Cell injury and cell death| reversible-irreversible| cellular swelling| Reactive oxygen species| free radical| necrosis| type of necrosis|

CELL INJURY & CELL DEATH 

An Insightful Introduction to Pathology: -

Pathology serves as a crucial gateway to understanding the intricate mechanisms of disease. It is the study of the nature and causes of illness, bridging the gap between clinical practice and laboratory science. This field delves deep into the cellular and tissue-level changes that occur in the body due to various pathological processes, unveiling the mysteries behind symptoms and diagnoses. Through histological examinations, biomolecular analyses, and a range of cutting-edge techniques, pathology illuminates how diseases disrupt the harmony of normal function, fostering insights that are vital for effective treatment and patient care. Pathology (definition): Pathology is the study of the structural, biochemical & functional changes in cells, tissues, and organs that underlie disease. 

Divisions of pathology: 

Traditionally, the study of pathology is divided into two parts- 

1. General pathology: General pathology is concerned with the common reactions of cells and tissues to injurious stimuli. 
2. Systemic pathology: Systemic pathology examines the alterations & underlying mechanisms in diseases of particular organ systems. 

Functions of pathology: 

1. By the use of morphologic, microbiologic, immunologic, and molecular techniques, pathology attempts to explain the whys and wherefores of the signs and symptoms manifested by patients while providing a rational basis for clinical care and therapy.
2. It thus serves as the bridge between the basic sciences and clinical medicine and is the scientific foundation for all of medicine. 

Aspects of pathology: 

The four aspects of a disease process that form the core of pathology, they are as follows- 

1. The causation (etiology).
2. Biochemical & molecular mechanisms (pathogenesis). 
3. The associated structural (morphologic changes) & functional alterations in cells & organs. 
4. The resulting clinical consequences (clinical manifestations).

Father of modern pathology: 

Virtually all diseases start with molecular or structural alterations in cells. This concept of the cellular basis of disease was first put forth in the nineteenth century by Rudolf Virchow, known as the father of modern pathology. Virchow emphasized the idea that individuals are sick because their cells are sick. 

Cell injury 

Define cell injury: -

Cell injury: If the limits of adaptive response to a stimulus are exceeded or if cells are exposed to damaging insults, deprived of essential nutrients, or compromised by mutations that affect essential cellular functions, a sequence of events follows that is termed as cell injury.

Causes of cell injury: -

1. Oxygen deprivation: 

• Hypoxia: Hypoxia is a deficiency of O2 to the tissue. It occurs due to cardio-respiratory failure, anemia, CO poisoning etc. 
• Ischemia: Ischemia means loss of blood supply from impeded arterial flow or reduced venous drainage in a tissue. 

     2. Physical agents: 

• Mechanical trauma. 
• Extremes of temperature (burn and deep cold). 
• Sudden changes in atmospheric pressure.
• Radiation. 
• Electric shock. 

3. Chemical agents and drugs: 

• Glucose or salt in hypertonic concentrations.
• Oxygen at high concentrations is toxic.
• Poisons, such as arsenic, cyanide, or mercury. 
• Environmental pollutants, insecticides, and herbicides. 
• Industrial and occupational hazards, such as carbon monoxide and asbestos. 
• Recreational drugs such as alcohol. 
• Variety of therapeutic drugs. 

4) Infectious agents: Virus, bacteria, fungi, protozoa and high forms of parasites. 

5) Immunological reactions: Anaphylactic reaction to foreign body and autoimmune diseases.

6) Genetic abnormalities: Genetic injury may result in congenital malformations e.g. Down syndrome, sickle cell anemia, enzymatic abnormalities etc. 

7) Nutritional imbalance: such as protein-calorie deficiency, vitamin deficiency, nutritional excess (atherosclerosis, obesity) etc.

Types of cell injury: 

🕮According to severity: 

1. Reversible / mild cell injury. 
2. Irreversible / severe cell injury. 

🕮Morphological types: 

1. Reversible cell injury: 

• Cellular swelling. 
• Fatty change. 

      2. Irreversible cell injury: 

• Necrosis. 
• Apoptosis. 

Reversible cell injury: 

Reversible cell injury is characterized by functional and structural alterations in early stages or mild forms of injury, which are correctable (reversible) if the damaging stimulus is removed. 

Features of reversible cell injury: 

🕮Gross changes / features: Two features are consistently seen in reversibly injured cells. 

1) Early alterations in reversible injury include: 

• Generalized swelling of the cell and its organelles, 
• Blebbing of the plasma membrane, 
• Detachment of ribosomes from the endoplasmic reticulum (ER), and 
• Clumping of nuclear chromatin. 

2) Fatty change occurs in organs that are actively involved in lipid metabolism (e.g. liver). 

🕮The ultra-structural changes: (Visible by electron microscopy) 

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. Accumulation of 'myelin figures' in the cytosol composed of phospholipids derived from damaged cellular membranes.
4. Dilation of the ER, with detachment of polysomes.
5. Nuclear alterations, with disaggregation of granular and fibrillar elements. 

Irreversible cell injury: 

If the injurious stimulus persists or is severe enough from the beginning, the cells or tissues are damaged to such extent that it becomes unable to recover when injurious stimulus is withdrawn, is called irreversible cell injury. 

🕮Pathogenesis / features / criteria of irreversible cell injury:

Irreversible cell injury occurs due to irreversible damage of cell membrane, mitochondria & nucleus. It is characterized by - 

• Increasing swelling of the cell. 
• Swelling and disruption of lysosomes.
• Vacuolization of mitochondria with reduced capacity to generate ATP. 
• Disruption of cellular membranes.
• Profound nuclear changes. 

🕮Hallmarks of reversible cell injury: 

1. Reduced oxidative phosphorylation.
2. ATP depletion.
3. Cellular swelling that causes changes in ion concentration & water influx. 

🕮Hallmarks of irreversible cell injury: 

Irreversible damage of cell membrane, mitochondria & nucleus. 

🕮 Reversible cell injuries become irreversible

Transformation of reversible cell injury to irreversible cell injury; It is useful to consider the possible events that determine when reversible injury becomes irreversible and progresses to cell death. Two phenomena consistently characterize irreversibility - 

1. The inability to reverse mitochondrial dysfunction (lack of oxidative phosphorylation and ATP generation) even after resolution of the original injury.
2. Profound disturbances in membrane function (of cell and cellular organelles of those membrane- bound organelles like mitochondria and endoplasmic reticulum). 

🕮Morphological pattern of irreversible cell injury/cell death: 

Irreversibly injured cells invariably undergo morphologic changes that are recognized as cell death. There are two morphological patterns of cell death - 

• Necrosis. 
• apoptosis. 

Cellular swelling:

Cellular swelling is the earliest manifestation of almost all forms of injury to cells 

🕮Causes of cellular swelling: 

1. Hypoxia.
2. Radiation.
3. Toxins. 

🕮Pathogenesis of cellular swelling: 

Hypoxia, mitochondrial damage by radiation or toxins, 

 ↓ 

Depletion of ATP.

↓ 

Failure of ATP-dependent Na*-K pump in the plasma membrane. 

 ↓ 

Influx of water into the cell. 

 ↓ 

Cellular swelling. 

Morphology of cellular swelling: 

• It causes pallor, increased turgor, and increased weight of the affected 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. 

Cellular responses to stresses & noxious stimuli: 

Cellular response to injury varies in its effects on cell structure and function according to the type of involved and the nature and severity of the agent responsible. They include - 

1) Cellular adaptations: 

• Atrophy. 
• Hypertrophy.
• Hyperplasia.
• Metaplasia. 

2) Cell injury: 

a) Acute reversible injury. 
b) Irreversible injury / cell death: 

✓ Necrosis. 

✓ Apoptosis. 

3) Intracellular accumulations. 

4) Pathological calcifications. 

5) Cellular aging. 

Figure: Stages of the cellular response to stress and injurious stimuli. 

Subcellular responses to cell injury 

 These conditions are associated with distinctive alterations in cell organelles or the cytoskeleton. The responses are - 

1) Lysosomal catabolism: Lysosomes are involved in the breakdown of phagocytosed materials in 

one of the following two ways - 

• ➤ Heterophagy: It is the process of lysosomal digestion of materials ingested from the extracellular environment, e.g. uptake and digestion of bacteria by neutrophils and removal of apoptotic cells by macrophages. 
• ➤ Autophagy: It refers to lysosomal digestion of the cell's own components, and involved in the removal of damaged organelles during cell injury. 

2) Induction (hypertrophy) of smooth endoplasmic reticulum. 

3) Mitochondrial alterations: 

• ➤ In cell hypertrophy: There is increase in number of mitochondria. 
• ➤ In cell atrophy: There is decrease in number of mitochondria. 
• ➤ Extremely large and abnormal shape mitochondria (mega-mitochondria) are seen in liver in alcoholic liver disease and in certain nutritional deficiencies. 

4) Cytoskeletal abnormalities: 

• ➤ Defects in the function such as cell locomotion and movements of intracellular organelles. 
• ➤ In some instances, there is intracellular accumulation of fibrillar material.  

Biochemical mechanisms of cell injury 

Mechanism/biochemical mechanism / intracellular targets of cell injury: 

1.  ATP depletion. 
2. Mitochondrial damage. 
3. Membrane damage & defects in membrane permeability.
4. Damage to DNA.
5. Oxidative stress: Accumulation of oxygen derived free radicals.
6. Disturbances in calcium homeostasis: Influx of intracellular calcium & loss of calcium homeostasis.
7. Endoplasmic reticulum stress: The unfolded protein response. 

Description of each mechanism of cell injury: 

1) Depletion of ATP: 

Causes of ATP depletion: 

✓ Ischemia & hypoxia: Reduced supply of oxygen (hypoxia) & nutrients.

✓ Mitochondrial damage: By radiation and toxins. 

Effects / consequences of ATP depletion: 

a) Failure of ATP dependent Na+-K+ pump & Ca2+ pump: 

• Na+-K+ pump failure → influx of Na* (and accumulate inside the cell) and K to diffuse out
• Failure of the Ca2+ pump → influx of calcium

↓  

Net gain of solute is accompanied by isosmotic gain of water 

↓ 

✓ Cellular swelling. 

✓ Dilatation of endoplasmic reticulum. 

✓ Loss of microvilli. 

✓ Cellular blebs. 

b) Cellular energy metabolism is altered: Oxidative phosphorylation, ATP & ↑AMP (adenosine monophosphate) → ↑Glycolysis & Glycogenolysis. 

c) Rapid depletion of glycogen store & increased anaerobic glycolysis: Lactic acid & 

inorganic phosphates →↓pH→↓activity of many cellular enzymes. 

d) Detachment of ribosome from ER: ↓Protein synthesis. 

e) Misfolding of proteins & their accumulation in the ER: Cell injury & cell death. 

f) Ultimately, there is irreversible damage to mitochondrial & lysosomal membranes, and 

the cell undergoes necrosis. 

2) Mitochondrial damage: 

Mitochondria have a critical role in all pathways leading to cell injury & cell death. 

Causes of mitochondrial damage: 

✓ ↑ Cytosolic calcium. 

✓ Reactive oxygen species (ROS). 

✓ Oxygen deprivation (make the mitochondria sensitive to virtually all types of injurious stimuli). 

✓ Mutation of mitochondrial gene (in some inherited diseases). 

Effects of mitochondrial damage: 

Major consequences of mitochondrial damage are- 

a) ATP depletion: (discussed earlier). 

b) Formation of a 'high-conductance channel (mitochondrial permeability transition pore)' in the mitochondrial membrane 

↓ 

Loss of mitochondrial membrane potential.

↓

• Failure of oxidative phosphorylation. 
• Progressive depletion of ATP. 
• Cell necrosis. 

c) Formation of reactive oxygen species & their effects (please see below). 

d) Leakage of apoptotic proteins, e.g. cytochrome c & caspases (situated in between outer & inner mitochondrial membrane) into the cytosol → apoptosis. 

3) Membrane damage & defects in membrane permeability: 

Causes/mechanism of membrane damage: 

Early loss of selective membrane permeability, leading ultimately to membrane damage. Causes are- 

✓Free radical (causes injury to the cell membrane).

✓Decreased phospholipid synthesis. 

✓Increased phospholipid breakdown. 

✓Cytoskeletal abnormalities. 

Consequences of membrane damage: 

Membrane damage is a consistent feature of most form of cell injury except apoptosis. 

a) Mitochondrial membrane damage: Discussed earlier. 

• ➤Failure of oxidative phosphorylation.
• ➤Progressive depletion of ATP.
• ➤Cell necrosis.
• ➤Apoptosis. 
• ➤ Formation of reactive oxygen species (ROS). 

b) Plasma membrane damage: 

• ➤Loss of osmotic balance → influx of fluid & ions. 
• ➤Loss of cellular contents.
• ➤"Leakage of metabolites, those are vital for the reconstitution of ATP → further depletion of energy stores. 

c) Injury to lysosomal membranes: Loss of lysosomal membrane

 ↓

Leakage of lysosomal enzymes into the cytoplasm; e.g. RNases, DNases, proteases, phosphatases & glucosidases

↓  

Enzymatic digestion of RNA, DNA, protein, phosphate & glycogen respectively

↓ 

Cells death & necrosis 

4) Damage to DNA: Damage to nuclear DNA activates sensors that trigger p53-dependent pathways. Cells have physiological mechanisms that repair damage to DNA, but, DNA damage persists. 

Causes of DNA damage: 

✓Exposure to radiation. 

✓Chemotherapeutic (anti-cancer) drugs. 

✓Oxidative stress (produces ROS). 

✓ Aging (may occur spontaneously as a part of aging). 

Effects of damage to DNA: 

a) Aging. 

b) Cell cycle arrest. 

c) If physiological repair mechanism fails → apoptosis (by activation of caspases). 

d) If DNA damage is too severe to be corrected or mutations in p53 → malignant transformation. 

5) Oxidative stress – accumulation of oxygen-derived free radicals: 

Causes of accumulation of O2-derived free radicals: 

✓The reduction-oxidation reactions that occur during normal metabolic processes. 

✓Absorption of radiant energy (e.g. UV light, X-rays etc.). 

✓Rapid bursts of reactive oxygen species (ROS). 

✓Enzymatic metabolism of exogenous chemicals or drugs (CCl, generates CCl3"). 

✓Transition metals (e.g. Fe & Cu) → donate/accept free electrons during intracellular reactions → catalyze free radical formation. 

✓Nitric oxide. 

Effects of accumulation of O2-derived free radicals: 

a) Lipid peroxidation in membranes. 

b) Oxidative modifications of proteins. 

c) Lesions in DNA (DNA damage) 

6) Disturbance in calcium homeostasis: 

Calcium ions normally serve as 2 2nd messengers in several signaling pathways. If released into the cytoplasm of cells in excessive amounts, are also important sources of cell injury. Most intracellular Ca2+ is sequestrated in mitochondria & ER. 

Causes of increased intracellular Ca2+ level: Initially Ca2+ releases from intracellular stores & later influxes across the cell membrane. 

✓Ischemia. 

✓Certain toxins. 

Effects of increased intracellular / cytosolic Ca2+ level: 

a) Opening of the 'mitochondrial permeability transition pore' → failure of ATP generation (discussed earlier). 

 b) Activation of potentially harmful enzymes: Increased cytosolic Ca** activates a number of enzymes, with potentially deleterious cellular effects, e.g. 

• ATPase→ Leads to ATP depletion.
• Phospholipase→ Membrane damage. 
• Protease → Breakdown of membrane and cytoskeletal proteins.
• Endonucleases → Fragmentation of DNA and chromatin. 

c) Direct activation of caspases & Mitochondrial permeability → apoptosis. 

7) Endoplasmic reticulum (ER) stress: 

The unfolded protein response Causes of intracellular accumulation of misfolded proteins: (Increase production or decreased ability to repair & elimination) 

✓Ischemia. 

✓Hypoxia. 

✓Radiation. 

✓Mutation. 

✓Viral infection. 

✓Insulin-resistant states. 

Effects of damage to DNA & proteins: 

a) Trigger apoptosis by activating pro-apoptotic proteins. 

b) Cellular aging. 

c) Neurodegenerative disease: 

• ➤ Alzheimer disease. 
• ➤ Familial hypercholesterolemia. 
• ➤ Cystic fibrosis. 
• ➤ⲁ-antitrypsin deficiency. 
• ➤Tay-Sachs disease. 
• ➤ Creutzfeldt-Jacob disease. 

ATP is produced in two ways: 

1) Oxidative phosphorylation: It is the major pathway in mammalian cells in non-dividing (e.g. brain and liver) cells by the mitochondrial electron transport system. 

2) Glycolysis. 

Roles of ATP in the body: 

✓High-energy phosphate in the form of ATP is required for virtually all synthetic and degradative processes within the cell. These include membrane transport, protein synthesis, lipogenesis, and the deacylation-reacylation reactions necessary for phospholipid turnover. 

✓Depletion of ATP to 5% to 10% of normal levels has widespread effects on many critical cellular systems.  

Oxidative stress: 

Increased production or decreased scavenging of reactive oxygen species (ROS) may lead to accumulation of an excess of free radicals, a condition called oxidative stress. 

Reactive oxygen species (ROS) / free radicals: 

❖ Free radicals are chemical species that have a single unpaired electron in an outer orbit.

❖ Unpaired electrons are highly reactive and 'attack' and modify adjacent molecules, such as inorganic or organic chemicals (carbohydrates, proteins, lipids, nucleic acids etc.), many of which are key components of cell membranes & nuclei. 

❖ Some of above reactions are autocatalytic, whereby molecules that react with free radicals are themselves converted into free radicals, thus propagating the chain of damage. 

Types of free radicals: 

1. Oxygen derived free radicals: Superoxide (O2), H2O2, OH 
2. Carbon derived free radicals: CCl2 Hoc 
3. Nitrogen derived free radicals: NO, NO2, NO3".
4. Transitional metal: Feat. 

Characteristics of free radical: 

1. Has unpaired electron (s) in the outer orbit.
2. Highly reactive chemical species.
3. Half-life is very short.
4. Interact with other substance to lose or gain electron.
5. Cause damage to the living cells, tissues and responsible for early aging process & different diseases (neurodegenerative diseases, atherosclerosis, IHD etc.). 

Sources/mechanism of formation of free radicals: 

1. The reduction-oxidation reactions that occur during normal metabolic processes: 

✓Small amount of free radicals produce spontaneously during mitochondrial respiration & energy generation by oxidative enzymes in the ER, cytosol, mitochondria, peroxisomes and lysosomes. 

✓Free radicals that produces normally: They are- 

• Superoxide anion (O2, one electron).
• Hydrogen peroxide (H2O2, two electrons), &
• Hydroxyl ions ("OH, three electrons). 

2. Absorption of radiant energy (e.g. UV light & X-rays): Ionizing radiation can hydrolyze water into 'OH & hydrogen (H) free radicals. 

3. Rapid bursts of reactive oxygen species (ROS): Produced in activated leukocytes during inflammation. 

4) Enzymatic metabolism of exogenous chemicals or drugs (e.g. CCL generates CC13'). 

5) Transition metals (e.g. Fe & Cu) → donate/accept free electrons during intracellular reactions → catalyze free radical formation. 

6) Nitric oxide (NO): It is an important chemical mediator generated by endothelial cells, macrophages, neurons & other cell types. It can act as a free radical and can also be converted to highly reactive peroxynitrite anion (ONOO) as well as NO, & NO. 

Cell injury caused by free radical / reactive oxygen species (ROS) / oxidative stress: 

Cell injury induced by free radicals (particularly by ROS), is an important mechanism of cell damage in many pathologic conditions, such as- 

1. Chemical and radiation injury.
2. Ischemia-reperfusion injury (induced by restoration of blood flow in ischemic tissue).
3. Cellular aging.
4. Cancer.
5. Microbial killing by phagocytes.
6. Some degenerative diseases (e.g. Alzheimer disease). 

Injuries caused by free radicals: 

1. Lipid peroxidation of membranes: 

O2-derived free radicals (particularly by 'OH) 

↓ 

Attack double bonds in unsaturated fatty acids of membrane lipids

↓ 

Initiation of oxidative damage 

↓ 

Peroxidation of lipids occurs within plasma and organellar membranes 

↓ 

The 'lipid-free radical interactions' yield peroxides, which are themselves unstable and reactive 

↓ 

An autocatalytic chain reaction ensues (called propagation) 

↓ 

Further extensive membrane damage occurs (viscous cycle continues) 

2) Oxidative modification of protein: 

Free radicals causes: 

• Oxidation of amino acid side chains. 
• Formation of covalent protein-protein cross-lins (e.g. disulfide bond), and 
• Oxidation of protein backbone resulting in protein fragmentation. 

The effects of oxidative modifications are- 

• Damage of active site of proteins. 
• Disruption of structural proteins. 
• Enhance proteasomal degradation of unfolded or misfolded proteins. 

3) Lesions in DNA: Free radicals are capable of causing- 

• Single- & double-strand breaks in DNA. 
• Cross linking of DNA strands. 
• Formation of adducts 

Inactivation / destruction of free radicals: Free radicals are destroyed by- 

1. Spontaneous destruction: Free radicals are inherently unstable and generally decay spontaneously.
2. Cellular mechanism: 

• ➤Antioxidants: They block synthesis of free radicals, inactivate them & terminate radical damage e.g. vitamin A, C & E (ACE) and glutathione etc. 
• ➤ Binding with storage and transport proteins: Iron and copper can catalyze the formation of reactive oxygen species. The levels of these reactive metals are minimized by binding of the ions to storage and transport proteins (e.g. transferrin, ferritin and caeruloplasmin), thereby minimizing formation of reactive oxygen species. 
• ➤Enzymes: 

✓Catalase: It breaks H2O2 (2 H2O2 → O2 + 2H2O). 

✓Superoxide dismutase: It converts superoxide (O2 ̄) to H2O2. 

202+ 2H⭑→ H2O2 + O2 

✓Glutathione peroxidase: 

H2O2+2GSH → GSSG (glutathione homodimer) + 2H2O 20H ̄ +2GSH → GSSG (glutathione homodimer) + 2H2O 

Example of cell injury: 

• ➤Ischemic &/or hypoxic cell injury. 
• ➤Reperfusion injury. 
• ➤Chemical (toxic) injury. 

Ischemic & Hypoxic cell injury 

Ischemia, the most common cause of cell injury in clinical medicine, results from hypoxia induced by reduced blood flow, most often due to a mechanical arterial obstruction. It can also occur due to reduced venous drainage. 

Mechanism: 

As intracellular oxygen tension falls.

↓ 

Oxidative phosphorylation fails and ATP generation decreases. 

↓ 

Loss of ATP results initially in reversible cell injury (cell and organelle swelling). 

↓ 

Later in cell death by necrosis 

Sequence of events / pathogenesis / mechanism of ischaemic &/or hypoxic cell injury: 

Reversible events/changes following ischaemic &/or hypoxic cell injury: 

1) Loss of oxidative phosphorylation & generation of ATP: 

a) Failure of ATP dependent Na' pump: 

• ↑ influx of Ca, H2O and Na*, Efflux of K → Cellular swelling, swelling of ER, loss of microvilli & cellular blebs. 

b) Increased anaerobic glycolysis: ↓ Glycogen, ↑ lactic acid, ↓ pH → Clumping of nuclear chromatin. 

c) Detachment of ribosomes: ↓ Protein synthesis. 

2) Mitochondrial membrane damage: 

• ➤ Failure of oxidative phosphorylation & progressive depletion of ATP. 
• ➤ Formation of reactive oxygen species (ROS). 
• ➤ Swelling and the appearance of small phospholipid-rich amorphous densities. 

3) Plasma membrane damage: 

• ➤ Loss of osmotic balance → influx of fluid & ions → cellular swelling, dilatation of ER, loss of microvilli & cellular blebs. 
• ➤ Leakage of metabolites, those are vital for the reconstitution of ATP→ further depletion of energy stores. 
• ➤ Loosening of intercellular attachments. 

4) Nuclear alterations: Clumping of chromatin. 

5) Gross changes / macroscopic changes: 

• ➤ Cellular swelling → pallor, ↑turgor & Weight of the organ. 
• ➤ Fatty changes. 
• ➤ Hydropic change or vacuolar degeneration. 

Irreversible changes following ischemic / hypoxic cell injury: 

1. Increased eosinophilia of cells. 
2. Loss of glycogen particles → more glassy homogeneous appearance than that of normal cells. 
3. Lysis of endoplasmic reticulum (ER) → damage of protein synthesis machinery. 
4. Cytoskeletal alteration.
5. Extensive damage of plasma membrane: 

• ➤ Loss of cellular contents.
• ➤ Giving rise to myelin figures. 

 6. Severe swelling of mitochondria & damage of mitochondrial cell wall: 

• ➤ Large, flocculent, amorphous densities develop in the mitochondrial matrix. 
• ➤Leakage of apoptic proteins, e.g. cytochrome c & caspases (situated in between outer & inner mitochondrial membrane) into the cytosol → apoptosis. 
• ➤Cell necrosis. 

7. Injury to the lysosomal membranes & swelling of the lysosomes: 

Loss of lysosomal membrane

 ↓ 

Leakage of lysosomal enzymes into the cytoplasm, e.g. RNases, DNases, proteases, phosphatases, lipase & glucosideases

↓ 

Enzymatic digestion of RNA, DNA, protein, phosphate, lipid & glycogen respectively

↓ 

Cell dies by necrosis 

8. Nuclear changes: 

• ➤ Pyknosis.
• ➤ Karyorrhexis. 
• ➤ Karyolysis. 

Ischaemia tends to cause more and severe cell and tissue injury than hypoxia: 

1. In hypoxia, there is only reduced oxygen availability to cells and tissues. On the other hand, in ischaemia, the supply of oxygen and nutrient is decreased.
2. In hypoxia, energy production by anaerobic glycolysis can continue. But in ischaemia, delivery of substances for glycolysis is compromised. So, in ischaemia, both aerobic and anaerobic glycolysis is inhibited.
3. Again, in ischaemia, there is accumulation of metabolites that would have been removed otherwise by blood flow. 

For these reasons, ischaemia tends to cause more rapid and severe injury than does hypoxia alone. 

Ischaemia-reperfusion injury / reperfusion injury: 

Paradoxical & progressive cell injury followed by cell death even after restoration of blood flow to ischaemic tissues in a reversibly injured cell (which was caused by ischaemia) is called ischaemia reperfusion injury. 

• ✓ Restoration of blood flow to ischemic tissues can promote recovery of cells if they are reversibly injured. 
• ✓But in case of ischaemia-reperfusion injury, reperfused tissues sustain & continue the cell injury and ultimately irreversible cell injury & cell death occurs. 

Importance: It is clinically important because it contributes to tissue damage during myocardial infarction, cerebral infarction following therapies to restore blood flow. 

Mechanism of reperfusion injury: 

1. Oxidative stress: New damage may be initiated during reoxygenation by increased generation of reactive oxygen and nitrogen species. These free radicals may be produced in reperfused tissue as a result of incomplete reduction of oxygen in leukocytes, and in damaged endothelial cells and parenchymal cells. Compromise of cellular antioxidant defense mechanisms during ischemia may sensitize cells to free radical damage.
2. Intracellular calcium overload. 

Intracellular & mitochondrial calcium overload begins during acute ischemia

 ↓ 

Ca2+ overload exacerbated during reperfusion due to influx of calcium resulting from- 

• Cell membrane damage and 
• ROS mediated injury to sarcoplasmic reticulum. 

 ↓ 

Oening of the mitochondrial permeability transition pore with resultant depletion of ATP.

 ↓ 

Further cell injury 

3) Inflammation: 

Ischemic injury is associated with inflammation caused by resident immune cells (e.g. macrophages) 

 ↓ 

Increased expression of adhesion molecules by hypoxic parenchymal & endothelial cells

 ↓

Reperfusion recruits more circulating neutrophils to the injured tissue

 ↓

Further tissue injury & cell death 

4) Activation of the complement system: 

Some IgM antibodies have a propensity to deposit in ischemic tissues 

 ↓

For unknown reasons & when blood flow is resumed, complement proteins bind to the antibodies, are activated

 ↓

Further cell injury and inflammation 

Chemical injury: 

Cell injury caused by chemicals (mostly environmental), drugs & toxins is called chemical injury. 

✓ Most of the drugs are metabolized in the liver & is a frequent target of drug toxicity. 

✓ In fact, toxic liver injury is perhaps the most frequent reason for terminating the therapeutic use or development of a drug. 

✓ Chemical injury remains a frequent problem in clinical medicine and is a major limitation to drug therapy. 

Mechanism of chemical toxicity: 

🕮Direct toxicity: Some chemicals can injure cells directly by combining with critical molecular components, e.g. 

• ➤In case of mercuric chloride poisoning, mercury binds to the sulfhydryl groups of cell membrane proteins → ↑membrane permeability & inhibition of ion transport → cell injury. 
• ➤Cyanide poison inhibits oxidative phosphorylation 

🕮Conversion to toxic metabolites: 

• Most toxic chemicals are not biologically active in their native form but must be converted to reactive toxic metabolites, which then act to target molecules.
• This modification is usually accomplished by the cytochrome P-450 mixed function oxidase (Cyt. P450 MFO) enzymes in the smooth ER of the liver and other organs.
• The toxic metabolites cause membrane damage and cell injury mainly by formation of free radicals and subsequent lipid peroxidation.
• Direct covalent binding to membrane proteins and lipids may also contribute. 

Example: 

✓ CCI, (widely used in the dry-cleaning industry), is converted by cytochrome P-450 to the highly reactive free radical ̊CCl3 → causes lipid peroxidation & damages many cellular structures. 

✓ Acetaminophen (Paracetamol) also converted to a toxic product during detoxification in the liver → cell injury. 

Necrosis: 

Necrosis is a pathologic process that is the consequences of severe injury, which is characterized by denaturation of cellular proteins, leakage of cellular contents through damaged membranes, local inflammation, and enzymatic digestion of the lethally injured cell. 

Necrosis refers to a spectrum of morphologic changes that follow cell death in living tissue, largely resulting from the progressive degradative action of enzymes on the lethally injured cell. 

Classification / types of necrosis with example: 

🕮Basic types of necrosis: 

1. Coagulative necrosis: e.g. Ischaemic necrosis of- 

• Heart (myocardial infarction),
• Kidney,
• Liver,
• Adrenal gland & 
• Other solid organs. 

2) Liquefactive / colliquative necrosis: [Mnemonic- BBA] 

• Ischaemic necrosis of brain tissue. 
• Boil.
• Abscess. 

🕮Special types of necrosis: 

1. Caseous necrosis: e.g. Granuloma of tuberculosis.
2. Fat necrosis: 

• Enzymatic fat necrosis: e.g. enzymatic fat necrosis of pancreas and omental tissue. 
• Traumatic fat necrosis: e.g. traumatic fat necrosis of breast. 

3) Gangrenous necrosis: Any necrosis with superadded putrefaction. 

4) Fibrinoid necrosis: Acute rheumatic fever, rheumatoid arthritis, systemic lupus erythematosus.

5) Necrosis of muscle: e.g. Zenker's degeneration. 

6) Osteonecrosis: Necrosis of bone can affect the medullary bone with its medullary cavity and trabecular bone or affect both medullary and cortical bone. 

Ischaemic necrosis: 

Ischaemic necrosis means necrosis of tissue that result from impaired arterial supply or venous drainage from that tissue. 

Morphology of ischaemic necrosis: 

🕮Ischaemic necrosis of all solid organs except brain 

🕮Ischaemic necrosis of brain 

Sequelae of necrosis/cell death: 

1. When small numbers of cells are involved, the cellular debris is removed by phagocytosis.
2. With larger numbers of dead cells, there is an inflmmatory response with organization and firous repair.
3. When the necrotic tissue cannot be completely removed or organised, deposition of calcium may be an additional feature, for example in tuberculous caseous necrosis. This feature is important in radiologic diagnosis. It is known as 'dystrophic calcifiation'.  

Autolysis:

When enzymatic digestion of a cell occurs by its own lysosomal enzyme, is called autolysis. 

Heterolysis:

When enzymatic digestion of a cell occurs by the lysosomal enzymes of immigrant leukocytes during inflammatory reaction, is called heterolysis. 

Morphological changes of necrosis 

🕮Cytological changes in necrosis: 

1. Staining: Increased cosinophilia in hematoxylin and eosin (H & E) stains.
2. Appearance: More glassy homogeneous appearance than do normal cells (due to loss of glycogen particles).
3. Cytoplasm: Enzymatic digestion of cytoplasmic organelles → cytoplasm becomes vacuolated and appears moth-eaten. 
4. Myelin figures: Replacement of dead cells by large, whorled phospholipid masses → called myelin figures (derived from damaged cell membranes). 

Fate of myelin figures are- 

🕮Mayelin figures may phagocytosed by other cells or 

🕮They may further degraded into fatty acids → calcification of fatty acid residues → generation of calcium soaps → thus, the dead cells may ultimately become calcified. 

Nuclear changes in necrosis (due to non-specific breakdown of DNA): 

1) Karyolysis: 

• The basophilia of the chromatin may fade, 
• This change presumably reflects loss of DNA because of enzymatic degradation by endonucleases. 

2) Pyknosis: 

• Pyknosis is also seen in apoptic cell death. 
• It is characterized by nuclear shrinkage and increased basophilia. 
• Here the chromatin condenses into a solid, shrunken basophilic mass. 

3) Karyorrhexis: 

• Here, the pyknotic nucleus undergoes fragmentation. 
• With the passage of time (a day or two), the nucleus in the necrotic cell totally disappears. 

Figure: -Nuclear and cytoplasmic changes in necrosis. 

Pathogenesis of inflammation in necrosis: 

Cellular contents leak through the damaged plasma membrane into the extracellular space, e.g. ATP (released from damaged mitochondria), Uric acid (a breakdown product of DNA) & other molecules that are released after severe cell injury.

↓

These molecules are recognized by receptors present in macrophages and most other cell types 

↓

Trigger phagocytosis of the debris and there is production of cytokines

↓

Cytokine induces inflammation 

Coagulative necrosis / structured necrosis: 

• ➤ Coagulative necrosis is a form of necrosis in which the basic cellular shape, outline & architecture of dead tissues is preserved for at least some days. 
• ➤The affected tissues exhibit a firm texture.
• ➤ A localized area of coagulative necrosis is called an infarct. 
• ➤Also called structured necrosis. 

Pathogenesis of coagulative necrosis: 

The injury denatures both structural & enzymatic proteins due to↑ Intracellular pH

↓ 

Proteolysis of the dead cells is blocked (due to lack of proteolytic enzymes)

↓

Eosinophilic, anucleate cells may persist for days or weeks 

↓

Ultimately the necrotic cells are removed by - 

• Phagocytosis of the cellular debris by infiltrating leukocytes &
• By digestion of the dead cells by the action of lysosomal enzymes of the leukocytes. 

Examples: Ischemic necrosis in all solid organs except the brain, such as - 

• ➤Heart (myocardial infarction).
• ➤Kidney.
• ➤Liver. 
• ➤Adrenal gland.
• ➤Gumma of tertiary syphilis. 

Morphology: 

🕮The affected tissues exhibit a firm, dry, opaque and homogenous architecture. 

🕮Microscopic: -

• Cellular outline: Basic cellular outline is preserved. 
• Cytoplasm: Becomes opaque and acidophilic.
• Nucleus: Nucleus shows pyknosis, karyorrhexis & karyolysis.  

Liquefactive / colliquative necrosis: 

• ➤Liquefactive necrosis is characterized by digestion of the dead cells, resulting in transformation of the tissue into a liquid viscous mass.
• ➤It is seen in focal bacterial & occasionally in fungal infections.
• ➤For unknown reasons, hypoxic death of cells within the CNS often manifests as liquefactive necrosis.
• ➤If the process had been initiated by acute inflammation → the material is frequently creamy yellow because of the presence of dead WBC & called 'pus'. 

Pathogenesis: 

Microorganisms (usually bacterial & occasionally fungal) stimulate the accumulation of leukocytes and the liberation of enzymes from these cells

↓

Enzymatic digestion of the dead cell completely

↓

Transformation of the tissue into a liquid viscous mass 

Example: 

• ➤CNS infarction: Ischaemic necrosis of brain tissue.
• ➤Suppurative inflammation: Abscess & boil. 

Morphology: 

🕮Macroscopic: 

• Softening of the necrosed area (liquid viscous mass).
• Colour: Creamy yellow (due to presence of dead WBC) if pus is formed. 

🕮Microscopic: Basic cellular outline is not preserved. 

Caseous necrosis: 

Caseous means 'cheese-like' 

• ➤ It is a distinctive form of coagulative necrosis.
• ➤It occurs most commonly in foci of tuberculous infection.
• ➤There is formation of soft, friable, amorphous granular debris resembling white & cheesy material (hence named as 'caseaous'). 

 Example:  

• ➤Granuloma of tuberculosis.
• ➤Cat scratch disease. 

Morphology: On microscopic examination-

➤The tissue architecture: Tissue architecture is lost & the cell outlines are not preserved. 

➤Formation of granuloma: The necrotic area appears as a structureless collection of fragmented or lysed cells and amorphous granular debris enclosed within a distinctive inflammatory border, this appearance is characteristic of a focus of inflammation known as a 'granuloma'. 

Gangrenous necrosis 

• ➤Gangrenous necrosis is not a specific pattern of cell death, but the term is commonly used in clinical practice.
• ➤It is usually applied to a limb, generally the lower leg.
• ➤Loss of blood supply in lower leg causes necrosis (typically coagulative necrosis) → dry gangrene.
• ➤When bacterial infection is superimposed → liquefactive necrosis occurs due to actions of degradative enzymes of bacteria and the attracted leukocytes → giving rise of wet gangrene. 

Fat necrosis:

It occurs in adipose tissue due to action of lipase. 

Classification of fat necrosis: 2 types. 

🕮Enzymatic fat necrosis: 

Site: Pancreas & omental tissue.

Pathogenesis: 

In milder form of acute interstitial pancreatitis 

↓ 

Pancreatic enzymes leak out of acinar cells 

↓ 

Release of activated pancreatic lipases into the pancreas, peripancreatic fat & peritoneal cavity 

↓ 

The released lipases split the triglycerides to free fatty acids 

↓ 

Free fatty acid combines with calcium to produce grossly visible chalky-white areas (fat saponification) 

Morphology: On histologic examination the necrosis takes the form of foci of shadowy outlines drop necrotic fat cells, with basophilic calcium deposits, surrounded by an inflammatory reaction. 

❖ Traumatic fat necrosis: 

Site: Breast, abdominal fat etc. 

Pathogenesis: Unlike pancreatic fat necrosis, it is not enzyme mediated. 

Surgery of blunt trauma of the breast tissues or abdominal fat 

↓

Acute lesions may be hemorrhagic and contain central areas of liquefactive fat necrosis with neutrophils and macrophages 

↓

Over the next few days proliferating fibroblasts and chronic inflammatory cells surround prio the injured area 

↓

Subsequently, giant cells, calcifications, and hemosiderin make their appearance 

↓

Eventually the focus is replaced by scar tissue or is encircled and walled off by fibrous tissue. 

Morphology: 

❖Macroscopically: Ill defined, firm, graywhite nodules containing small chalky-white foci are seen grossly. 

❖Microscopically: Nutrophil, lipid-laden macrophages, giant cells & fibroblast is seen macroscopically.  

Fibrinoid necrosis: 

❖Fibrinoid necrosis is a special form of necrosis usually seen in immune reactions involving blood vessels. 

Site: -

• ✓ The immune-complex mediated vasculitis syndromes, e.g. polyarteritis nodosa. 
• ✓ Acute rheumatic fever.
• ✓ Connective tissue disorder, e.g. Rheumatoid arthritis (RA), SLE etc.
• ✓ Arthus phenomenon.
• ✓ Malignant hypertension in arterioles. 

Pathogenesis: 

Antigens & antibody complexes are deposited in the walls of arteries

↓

Deposition of these "immune complexes", together with fibrin that has leaked out of vessels 

↓

Result in a bright pink and amorphous appearance in H&E stains, called 'fibrinoid' (fibrin-like) necrosis 

Reference: -

🖋Robbins 10th

🖋Pathology Tutorial 

THANK YOU STAY CONNECTED.........

An introduction to pathology | basis of pathology | cellular response to injury | Hypoxia | and...

 Introduction to Pathology

In the preface of the very first edition of Patho- logic Basis of Disease (1957), Stanley Robbins wrote: "The pathologist is interested not only in the recognition of structural alterations, but also in their significance, i.e., the effects of these changes on cellular and tissue function and ultimately the effect of these changes on the patient. It is not a discipline isolated from the living patient, but rather a basic approach to a better understanding of disease and there-fore a foundation of sound clinical medicine". 

The study of the origin & development of disease 

📙Pathologists investigate etiology & pathogenesis of disease by way of morphology (morphe form), using techniques of histology (histo tissue) as well as biochemistry and molecular biology with a goal of easing clinical signs and symptoms where the clinician begins.

📙Pathology means scientific study of sufferings or disease. (Pathos = suffering. Logos = study) 

📙The term pathology is used when one refers to the "scientific study of disease" or the alterations that occur when abnormal influences (bacteria, viruses, etc.) affect cells, tissues, or body systems. More specifically, pathology may be defined as the "scientific study of the molecular, cellular, tissue, or organ system response to injurious agents or adverse influences." Pathology is the study & diagnosis of disease through examination of organs, tissues, body fluids & whole bodies (autopsies).

📙Pathology has been one of the "keystones" of medicine & it serves as a "bridge" or "link" between the preclinical subjects (anatomy, physiology, etc.) & the courses in clinical medicine. Actually, pathology provides a logical means of relating the knowledge of normal structure & function (anatomy & physiology) to abnormal structure & function as encountered in a diseased person. 

It should be emphasized that pathology, as the scientific study of disease, follows the morbid process from its inception to its termination, & investigates the lesions produced. Therefore, a sound knowledge of pathology is the foundation of a solid understanding of disease as it occurs in the living patient. Pathology actually deals with etiology, pathogenesis, morphologic changes and clinical course of disease.

📙Pathology is devoted to the study of the structural, biochemical & functional changes in cells, tissues, and organs that underlie disease. By the use of molecular, microbiologic, immunologic and morphologic techniques, pathology attempts to explain the whys and wherefores of the signs and symptoms manifested by patients while providing a rational basis for clinical care and therapy. Traditionally the study of pathology is divided into general pathology and systemic pathology. The four aspects of a disease process that form the core of pathology are its cause (etiology), the biochemical and molecular mechanisms of its development (pathogenesis), the structural alterations induced in the cells and organs of the body (morphologic changes), & the functional consequences of these changes (clinical manifestations). 

📙General Pathology: General pathology is concerned with the common reactions of cells & tissues to injurious stimuli. Such reactions are often not tissue specific: thus acute inflammation in response to bacterial infections produces a very similar reaction in most tissues. General pathology is a broad and complex scientific field which seeks to understand the mechanisms of injury to cells and tissues, as well as the body's means of responding to and repairing injury. Areas of study include cellular adaptation to injury, necrosis, inflammation, wound healing and neoplasia. It forms the foundation of pathology, the application of this knowledge to diagnose diseases in humans and animals.

📙Systemic Pathology: Refers to the study of the diseases of the organ systems of the body such as the respiratory system, digestive system and nervous system. Systemic pathology examines the alterations & underlying mechanisms in organ specific diseases such as ischemic heart disease.

📙Anatomic Pathology: Anatomic pathology is a medical specialty that is concerned with the diagnosis of disease based on the gross, micro- scopic, chemical, immunologic and molecular examination of organs, tissues & whole bodies (autopsy). Anatomic pathology is itself divided in sub specialties, the main ones being surgical pathology, cytopathology & forensic pathology.

📙Clinical Pathology: Clinical pathology or Laboratory medicine, is a medical specialty that is concerned with the diagnosis of disease, based on the laboratory analysis of body fluids such as blood and urine, and tissues using the tools of chemistry, microbiology, hematology & molecular pathology. Clinical pathologists work in close collaboration with medical technologists, hospital administrations, & referring physicians to ensure the accuracy and optimal utilization of laboratory testing.

📙Forensic Pathology: Forensic pathology is a branch of pathology concerned with determining the cause of death by examination of a cadaver. The autopsy is performed by the pathologist at the request of a coroner usually during the investigation of criminal law cases and civil law cases in some jurisdictions. Forensic pathologists are also frequently asked to confirm the identity of a cadaver. 

Pathology as a Medical Specialty

🖋 Pathologists are physicians who diagnose and characterize disease in living patients by exam- ining biopsies or body fluid. The vast majority of cancer diagnoses are made or confirmed by a pathologist. Pathologists may also conduct autopsies to investigate causes of death. Pathol- ogy is a core discipline of medical school and many pathologists are also teachers. As manag- ers of medical laboratories, pathologists play an important role in the development of laboratory information systems. 

🖋 Pathology is a unique medical specialty in that pathologists typically do not see patients directly, but rather serve as consultants to other physicians (often referred to as "clinicians" within the pathology community). 

🖋 In summary, pathology is one of several mechanisms employed to solve those problems encountered in clinical situations. Thus, the student is required to make practical use of information accumulated in the General and Special Pathology courses. The compartmental- ization & storage of knowledge for examination purposes is an exercise in futility. However, the utilization of accumulated knowledge in under- standing clinical problems is an educational reality. 

Basic Language of Pathology 

In order for a subject or course to be meaningful, one should become familiar with the basic terminology applicable to that subject. Listed below are a few basic terms used repeatedly in pathology and/or veterinary medicine. The student should become familiar with these terms and their definitions. 

Disease: A disease may be defined as a "state in which an individual exhibits an anatomical, physiological, or biochemical deviation from the normal." As generally used, the term "disease" is employed to describe a state in which there is sufficient departure from the normal for clinical signs or symptoms to be produced. 

Lesions: The term lesion is generally used to prefer to "structural or morphological altera- tions associated with a diseased state in an individual." It is the objective deviation from the normal. Lesions may be recognized with the naked-eye (gross lesions), with the aid of a light microscope (microscopic lesions), or with the aid of the electron microscope (ultrastructural lesions). Biochemical or functional lesions are recognized as changes which result from disturbed function. 

Etiology or Cause: The term "etiology" refers to a "study of the cause of a disease." An me etiologic agent is the factor (bacterium, virus, etc.) responsible for lesions or a disease state. 

Predisposing causes of disease: Refer to those factors which make an individual more suscep- tible to a disease (damp weather, poor ventila- tion, etc.) 

Exciting causes of disease: Refer to those factors which are directly responsible for a disease (bacteria, viruses, hypoxia, chemical agents, etc.). Etiologic factors can all be grouped into two classes: genetic (e.g., inherited muta- tions and disease associated gene variants, or polymorphisms) and acquired (e.g., infectious, nutritional, chemical, physical). 

Pathogenesis: Pathogenesis refers to the sequence of cellular, biochemical, and molecu- lar events that follow the exposure of cells or tissues to an injurious agent. The term "patho- genesis" refers to the "progressive development (sequence of events) of a disease from the time it is initiated to its final conclusion in recovery or death." 

Morphologic Changes: Morphologic changes refer to the structural alterations in cells or tissues that are either characteristic of a disease or diagnostic of an etiologic process. 

Pathognomonic Lesio: Refers to a change which is specifically characteristic of a disease. When one sees a pathognomonic lesion, he knows that a particular disease is present. 

Health: As generally used, the term "health" refers to the "state in which an individual is living in complete harmony with his environ- ment," it is a relative state. All body functions are performed normally even though lesions may be present in organs and/or tissues. It should be remembered that the transitional zone between health and disease is difficult to define. 

Clinical Signs : Refer to any "functional evidence of disease which can be determined objectively or by the observer" (lameness, salivation, increased respiratory efforts etc.). Remember, clinical signs are seen only in the living individual. The term clinical symptoms should be reserved for any "functional evidence of disease that can be determined subjectively or by the patient" (feeling of abdominal discomfort etc.). The end results of genetic, biochemical, and structural changes in cells and tissues are func- tional abnormalities, which lead to the clinical manifestations (symptoms & signs) of disease, as well as its progress (clinical course and outcome). Hence, clinicopathologic correlations are very important in the study of disease

The end results of genetic, biochemical, and structural changes in cells and tissues are func- tional abnormalities, which lead to the clinical manifestations (symptoms & signs) of disease, as well as its progress (clinical course and outcome). Hence, clinicopathologic correlations are very important in the study of disease

Diagnosis: The term "diagnosis" refers to the "determination of the nature of a disease expressed in a concise manner." 

A morphologic or anatomic diagnosis is based on the location and nature of the lesion (hemorrhagic enteritis, etc.). Etiologic diagnosis is made on the basis of the cause (dirofilariasis, etc.). Definitive diagnosis is made on the basis of the specific disease entity involved (canine distemper, animal disease etc.). A clinical diag- nosis is made on the basis of clinical signs observed in the living animal. 

Biopsy: Removal or collection of tissue or specimen obtained from the living body for examination is known as biopsy.

Necrosis: Refers to the morphological changes caused by the progressive degradative action of enzymes on the lethally injured cell within the living body. After a cell dies, lysosomes rupture and their hydrolytic enzymes are released. The release and activation of these lysosomal enzymes are responsible for cell necrosis. Remember, necrotic cells are dead cells, but dead cells are not necessarily necrotic. 

Postmortem change: Refer to cell death which accompanies or occurs after death of the entire body (somatic death), whereas antemortem changes refer to those alterations that occur in cells, tissues, organs, etc. prior to somatic death or in the living individual. It is important to differentiate postmortem changes from ante- mortem changes in order to interpret correctly those lesions encountered at necropsy. 

Gross Pathology (macroscopic pathology, patho- logical anatomy, morbid anatomy): Refers to the study of disease in which tissues and organs are examined with the unaided eye. 

Cellular Pathology (microscopic pathology, histopathology): Refers to the study of diseased tissues & organs with the aid of a micro-scope. 

Surgical Pathology: Refers to the study of tissues removed at the time of surgery

Clinical Pathology: Refers to the study of disease by examination of blood, urine, feces, skin scrapings, etc. 

Immunopathology: Refers to the study of diseases associated with abnormalities of the immune mechanisms of the body. 

Chemical Pathology: Refers to the study of chemical changes in the fluids and tissues of the body as the result of disease. This branch of pathology is merely a portion of clinical pathology. 

Morphologic Terminologies

• Follicular pattern: Forming follicle-like structure. 
• Diffuse pattern: Distributed diffusely.
• Glandular pattern: Arranged in a gland like structure.
• Acinar pattern: Cells arranged in gland having lumen.
• Tubular pattern: Arranged in tubule like structure.
• Villous pattern: Forming villi-like structure
• Papillary pattern: Arranged in finger like structure.
• Stag-horn or antler-horn pattern: Arranged in tight clusters or monolayered sheets of cells taking the pattern of antler horn
• Cyst: - Abnormal closed sac like structure containing liquid or semi solid substances. lined by epithelium.
• Pseudocyst: Abnormal closed sac containing fluid covered by fibrocollagenous tissue
• Adenoma: Benign epithelial neoplasms forming glands or formed from the glands (but not necessarily exhibiting gland pattern).
• Papilloma: Benign epithelial neoplasm growing on any surface that produce micro- scopic or macroscopic finger-like projections. 
• Polyp: A mass that projects above a muco- sal surface to form a macroscopically visible structure. 
• Abscess: Collection of pus lined by pyogenic membrane.
• Ulcer: Ulcer is a local defect or excavation of the surface of an organ or tissue that is produced by the sloughing (shedding) of inflamed necrotic tissue (breech in the tissue continuum) e.g. Peptic ulcer, Typhoid ulcer. 

Cellular response to stress

Cells are the basic unit of tissues, which form organs and systems in human body.Normal cell is in a steady state "Homeostasis". Change in Homeostasis due to stimuli "Injury". When a cell is exposed to an injurious agent, it achieves a new steady state to survive is called cellular adaptation. If the adaptive capability is exceeded or if the external stress is inherently harmful or excessive, adaptation fails and thus cell injury develops. 

When stimuli or stress is mild to moderate the injured cell may recover or revert to normal (reversible cell injury). When stimuli or stress is persistent or severe the injured cell cannot recover or revert to normal (irreversible cell injury or cell death). The effects of reversible cell injury may persist in the cell as evidence of cell injury at subcellular level (subcellular alteration). Various metabolites may accumulate within the cells (intracellular accumulation). 

The effects of injury depend on : 

a. Type, duration and severity of injury.

b. Type of injured tissue, its adaptability and genetic makeup. 

■Brain tissue is very sensitive to hypoxia (3-4 min). 

■ Myocardial tissue is also sensitive to hypoxia (20-30 min).

■ Skeletal muscles can adapt hypoxia for 2-6 hours. 

■Reversibility depends on the type, severity and duration of injury. 

■Cell death is the result of irreversible injury. 

Injurious Stimulus / Stress factors (Causes of cell injury) 

The causes of cell injury range from gross physical trauma (such as after a motor vehicle accident) to a single gene defect (results in a nonfunctional enzyme in metabolic disease). 

Most injurious stimuli can be grouped into the following categories: 

1. Oxygen Deprivation: 

• Hypoxia (deficiency of oxygen in tissue level). 
• Hypoxemia (deficiency of oxygen in arterial blood). 
• Ischemia (loss of blood supply).
• Anoxia (complete lack of oxygen).

2. Physical agents: 

•  Mechanical trauma.
• Extremes of temperature (burn or deep cold). 
• Sudden change in atmospheric pressure.
• Radiation.
• Electric shock. 

3. Chemical agents & drugs: 

• Glucose or salt in high concentration. 
• High concentration of oxygen.
• Trace amounts of poisons (e.g. Arsenic, cyanide, mercuric salt).
• Environmental and air pollution.
• Insecticides and herbicides.
• Industrial & occupational hazards (e.g. asbestos).
• Alcohol. 

4. Infectious/microbiological agent: 

• Virus.
• Bacteria.
• Fungi.
• Protozoa. 
• Parasites. 

5. Immunologic reactions (several autoimmune diseases discuss in lesson 20). 

6. Genetic defects (discuss in lesson 19) 

7. Nutritional imbalance (discuss in lesson 22A). 

8. Free radical damage (discuss in lesson 2C). 

9. Reperfusion injury. 

Hypoxia

Definition: 

Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. 

There are Four Types of Hypoxia : 

1. Hypoxic hypoxia: Due to low oxygen in arterial blood. 
2. Anemic hypoxia: Due to low level of hemo-globin in blood. 
3. Stagnant hypoxia: Due to inadequate blood supply.
4. Histotoxic hypoxia: Low oxygen uptake due to cellular toxicity. 

Clinical Manifestations: 

1. Cyanosis (bluish discoloration of skin and mucous membrane) 
2. Confusion / disorientation / hallucinations / behavioral change, 
3. Dyspnea (Breathlessness) 
4. Lethargy. 

Hypoxemia

Definition: 

Hypoxemia is defined as reduced partial pressure of oxygen (mmHg) in arterial blood. Hypoxemia can cause hypoxia (hypoxemic hypoxia), but hypoxia can also occur via other mechanisms, such as anemia. 

■It can be estimated by measuring the oxygen saturation of blood using a pulse oximeter, a small device that clips to a finger. 

■Normal pulse oximeter readings usually range from 95 to 100 percent. 

■Values under 90 percent are considered low. Normal arterial oxygen is approximately 75 to 100 millimeters of mercury (mm Hg). 

■Values under 60 mm Hg usually indicate the need for supplemental oxygen.

Causes: 

1. Decrease concentration of oxygen in air. 
2. Respiratory acidosis. 
3. Ventilation defect 
4. Perfusion defect 
5. Hemoglobin related abnormalities e.g. anemia. 

Ischemia 

Definition: 

Ischemia is a decrease in blood supply to tissues, causing a shortage of oxygen and glucose needed for cellular metabolism. 

Causes : Coronary artery atherosclerosis, decreased cardiac output, Thrombosis. 

Consequences: 

1. Atrophy 
2. Infarction of tissue 
3. Organ dysfunction 

Clinical manifestations are acute limb ischemia include pain, pallor, pulseless, paresthesia, paralysis. 

Major structural targets for cell damage: 

• Cell membranes : 

• Plasma membrane 
• Organelle membranes 

• DNA. 

•Structural protein 

• Enzymes 

• Mitochondria: 

• Oxidative phosphorylation 

Cellular responses to Injury 

1. Cellular adaptation: 

• Atrophy. 
• Hypertrophy. 
• Hyperplasia. 
• Metaplasia. 

2. Cell injury: 

• Reversible (Cellular swelling, Fatty change). 
• Irreversible (Necrosis, Apoptosis). 

3. Subcellular alterations- in sublethal & chronic injury. 

4. Intracellular accumulations. 

5. Pathologic calcification. 

6. Cell aging. 

Cellular response to injurious stimuli

Reference: -

🖎Text book of pathology by Harsh Mohan

🖎 Pathology tutorial by Mohammad Zillur Rahman