarette smoke, ethanol, and drugs.
 Many drugs in therapeutic doses can cause cell or tissue injury in a susceptible patient or in
many individuals if used excessively or inappropriately.
 Even substances, which are not harmful, such as glucose, salt, water and oxygen, also can be
toxic.

9. INFECTIOUS AGENTS:
 All types of disease-causing pathogens, including viruses, bacteria, fungi, and protozoans,
injure cells.
IMMUNOLOGIC REACTIONS:
 Although the immune system defends the body against pathogenic microbes, immune
reactions also can result in cell and tissue injury.
 Examples are autoimmune reactions against one’s own tissues, allergic reactions against
environmental substances, and excessive or chronic immune responses to microbes.
 In all of these situations, immune responses elicit inflammatory reactions, which are often the
cause of damage to cells and tissues.

10. GENETIC ABNORMALITIES:
 Genetic aberrations can result in pathologic changes like congenital malformations associated
with Down syndrome or the single amino acid substitution in hemoglobin giving rise to
sickle cell anemia. Genetic defects may cause cell injury as a consequence of deficiency of
functional proteins, such as enzymes in inborn errors of metabolism, or accumulation of
damaged DNA or misfolded proteins, both of which trigger cell death when they are beyond
repair.
PHYSICALAGENTS:
 Trauma, extremes of temperature, radiation, electric shock, and sudden changes in
atmospheric pressure all have wide ranging effects on cells.

11. 1. Type, duration and severity of injurious agent: The extent of cellular injury depends upon
type, duration and severity of the stimulus e.g. small dose of chemical toxin or short duration
of ischemia cause reversible cell injury while large dose of the same chemical agent or
persistent ischemia cause cell death.
2. Nature, status and adaptability of target cell: The type of cell as regards its susceptibility to
injury, its nutritional and metabolic status, and adaptation of the cell to hostile environment
determine the extent of cell injury e.g. skeletal muscle can withstand hypoxic injury for
longer time while cardiac muscle suffers irreversible cell injury after 20-30 minutes of
persistent ischemia.

12. 3. Underlying intracellular phenomena: Irrespective of other factors, following essential
biochemical phenomena underlie all forms of cell injury:
 Mitochondrial damage causing ATP depletion.
 Cell membrane damage disturbing the metabolic and trans-membrane exchanges.
 Release of toxic free radicals.

14.  Adaptations are reversible changes in the number, size, phenotype, metabolic activity, or
functions of cells in response to changes in their environment.
 Physiologic adaptations usually represent responses of cells to normal stimulation by
hormones or endogenous chemical mediators (e.g., the hormone-induced enlargement of the
breast and uterus during pregnancy).
 Pathologic adaptations are responses to stress that allow cells to modulate their structure and
function and thus escape injury. Such adaptations can take several distinct forms.

15.  Hypertrophy is an increase in the size of the cells resulting in increase in the size of the
organ.
 Increased workload leads to increased protein synthesis and increased size and number of
intracellular organelles and which in turn leads to increased cell size.
 Seen in cells, which cannot divide (nerve cells, myocardial cells, skeletal muscle cells).
 Increase in the cell size results in the increased size of the specific organ.
 Hypertrophy can be physiologic or pathologic and is caused either by increased functional
demand or by specific hormonal stimulation.

16.  Examples of physiologic cellular hypertrophy include physiologic enlargement of the breast
and uterus during pregnancy occurs as a consequence of estrogen stimulated smooth muscle
hypertrophy and a rippled physique of weightlifter due to the hypertrophy of individual
skeletal muscle cells induced by an increased workload.
 Examples of pathologic cellular hypertrophy include the cardiac enlargement that occurs with
hypertension or aortic valve disease.

18.  Increase in the number of cells.
 Result of increased mitosis.
 Seen in cells, which can divide.
 It can leads to an increase in the size of the organ.
 Usually caused by hormonal stimulation.

19.  It can be physiological or pathological.
 The two types of physiologic hyperplasia are (1) hormonal hyperplasia, exemplified by the
proliferation of the glandular epithelium of the female breast at puberty and during
pregnancy; and (2) compensatory hyperplasia, that is, hyperplasia that occurs when a portion
of the tissue is removed or diseased. For example, when a liver is partially resected, mitotic
activity in the remaining cells begins as early as 12 hours later, eventually restoring the liver
to its normal weight.

20.  Most forms of pathologic hyperplasia are caused by excessive hormonal or growth factor
stimulation.
 For example, after a normal menstrual period there is a burst of uterine epithelial proliferation
that is normally tightly regulated by stimulation through pituitary hormones and ovarian
estrogen and by inhibition through progesterone. However, if the balance between estrogen
and progesterone is disturbed, endometrial hyperplasia occurs which is a common cause of
abnormal menstrual bleeding.

21.  Decrease in the size of the cell due to the loss of substances.
 Atrophy results in the shrinkage of organs.
 Although atrophic cells may have diminished function, they are not dead.
 Causes of atrophy include a decreased workload (e.g., immobilization of a limb to permit
healing of a fracture), loss of nerve supply, diminished blood supply, inadequate nutrition,
loss of endocrine stimulation, and aging (senile atrophy).
 Although some of these stimuli are physiologic (e.g., the loss of hormone stimulation in
menopause), the fundamental cellular changes are identical.

22.  Atrophy results from decreased protein synthesis and increased protein degradation in cells.
Protein synthesis decreases because of reduced metabolic activity.
 In many situations, atrophy is also accompanied by increased autophagy, with resulting
increases in the number of autophagic vacuoles. Autophagy (“self- eating”) is the process in
which the starved cell eats its own components in an attempt to find nutrients and survive.

24.  Metaplasia is a reversible change in which one adult cell type (epithelial or mesenchymal) is
replaced by another adult cell type.
 In this type of cellular adaptation, cells sensitive to a particular stress are replaced by other cell
types, which can withstand the adverse environment.
 Metaplasia is thought to arise by genetic “reprogramming” of stem cells rather than trans
differentiation of already differentiated cells.
 Epithelial metaplasia is exemplified by the squamous change that occurs in the respiratory
epithelium in habitual cigarette smokers.
 The normal ciliated columnar epithelial cells of the trachea and bronchi are focally or widely
replaced by stratified squamous epithelial cells.

26.  Dysplasia means ‘disordered cellular development’, often accompanied with metaplasia and
hyperplasia; it is therefore also referred to as atypical hyperplasia.
 Dysplasia occurs most often in epithelial cells.
 Epithelial dysplasia is characterized by cellular proliferation and cytological changes.
 The two most common examples of dysplastic changes are the uterine cervix and respiratory
tract.
 Dysplastic changes often occur due to chronic irritation or prolonged inflammation.

27.  On removal of the inciting stimulus, the changes may disappear.
 In a proportion of cases, however, dysplasia progresses into carcinoma in situ (cancer
confined to layers superficial to basement membrane) or invasive cancer.

28.  Cell injury results when cells are stressed so severely that they are no longer able to adapt or
when cells are exposed to inherently damaging agents or suffer from intrinsic abnormalities.
 In early stages or mild forms of injury the functional and morphologic changes are reversible
if the damaging stimulus is removed.
 At this stage, although there may be significant structural and functional abnormalities, the
injury has typically not progressed to severe membrane damage and nuclear dissolution.
 The main morphologic correlates of reversible cell injury are cellular swelling (hydropic
change), fatty change and intracellular accumulation.

30.  A mild lethal injury results in mild ischemia, which in turn results in a mild hypoxia.
 Hypoxic condition in cells leads to anaerobic glycolysis and decreased ATP production.
 Normally the cell membrane maintains intracellular sodium at a lower concentration than in
the extracellular fluid, by Na-K ATPase pump, a job that requires expenditure of energy.
 Due to decreased ATP production the Na-K ATPase pump will not function and results in
increased level of sodium in the intracellular fluid.
 Injury that is mild and lasts for a few minutes or hours may damage this mechanism and
allow intracellular sodium to rise, which attracts water and causes the cytoplasm and
intracellular organelles to swell. The result is Hydropic Change (vacuolar degeneration).
Substances other than water can also accumulate in cells.

31.  Endoplasmic reticulum swells (due to the swelling of intracellular organelles in hydropic
swelling) results in the detachment of ribosomes from ER, which leads to decreased protein
synthesis.
 When proteins are not available for energy, triglycerides are broken down to free fatty acid
for energy production.
 Results in the accumulation of free fatty acid in cells and is called Fatty Change.
 Manifested by the appearance of lipid vacuoles in the cytoplasm.
 Fatty changes are also reversible.

32. HYDROPIC CHANGES:
• The first manifestation of almost all forms of injury to cells.
• Increase in weight of the organ.
• Loss of microvilli.
• Cytoplasmic bleb formation.
• Intracellular organelle swelling.
• Microscopic examination may reveal small, clear vacuoles
within the cytoplasm and is called vacuolar degeneration.
• The cell will not take the usual pink color when stained due to
the water content and it will appear as cloudy pale color and is
called cloudy swelling.
• Swelling of cells is reversible.

33.  Cholesterol: The most extensive and most damaging intracellular accumulation is cholesterol, deposited in
the cells of arteries in atherosclerosis. Cholesterol first appears in macrophages and smooth muscle cells in the
arterial wall and later accumulates into large, extracellular pools in the arterial wall.
 Protein: Protein accumulations can occur in cells. An important feature of normal proteins is that they are
long molecules that must be folded into correct shape for normal function. Microscopically visible
cytoplasmic accumulations of misfolded or otherwise abnormal proteins occur in a variety of diseases.
 Glycogen: Glycogen is a long chain of glucose molecules formed and stored in liver and muscle as a glucose
reserve. Glycogen synthesis is regulated by blood glucose concentration. For example, patients with diabetes;
have high blood glucose levels, and, as a consequence, hepatocytes and kidney cells in people with diabetes
are often stuffed with glycogen.
 Pigments: The most widely occurring cell pigment accumulation is Lipofuscin, a golden brown substance
most notable in brain neurons and myocardial muscle cells, both of which are permanent, non-reproducing
cells, and in hepatocytes, which are slow dividing, stable cells. Melanin is a dark-brown compound that gives
skin its color. It is synthesized by melanocytes in the epidermis and deposited in the cytoplasm of cells in the
basal layer of the epidermis. Inhaled carbon particles from cigarette smoke or polluted air is ingested by
macrophages of bronchial lymph nodes and remains permanently with little damage. Hemosiderin and
ferritin are brownish-pigmented normal iron-storage compounds important in iron and hemoglobin
metabolism.

34.  With continuing damage, the injury becomes irreversible, where the cell cannot recover and it dies.
 There are two types of cell death, which differ in their morphology.
1. NECROSIS
2. APOPTOSIS
NECROSIS
 Necrosis is defined as a localized area of death of tissue followed by degradation of tissue by hydrolytic
enzymes liberated from dead cells; it is invariably accompanied by inflammatory reaction.
 Necrosis can be caused by various agents like hypoxia, chemical and physical agents, microbial agents,
immunological injury, etc.

38. 1. COAGULATIVE NECROSIS
2. LIQUIFACTIVE NECROSIS
3. GANGRENE NECROSIS
4. CASSEOUS NECROSIS
5. FATTY NECROSIS
6. FIBRINOID NECROSIS

39.  The decreased pH due to the lactic acid production in anaerobic glycolysis inactivates the hydrolytic
enzymes (Protease & Phospholipase).
 So the cell outline (collagen) is maintained even after hydrolytic enzymes damage the plasma
membrane.
 Neutrophils infiltrate can be seen in and around the remaining of plasma membrane (Tomb stone
appearance).

40.  After few days the enzymes reactivate and the entire outline is dissolved.
 Coagulative necrosis is commonly seen in ischemia and thermal injury of solid organs except CNS and
brain.

41.  When the Coagulative necrosis remains for few days, the remaining outline of plasma membrane will
liquefy by reactivated hydrolytic enzymes.
 The liquefied necrosed tissue along with the neutrophils is called pus.
 Localized collection of pus is called as abscess.
 Seen in cells, which contain very less collagen framework (Brain and spinal cord-CNS).
 Since there is no collagen in the cell membrane, the cell completely liquefies when acted by hydrolytic
enzymes.
 A bacterial infection along with Coagulative necrosis can also results in Liquifactive necrosis since
bacteria produces proteolytic enzymes.

42.  Necrosis with white cheese like deposits.
 Examples: Tuberculosis, Syphilis, and Histoplasma.
 Most commonly appear as Coagulative necrosis and less often as
Liquefactive necrosis.
 Presence of granuloma is significant of casseous necrosis.

43.  Necrosis of fat cells (adipocytes).
 Fatty necrosis can be of two types
1. Enzymatic
2. Traumatic
 Both enzymatic and traumatic types result in the release of proteolytic
enzymes and lipases.
 The proteolytic enzymes cause tissue damage.
 Lipases break the triglycerides present in the adipocytes into free fatty
acids, which will combine with the calcium and form white chalky deposit.

45.  Homogenous pink color fibrin like morphology.
 Seen in blood vessels.
 Commonly seen in conditions where blood vessels are damaged
like vasculitis, malignant hypertension, immune complex
deposition, etc.

46. APOPTOSIS
 Programmed cell death.
 Involves activation of intrinsic enzymes by a
programming in cells.
 The cells are destroyed by these enzymes to form
fragmented cells called apoptotic bodies.
 The apoptotic bodies are engulfed by macrophages.

47. PHYSIOLOGICAL
1. During embryogenesis: any cells, which are not required, are destroyed in organogenesis by
apoptosis.
2. Hormonal: Endometrial cell breakdown in menstrual cycle and menopause.
3. Removal of self-reactive lymphocytes in thymus (central tolerance).
4. Post inflammatory response: the excess inflammatory cells are removed by apoptosis.

48. PATHOLOGICAL
1. DNA damaged cells: DNA damaged cells due to radiation, chemotherapy or inflammation are
removed by apoptosis.
2. Accumulation of misfolded proteins.
3. Cell death in infection: viral infections like HIV or hepatitis.
4. In organs due to duct obstruction (parotid gland, pancreas).

49. 2 pathways
1. Intrinsic pathway (mitochondrial)
2. Extrinsic pathway (death receptor-FAS)
Extrinsic Pathway
 This pathway triggers apoptosis in response to external stimuli, like ligand binding at death receptors on
the cell surface.
 The FAS ligand (TNF) acts on FAS receptor (TNF receptor) to activate the FADD (FAS Associated
Death Domain).
 This initiates the caspase-8 activation.
 Caspase-8 activates the executioner caspase (caspase-3, 6).

50. Intrinsic Pathway
 This pathway triggers apoptosis in response to internal stimuli such as biochemical stress,
DNA damage and lack of growth factors.
 The cell injury is sensed by apoptotic sensors (BH3 only protein-BIM, BID, BAD, NOXA,
PUMA).
 These groups of molecules determine whether a cell will survive or undergo apoptosis in
response to the stimuli.
 If the injury is less severe, apoptotic sensor decreases the pro-apoptosis genes (BAK, BAX)
and increases the anti-apoptosis genes (BCL-1, BCL-2, BCL-XL, MCL) and prevents cell
death.
 If the injury is severe, the apoptotic sensor increases the pro-apoptosis genes (BAK, BAX)
and decreases the anti-apoptosis gene (BCL-1, BCL-2, BCL-XL, MCL).

51.  The pro-apoptosis genes and anti-apoptosis genes act on mitochondria and increase the
permeability of mitochondria.
 Due to the increased permeability, the enzyme Cytochrome-C will leak out from
mitochondria.
 The Cytochrome-C combines with APAF-1 (Apoptosis Activator Factor-1) and form
Apoptosome (Cytochrome-C + APAF-1) and activates an enzyme called Caspases-9 (initiator
caspase).
 Caspase-9 activates executioner caspase (caspase-3, 6).
 The executioner caspase activate the endonuclease and cytoskeleton protein damage results in
the formation of Apoptotic Bodies (cytoplasmic bleb with damaged organelles and nuclear
fragments).
 The apoptotic bodies are separated out from main cell and are engulfed and removed by
macrophages.

53. NECROSIS APOPTOSIS
Mechanism Pathological Physiological
Cell size Cell swelling Cell shrinkage
Chromatin Pyknosis, Karyorrhexis, Karyolysis Pyknosis only
Cell membrane Disintegrated Intact
Neutrophils Present Absent