Cell injury summary 2017

Heyam Awad

Doctor 2016

• Cells normally maintain a steady state called Homeostasis.

• This means the intracellular environment is kept within a narrow range of physiologic state

• Examples: temperature, pH, chemical reactions, electrolyte concentration, water content… All are regulated and kept constant.

• If cells are subjected to stresses ( like changes in electrolyte balance) then cells adapt and reach a new homeostatic state that will preserve cell function.

• However if the stress is more severe and is beyond capability of adaptation then this will result in cell injury.

• Cell injury: at the beginning it is reversible within certain limits

• Then it becomes irreversible.

• Irreversible injury ends in cell death.

• Two types of cell death: necrosis and apoptosis ( more details later)

Adaptation in cells

• Adaptation means: reversible changes in in the number, size, phenotype, metabolic activity or function of cells in response to changes in their environment.

• Adaptive changes are reversible.: if the insult is removed, things can go back to normal.

• Can be physiologic or pathologic; depending on the cause

Adaptive responses

• Hypertrophy: Increased cell size.

• Hyperplasia: increased number of cells.. Cell division.

• Metaplasia: change from one adult cell type to another

• Atrophy: decreased size.

Hypertrophy

• Increased cell size.

• Due to increased organelles and proteins.

• Increased intracellular synthesis of the proteins and organelles.

Caused by: increased demands, hormones or growth factors.

Examples of Physiologic hypertrophy

• Uterus during pregnancy… due to oestrogen effect on smooth muscles of the uterus.

• Skeletal muscle in body builders… due to increased demand.

• Note: smooth muscles can divide.. So in the pregnant uterus there is hypertrophy and hyperplasia… whereas skeletal muscle is permanent tissue that can not divide, so there is pure hypertrophy without hyperplasia.

Pathologic hypertrophy

• Cardiac enlargement due to hypertension= Hypertensive heart disease

• Pathogenesis.. Two types of signals: mechanical: stretch and trophic: growth factors and androgenic hormones cause the hypertrophy in the cardiac muscle

• Note: the cardiac muscle can not undergo hyperplasia.

Hyperplasia: increased cell proliferation and division • Only in tissues that can replicate.

• Can be physiologic or pathologic.

• Occurs due to proliferation of both differentiated cells and stem cells.

• Examples of physiologic hyperplasia :1. Hormonal: uterus, breast. 2. Compensatory: after removal or loss of part of tissue.

Pathologic hyperplasia

• Due to excess in hormones or growth factors.

• E:g endometrial hyperplasia due to oestrogen- progesterone imbalance.

• Hyperplasia is controlled.. it responds to normal growth stimuli and inhibitors .This differentiates it from cancer because cancer keeps growing regardless of normal growth mechanisms.

• However.. Because hyperplastic tissue divides there is more chance that it will acquire mutations that can lead to cancer. This occurs more in pathologic hyperplasia, because the stimuli are long lived

atrophy

• Shrinkage in cell size due to loss of cell substance.

Causes

• decreased work load.

• Loss of innervation

• Loss of endocrine stimulation.

• Aging

atrophy

Mechanisms:

• Decreased protein synthesis.

• Degradation of cellular proteins, mainly by ubiquitin- proteasome pathway.. Proteins are attached to ubiquitin which targets them to be degraded by proteasome.

• Autophagy…. Literally means self eating. ( will be discussed later)

metaplasia

• Adult cell type replaced by another adult cell type.

• The cell type sensitive to a certain stress changes to another type which can better tolerate this particular stress.

• Arise due to reprogramming of stem cells to differentiate along a new pathway.

Epithelial metaplasia/ example

• Respiratory epithelium which is normally glandular, if exposed to smoking it changes to squamous epithelium, which can cope better with the smoke

• However, this change means we will loose the goblet cells in the columnar glandular epithelium.. So no mucus secretion

• Also we will loose the cilia.. So there is loss of important function happening in metaplastic tissue.

Metaplasia and cancer

• The stimuli that cause metaplasia if persist for a long time can cause cancer

• So metaplasia is considered premalignant.

• NOTE: vitamin A deficiency can cause metaplasia

Cell injury

.Cell injury results from functional and biochemical

abnormalities in one or more of essential cellular

components which are :

1. Mitochondria and their ability to generate ATP

2. Disturbance in calcium homeostasis

3. Damage to cellular (plasma and lysosomal) membranes

4. Damage to DNA and misfolding of proteins

Biochemical mechanisms of cell injury: 1. ATP depletion -ATP is the main energy store of cells. It is produced by oxidative

phosphorylation

The major causes of ATP depletion in cell injury are:

a. Reduced supply of oxygen and nutrients,

b. Mitochondrial damage,

c. The actions of some toxins (e.g., cyanide)

Effects of ATP depletion 1. The activity of plasma membrane ATP-dependent sodium pump is reduced,

resulting in Intracellular accumulation of Na and efflux of Ka accompanied by osmotic gain of water, causing cell swelling

2. Compensatory increase in anaerobic glycolysis resulting in rapid depletion of intracellular glycogen stores , and lactic acid accumulates, leading to decreased intracellular pH and decreased activity of many cellular enzymes

3. Failure of ATP-dependent Ca2+ pumps leads to influx of Ca2+ with damaging effects on many cell components

4. Structural disruption of the protein synthetic apparatus, manifested as detachment of ribosomes from the rough ER with a consequent reduction in protein synthesis

5. Ultimately, there is irreversible damage to mitochondrial and lysosomal membranes, and the cell undergoes necrosis.

Biochemical mechanisms of cell injury 2. Mitochondrial Damage and Dysfunction

- Mitochondria are sensitive to many types of injurious stimuli,

including hypoxia, chemical toxins, and radiation.

- Mitochondrial injury may result in several abnormalities:

A. Failure of oxidative phosphorylation leads to progressive depletion of ATP, culminating in cell necrosis

B. Abnormal oxidative phosphorylation leads to formation of reactive oxygen species with deleterious effects

Mechanism of mitochondrial dysfunction 1. Damage to mitochondria is often associated with formation of a

high-conductance channel in the mitochondrial membrane, called the mitochondrial permeability transition pore

2. The opening of this channel leads to the loss of mitochondrial membrane potential and pH changes, further compromising oxidative phosphorylation.

3. The mitochondria also contain several proteins that, when released into the cytoplasm, activating apoptosis

Biochemical mechanisms of cell injury 3. Influx of Calcium - Cytosolic free calcium is normally maintained at concentrations as

much as 10,000 times lower than the concentration of extracellular calcium or of sequestered intracellular mitochondrial and ER calcium

- Ischemia and certain toxins cause an increase in cytosolic calcium , initially because of release of Ca2+ from the intracellular stores, and later resulting from increased influx across the plasma membrane.

- Increased cytosolic Ca2+ activates a number of enzymes, with potentially deleterious cellular effects

Enzymes activated by increased cytosolic calcium a. Phospholipases (which cause membrane damage),

b. Proteases (which break down both membrane and cytoskeletal proteins)

c. Endonuclease (cause DNA and chromatin fragmentation),

d. Adenosine triphosphatases (ATPases) (thereby hastening ATP depletion).

Biochemical mechanisms of cell injury. 4.Disturbed membrane integrity

- Increased membrane permeability leading ultimately to overt membrane damage is a consistent feature of most forms of cell injury that culminate in necrosis.

- The plasma membrane can be damaged by ischemia, microbial toxins, physical and chemical agents.

Several biochemical mechanisms may contribute to membrane damage 1. Decreased phospholipid synthesis:due to fall in ATP levels. The reduced

phospholipid synthesis may affect all cellular membranes, including the membranes of mitochondria, thus exacerbating the loss of ATP.

2. Increased phospholipid breakdown- by activation of endogenous phospholipases by increased levels of cytosolic Ca2+.

3. Cytoskeletal abnormalities- Cytoskeletal filaments act as anchors connecting the plasma membrane to the cell interior, and maintain normal cellular architecture , motility, and signaling .Activation of proteases by increased Ca2+ cause damage to cytoskeletal elements leading to membrane damage

4. ROS.: Cause cell injury to by lipid peroxidation,

5. Lipid breakdown products:. These may insert into the lipid bilayer of the membrane or exchange with membrane phospholipids causing changes in permeability .

- Reactive oxygen species cause cell injury by :

a. Lipid peroxidation of membranes.

Double bonds in membrane polyunsaturated lipids are vulnerable to attack by oxygen-derived free radicals

b. Free radicals promote sulfhydryl-mediated protein cross-linking resulting in enhanced degradation or loss of enzymatic activity

-Free radicals may directly cause polypeptide fragmentation.

c. DNA damage. Free radical reactions with thymine in nuclear and mitochondrial DNA produce single-strand breaks. Such DNA damage has been implicated in cell death, aging, and malignant transformation of cells

Morphology of cell injury

• The first effect of all injuries is on the biochemical and molecular level

• Functional derangement happens next

• Ultrastructural changes seen by electron microscopy follow

• Then light microscopic changes occur

• The last visible change is at the gross; macroscopic level.

Morphology of reversible cell injury

• The main two morphologic changes in reversible cell injury are:

• 1. cellular swelling

• 2. fatty change

swelling

• Results from failure of the sodium potassium pump due to ATP depletion

• It is the first manifestation of all forms of injury

• It is reversible

• The organ affected will have increased weight

• Microscopy shows small clear vacuoles within the cytoplasm this is called hydropic change or vacular degeneration

• The organelles within the cells are also swollen

Fatty change

• Occurs mainly in hypoxic injury and in toxic and metabolic injury.

• Microscopy: lipid vacuoles in the cytoplasm

• Seen mainly in cells that participate in fat metabolism like hepatocytes and myocardial cells

• It is reversible.

Electron microscopic changes in reversible injury • Plasma membrane blebbing

• Mitochondrial swelling with appearance of amorphous densities

• Dilation of endoplasmic reticulum with detachment of ribosomes and dissociation with polysomes

• Nuclear changes with clumping of chromatin.. This happens due to changes in pH and can be reversible.

• Formation of phospholipid aggregates called myelin figures which are derived from damaged cellular membranes

Necrosis

• Necrosis is the type of cell death that is associated with loss of cell membrane integrity and leakage of cellular contents causing dissolution of cells.

• The dissolution occurs due to enzymatic action.

• Leakage of cellular content causes inflammation which aims at getting rid f the dead necrotic tissue

NECROSIS It is caused by:

• Denaturation of intracellular proteins.

• Digestion of cells by lysosomal enzymes of dying cells ( autolysis) and leukocytes (heterolysis).

Morphology of necrosis

• There are cytoplasmic and nuclear changes.

• Cytoplasmic changes: increased eosinophilia (pink staining from the eosin dye).

• Increased eosinophilia is due to 1. increased binding of eosin to dentures proteins and 2. to loss of basophilia (decreases hematoxylin binding to RNA, because RNA in the cytoplasm decreases.

• Due to these changes cells appear homogenous and glassy

• Myelin figures can be a nidus for calcium deposition and calcifications might occur.

Nuclear changes in necrosis one of three patterns

1. karyolysis: decreased chromatin basophilia secondary to deoxyribonuclease (DNAase) activity.

2. pyknosis: nuclear shrinkage and increased basophilia (DNA condenses into a solid shrunken mass.

3. karyorrhexis, fragmentation then disappearance of nucleus.

Myelin figures

• Myelin figures: aggregates of damaged cell membranes (phospholipids).

Fate of Myelin figures:

• phagocytosed by other cells

• or further degraded into fatty acids and calcify

Patterns of necrosis

• Denaturation of protein predominates…. Coagulative necrosis.

• Enzymatic digestion predominates… liquefactive necrosis.

• Special circumstances: caseous necrosis and fat necrosis.

Coagulative necrosis

• preserved architecture of dead tissue .

• Denaturation of structural proteins and enzymes… so no cellular proteolysis.

• Eosiniphilic anucleated cells

• Cells are removed by inflammatory cells.

• Ischemia in all solid organs except the brain may lead to coagulative necrosis

Liquifactive necrosis

• digestion of the dead cells resulting into a liquid jelly-like mass.

• In focal bacterial or fungal infections and in hypoxic death in central nervous system.

Caseous necrosis

• White cheese like friable necrosis.

• Prototype: Tuberculosis

• Typical finding is granuloma :Collection of fragmented or lysed cells with amorphous granular eosinophilic debris surrounded by macrophages.

Apoptosis

• cell death induced by a tightly regulated suicide program in which cells activate enzymes capable of degrading the cells' own nuclear DNA and nuclear and cytoplasmic proteins.

apoptosis

• The plasma membrane remains intact.

• Apoptotic bodies (contain portions of the cytoplasm and nucleus) become targets for phagocytosis before their contents leak out and so there would be no inflammatory reaction.

• So in apoptosis there is no damade to surrounding cells.

Causes of Apoptosis

• Physiologic situations:

To eliminate cells that are no longer needed OR to maintain a steady number of various cell populations in tissues.

Physiologic apoptosis

• Embryogenesis.

• involution of hormone-dependent tissues upon hormone withdrawal. (endometrium and breast after pregnancy)

• Cell loss in proliferating cell populations. (gastrointestinal tract, skin…)

• Death of host cells after serving their useful function. (neutrophils and lymphocytes in inflammation)

• Elimination of potentially harmful self-reactive lymphocytes.

• Cell death induced by cytotoxic T lymphocytes (tumor cells and virally infected cells)

Pathologic situations

• DNA damaged cells, if DNA damage is severe and cannot be repaired the cell dies by apoptosis.

• Cells with accumulation of misfolded proteins,

• Certain infections (viral ones): may be induced by the virus (as in human immunodeficiency virus infections) or by the host immune response (as in viral hepatitis).

• Pathologic atrophy in parenchymal organs after duct obstruction (pancreas, parotid and kidney)

Morphology of apoptosis

• Cell shrinkage: dense cytoplasm, tightly packed organelles.

• Chromatin condensation: peripherally under the nuclear membrane.

• Formation of cytoplasmic blebs

• apoptotic bodies: blebbing then fragmentation into membrane bound apoptotic bodies composed of cytoplasm and tightly packed organelles with or without nuclear fragments.

• Phagocytosis of apoptotic cells or cell bodies by macrophages (quickly hence no inflammation).

Mechanisms of Apoptosis

• Activation of enzymes called caspases .

• Two main pathways:

• 1- Mitochondrial pathway (intrinsic)

• 2- Death receptor pathway (extrinsic)

• 1- mitochondrial pathway (intrinsic)

• Leak of cytochrome c out of mitochondria and activation of caspase 9…

• 2- death receptor pathway (extrinsic)

• Involved in elimination of self-reactive lymphocytes and in killing of target cells by some cytotoxic T lymphocytes.

• Activation of caspase 8.

Intrinsic pathway = mitochondrial pathway

• Mitochondria contains several proteins that can induce apoptosis

• The most important of these is cytochrome C

• Stimulation of apoptosis depends on mitochondrial permeability

• Mitochondrial permeability is controlled by a family of more than 20 proteins ( Bcl2 family)

• When cells are deprived of growth signals, or exposed to severe DNA damage or have misfolded proteins.. In all these situations certain sensors are activated

• These sensors are called BH3 proteins ( they are part of the bcl2 family)

• BH3 now activate proapoptotic members of the family= Bax and Bak

• When bax and bak are stimulated they dimerize and insert into the mitochondrial membrane

• They form channels through which cytochrome c escapes into cytosol

• BH3 also inhibit anti inhibitory members of the family (BCL-2 and BCL-xl)

• So BH3 stimulate proapptotic and inhibit antiapoptotic signals.. Net result it leakage of cytochrome c from the mitochondria to cytosol

• Once cytochrome c is in the cytosol it stimulates caspase 9

• Caspase cascade is stimulated leading to nuclear fragmentation by executioner caspases.

Summary of intrinsic pathway

• BH3 stimulates pro-apoptotic (bax, bak), and inhibit anti apoptotic proteins (bcl-2, bcl-xl)

• Cytochrome c leaks out

• Stimulates caspase 9

• Stimulates executioner caspases that degrade cell components

Extrinsic pathway= death receptor pathway

• This pathway is triggered by death receptors, which are members of the TNF ( tumor necrosis factor) receptor family

• The most important types of death receptors are: TNF type 1 receptor and Fas receptor (CD95)

• FasL = fas ligand is a membrane protein expressed mainly on T lymphocytes

• When T cells recognize fas expressing target , fas molecules are cross linked by fasl to activate caspase 8

• Caspase 8 activates executioner caspases that degrade cell components

Clearance of apoptotic cells

• When apoptotic cells fragment they are phagocytosed without eliciting inflammation

• In normal cells phosphatidyl serine is present in the inner surface of cell membrane.in apoptotic cells it flips to outside the membrane and acts as a signal recognized by macrophages to phagocytose the apoptotic cell fragment

• So the apoptotic body is phagocytosed without inflammation

Feature Necrosis Apoptosis

Cell size

Enlarged (swelling) Reduced (shrinkage)

Nucleus

Pyknosis → karyorrhexis →

karyolysis

Fragmentation into

nucleosome-size fragments

Plasma membrane

Disrupted Intact; altered structure,

especially orientation of

lipids

Cellular content

Enzymatic digestion; may

leak out of cell

Intact; altered structure,

especially orientation of

lipids

Adjacent inflammation

Frequent No

Physiologic or pathologic role Invariably pathologic

(culmination of irreversible

cell injury)

Often physiologic, means of

eliminating unwanted cells;

may be pathologic after

some forms of cell injury,

especially DNA damage

autophagy

• Autophagy means lysosomal digestion of the cell’s own components

• Mechanism: intracellular organelles and proteins of the cytosol are sequestered within a phagocytic vacuole which is derived from the endoplasmic reticulum. This vacuole fuses with the lysosome to form the autophagolysosome , in which lysosomal enzymes digest th cellular content.

• Aim of autophagy: to recycle cellular contents as building blocks when needed.

A survival mechanism in times of nutrient deprivation

Involved in the clearance of misfolded proteins (in neurons and hepatocytes)

With inflammatory bowel disease(?!)

Role in cancer: tumor cells turn off autophagy at the beginning of their development in order to survive and proliferate but at times of stress like during giving chemotherapy the tumor cells use autophagy as a survival mechanism to recycle their organelles.

Roles of autophagy

• Age is one of the strongest independent risk factors for many chronic diseases, such as cancer,

Alzheimer disease, and ischemic heart disease

• Cellular aging is the result of a progressive decline in the life span and functional capacity of cells.

• Several mechanisms (cumulative DNA damage, decreased cellular replication capacity, Defective

protein homeostasis

CELLULAR AGING

telomeres

• Each cell has a limited replicative potential. • This is because chromosomes have repeated nucleotide sequences

at the ends of each chromosome. • With each cell replication, telomeres shorten.. Till they become too

short and the chromosomal ends fuse together which causes cell death by apoptosis.

• Stem cells have limitless replicative potential because they have telomerase enzyme which uses its RNA nucleotide sequence to replace the lost telomeres.

• Cancer cells upregulate telomerase transcription and become immortal.

• Abnormal deposition of calcium salts, together with smaller amounts of iron, magnesium, and other

mineral

• Dystrophic Calcification

• Deposition in dead/dying tissues

• Normal Ca2+ metabolism

• Exacerbated by Hypercalcemia

• Metastatic Calcification

• Deposition in normal tissues

• Almost always abnormal Ca2+ metabolism (hypercalcemia)

PATHOLOGIC CALCIFICATION

• GOOD LUCK