Difference between Necrosis and apoptosis

Cell Injury I & II

A.  Definitions.  Be able to recognize examples of these conditions and vignettes and in pathologic images from text, lecture outlines, and case studies.  Several of these conditions are discussed in greater detail in later pages of the text.

Necrosis:  irreversible cell injury, sequence of morphological changes that follow cell death in living tissue.  Occurs after a loss of blood supply or after an exposure to toxins.  Characterized by cell swelling, protein denaturation, and organelle breakdown.  Leads to tissue dysfunction.  Inflammatory response initiated.
Apoptosis:  programmed cell death, dead cells removed without much disruption to surrounding tissues.  Where unwanted cells (ex: embryogenesis) or pathological cells (ex: irreparable mutations) are eliminated.  Inflammatory response is not initiated.  Cells shrink in size.
Hypertrophy:  increase in the size of the cells due to increased workload (ex: myocardial cells increase in size if have excessive HTN or aortic stenosis because of the increased work load)
Atrophy:  decrease in cell size without change in # of cells (ex: casted broken leg---don’t use will see atrophy, remember use it or lose it)
Infarction:  irreversible injury after complete or prolonged occlusion, cells are dead (ex: myocardial infarction aka “heart attack”), irreversible
Cachexia:  weight loss after periods of prolonged starvation (ex: the setting of malignant tumors)
Hyperplasia:  increase in the number of cells, can be physiologic or pathologic, associated with hypertrophy at times (exception:  renal epithelial cells can undergo hypertrophy but not hyperplasia).   Physiological hyperplasia: a)  hormonally induced (ex: during breast development and during pregnancy have breast and uterine development) or b)  compensatory hyperplasia (occurs when tissue is removed or diseased ex: partial liver resection: cells will proliferate to fill in space where liver tissue once resided).  Hyperplasia is also important in wound healing.  Pathological hyperplasia:  mainly due to excessive hormonal stimulation or growth factor stimulation (ex: warts caused by the papillovirus, tumors)

Aplasia: incomplete or underdeveloped cells/organs

Hypoplasia: underdeveloped cells/organs

Metaplasia:  reversible change in which one adult cell type (epith or mesenchymal) is replaced by another adult cell type

Hypoxia:  oxygen deficiency which interferes with aerobic oxidative respiration, common cause of cell injury/death

Hypoxemia: low oxygenation of blood

Anorexia: self induced starvation, results in cachexia, anemia may result leading to hypoxic tissues and hypoxemia
Ischemia: loss of blood supply due to impeded arterial flow (due to occlusion) or decreased venous drainage

Phagocytosis:  engulfment of bacteria via invagination of membrane of pathogen to make phagosome which later fuses with lysosome to make phagolysosome.  With this fusion, the pathogen is ingested via heterophagy or autophagy.

Autophagy:  intracellular organelles (not outside sources) and portions of cytosol are sequestered from the cytoplasm in the autophagic vacuole form from the ribosome-free parts of the RER (prominent n cells undergoing atrophy).  The vacuole fuses with a primary lysosome to form a autophagolysosome.  This will completely digest proteins, carbs, but some lipids remain undigested.

Heterophagy:  materials from the external environment are taken up through a process generically called endocytosis; uptake of larger particular matter is phagocytosis; and uptake of smaller macromolecules is denoted pinocytosis.  Mainly limited to phagocytotic cells: PMNs and macrophages

B. (pg 4-8)  Understand causes and biochemical mechanisms of cell injury.  Be able to recognize specific examples and what happens at the subcellular level

Causes:                                  Examples:                                     Subcellular events:
1. O2 deprivation                  anemia, CO poisoning           *  ↓ Oxidative phosph = ↓ ATP
Leads to mito and cell swelling increase intracell Ca++ which activates endonucleases, proteases, ATPases via activation of phospholipases

2. Chemical agents   glucose, salt, high partial      * leads to free radical formation
                                              pressures of O2, asbestos,        causing lipid peroxidation
                                              drugs, insecticides

3. Infectious agents  tapeworm, bacteria, viruses,    *viruses take over nuclear
                                              fungi, protozoa                          activity making its own DNA

4. Immunologic rxts   incidental or intended,             *increase mast cell degranul.
   anaphylaxis, autoimmune dz     immune complexes deposited
          in blood vessel walls etc

5. Genetic defects          Down’s syndrome, sickle cell         *cells misshaped in sickle cell

6. Nutritional imbalances     protein insufficiency, vitamin     *decreased cell function
 deficiencies, DM, atherosclerosis

7. Physical agents          trauma, extreme temps, radiation,  *decreased cell function
                       electric shock, sudden changes in
                       atmospheric pressure

8. Aging          repeated injury                 *decreased cell function

**NOTE: there subcellular events are difficult to find**

C.  Understand the sequence of events in ischemic injury in the mitochondria

Decreased Oxidative       Phosporylation
    Decreased ATP

Decreased Na Pump

Increased influx of calcium, water, and, Na, efflux of K


Cellular swelling, loss of microvilli, blebs, ER swelling     Increased Glycolysis

      ↓                       ↓

Decreased       Decreased
Glycogen         pH


                        Clumping of
                        Chormatin Other Effects


Detachment of ribosomes, etc.


Decreased protein synthesis


Lipid deposition

D.   Understand how reperfusion of ischemic tissue can lead to further tissue damage

The restoration of blood flow to ischemic but otherwise viable tissues results in exacerbated and accelerated injury.  As a result, tissues sustain the loss of cells in addition to those that are irreversibly damaged at the end of the ischemic episode.  This is call ischemia/reperfusion injury that contributes to tissue damage in myocardial and cerebral infarctions, but is also amendable to therapeutic intervention.

Reperfusion to ischemic tissues may cause further damage by the following means:

1. Restoration of blood flow bathes compromised cells in high concentrations of Ca++ when the cells themselves cannot regulate themselves.  With high intracellular Ca++, this activates proteases (decreased cytoskeletal and disruption of membrane), endonucleases (nuclear chromatin damage), phospholipases (decrease phospholipids in membrane), and ATPases (decreased ATP).  This all culminates into the loss of cell integrity.

2. Increase in blood flow will also recruit inflammatory cells which release high levels of oxygen derived reactive species and damage cell membrane and mitochondrial membrane permeability.

3. Damaged mitochondria yield incomplete oxygen production and thus increase the production of free radicals.  Cells also have compromised antioxidant effects.

E. (pg 9-10)  Know what free radicals are, how they are formed, how they injure cells, and examples of diseases in which free radicals are involved in pathogenesis

Free radical damage underlies chemical and radiation injury, toxicity from oxygen and other gases, cellular aging, microbial killing by phagocytic cells, inflammatory cell damage, tumor destruction by macrophages, and other injurious processes.  Free radical generation is also a part of respiration and other normal functions like microbial defense.  (Superoxide breaks down water into O2 and hydrogen peroxide)

Free radicals:  chemical species with a single unpaired electron in the outer orbital.  These states are unstable and react readily.  When made inside cells they attack and degrade nucleic acids and membrane molecules.  They initiate autocatalytic reactions: molecules that react with free radicals are converted into free radicals (chain reaction).

Free radicals generated by:  1. redox reactions:  that occur during normal processes like respiration (reduce 02 by adding 4 electrons to generate water) intermediates are formed such as hydrogen peroxide, superoxide radicals (also produced by intracellular oxidases), and OH∙.  Fenton reaction forms free radicals and is catalyzed by transition metals (Cu and Fe) who donate or accept free electrons. The reduction step of the Fenton reaction is catalyzed by superoxide ion.

NO can act as a free radical.  Absorbing UV light, X-Rays  (radiant energy) can hydrolyze water into hydroxyl (OH∙) and hydrogen (H∙) free radicals.  Metabolizing exogenous chemical by enzymes can produce free radicals.

Lipid peroxidation of membranes: double bonds in membrane lipids are vulnerable to attack by O2 deprived free radicals.  This reaction yields peroxides which are unable and can lead to chain autocatalysis.

DNA fragmentation:  free radicals react with the thymidine in the nuclear/mitochondrial DNA produce single strand breaks.  This DNA damage has been seen in cell killing and malignant transformation of cells

Cross-linking of proteins:  Cross-linking promoted by free radicals result in enhanced rates of degradation or loss of enzymatic activity.

F. (pg 11)  Describe and understand 2 examples of how chemical agents injure cells.

Some chemicals act directly by combining with a critical molecular component or cellular component or cellular organelle.  Ex: mercuric chloride poisoning, mercury binds to the sulfhydryl groups of various cell membrane proteins, causing inhibition of ATPase –dependent transport and increased membrane permeability. (How antineoplastic agents and antibiotics work by cell damage).  The greatest damage is sustained by the cells that use, absorb, excrete, or concentrate the compounds.

Many other chemicals are not intrinsically biologically active but must be converted to reactive toxic metabolites, which will then act on target cells.   This usually happens by P-450 oxidases in the SER of the liver and other organs.   The metabolites may bind covelantly to proteins and lipids in the membrane and damage/injuring cells, however free radicals injure the cell the greatest.  For instance CCl4  used in the dry cleaning industry can be converted to free radical CCl3 ∙ in the liver causing phospholipids peroxidation breaking down the ER.  Will see less export of lipids because they cannot make apoprotein (a protein) that hooks triglycerides to be secreted by liver.  These patients get fatty liver from CCl4  poisening. Eventually the mitochondria decrease ATP production and the mitochondria start to swell with subsequent calcium influx and cell death.

G. (pg 14)  Define and understand processes of phagocytosis, endocytosis, and pinocytosis

Phagocytosis:  Uptake of large matter via engulfment by the cell.

Endocytosis: Uptake of materials from the external environment.

Pinocytosis:  Uptake of soluable smaller macromolecules

With the above the endocytosed vacuoles and their contents fuse with a lysosomes creating phaogolysomes and degrade by heterophagy (pinocytosis, endocytosis, or phagocytosis) or autophagy (intracellular organelles/cytosol are put into an autophagic vacuole and form from ribosome free parts of the RER.  The autophagic vacuole then fuses with lysosome creating a autophagolysosome---this is to remove damages organelles or to remodel—seen in cells undergoing atrophy.)  Lysosomes have enzymes that degrade proteins, carbohydrates, while some lipid remain undigested.  When undigested debris occurs (lipofuscin pigments, tattoo ink, etc) lysosomes store these in residual bodies

Lysosomal storage disorders caused by deficiencies in enzymes thus, will see an abnormal accumulation of intermediate metabolites all over the body (especially neurons).

H. (pg 15)  Understand the role of lipofuscin and its role in the aging process

Lipofuscin pigment granules represent indigestible material from intracellular lipid peroxidation.  Also known as the “wear and tear” pigment.  It is an insoluable brownish-yellow granular intracellular material that accumulates tissues (heart, brain, and liver) as a function of age or atrophy.  It is a complex of lipid and protein that results from free radical catalyzed perioxidation of lipids of subcellular membranes.  It is NOT injurious to cells but is a marker of past free radical injury.  Aka brown atrophy when you can see the pigment on gross specimen examination.

I. (pg 16-17)  Understand the pathophysiology and causes of hepatic steatosis, recognize examples in lab case studies

Hepatic steatosis:  aka fatty change which is an abnormal accumulation of triglycerides within the parenchymal cells.  Indication of reversible injury, but can be found adjacent to necrosed cells.  Most often seen in the liver since it is the major organ involved in fat metabolism but also seen in kidney, heart, and skeletal muscle.  Causes include: toxins, protein malnutrition, diabetes, obesity, and anorexia, but the MOST COMMON cause is alcoholism.
Excess accumulation of trigyclerides may result in defects at any step from fatty acid entry to the lipoprotein exit.  (last paragraph pg 16 tell about normal fat metabolism pathway)   Hepatotoxins (alcohol etc) alter mitochondrial and SER function; CCl4 and protein malnutrition decrease the synthesis of apoproteins; anorexia inhibits fatty acid oxidation; and starvation increases fatty acid mobilization from peripheral stores.
Mild fatty change: no effect on cellular function
Moderate fatty change: transiently impaired cellular function, reversible unless some vital intracellular process is impaired
Severe fatty change: fatty change may precede cell death but cells can also die without undergoing fatty change.

J. (pg 20)  Define hemosiderin and hemosiderosis and contrast with hemochromatosis

Hemosiderin:  a yellow to brown hemoglobin derived granular pigment that accumulates in tissues when there is an excess of iron.  Iron is normally stored within cells in association with the protein apoferritin forming micelles.  Small amounts are normal in mononuclear phagocytes of bone marrow, liver, and spleen where excessive RBC breakdown occurs.  However, usually if present they are pathologic.   Excess of local iron and hemosiderin results from hemorrhage like with bruising.  After lysis of RBCs in area of bruise, the RBS are phagocytosed by macrophages, hemoglobin is catabolized by lysosomes and the heme iron accumulates in hemosiderin.
Hemosiderosis:  Deposition of hemosiderin in orgasm in tissues due to overload of iron.  Found at first in mononuclear phagocytes of liver, marrow, spleen, and lymph nodes.  These deposits then progressively accumulate the liver, pancreas, heart, and endocrine organs and then the organs become “bronzed” with this pigment.  This occurs when 1) increased absorption of iron  2)  impaired ultilization of iron  3) hemolytic anemias  4) transfusions.   Most of the time the accumulation does not damage the parenchymal cells or impair organ function.  However excessive accumulation results in hemochromatosis.
Hemochromatosis:  Excessive accumulations of iron (hemosiderin) that results in tissue injury including liver fibrosis, heart failure, and diabetes.

K. (pg 21-22)  Define, understand the pathologic process, and recognize diseases associated with metastatic versus dystrophic calcification

Dystrophic calcification:  deposition of calcium salts (along with small amounts of iron, magnesium, and other minerals) in dead or dying tissues.  It occurs in the absence of calcium metabolic derangements (ie: with NORMAL calcium levels).  Comes with any necrotic areas (in atheromas seen in advance atherosclerosis where the initima is damaged in large arteries and see accumulation of lipids).  May be seen with organ dysfunction or cell injury.  It is a cause of aortic stenosis in older people when heart valves are damage causing cuspal calcification.

Morphology:  calcium salts occur as fine white granules or clumps and feel gritty.  On histo slides, will see intracellular and/or extracellular basophilic deposits.  In time, heterotrophic bone may develop in those areas.

Pathogenesis:  Iniation (nucleation) and propagation (intra or extracellular processes) start with end product forming calcium phosphate.  Initiation in extracellular sites occurs in membrane bound vesicles (in bone they occur in matrix vesicle and in pathologic calcification they derive from degenerating cells where the calcium is concentrated because calcium has affinity for membrane phospholipids and phosphatases accumulate as a result of membrane bound phosphatases.  Initiation in intracellular calcification occurs in the mitochondria of dead/dying cells that cannot regulate intracellular calcium anymore. Propagation then occurs and is dependent on the concentration of calcium and phosphate in the extracellular spaces, the presence of mineral inhibitors, and degree of collagenization.  Collagen enhances the rate of crystal growth but osteopontin (protein that binds calcium) is also involved.

Metastatic calcification: deposition of calcium salts (along with small amounts of iron, magnesium, and other minerals) in normal tissue.  It almost ALWAYS reflects some derangement in calcium metabolism (ie: hypercalcemia).  Thus, high calcium levels exacerbate metastatic calcification.

4 causes of hypercalcemia:  1) increased PTH due to parathyroid tumors or other malignant tumors  2)  destruction of bone due to accelerated bone turnover (Paget’s dz), immobilization, or tumors (increased bone breakdown associated with leukemias, bone cancer, or multiple myeloma)  3) vitamin D disorders like Vit D intoxication and sarcoidosis where macrophages activate vitamin D precursor  4)  renal failure which you retain phosphate which leads to secondary hyperparathyroidism

Morphology:  Mainly occurs interstitial tissues of the vasculature, kidneys, lungs, and gastric mucosa.  Granules look like dystrophic granules, but do NOT generally cause clinical dysfunction.  However, extensive calcifications in lungs and kidneys may lead to damage.

L. (pg 22-24 Fig 1.19)  Provide examples and understand the processes of reversible versus irreversible cell injury

Within limits, the cell can compensate for disturbance to ATP production, cell membrane, protein synthesis and DNA and can return the cell to normalcy.  However, after repeated and persistent injury causes cells to surpass the threshold and lead to irreversible cell injury.  Irreversible injury causes problems with oxidative phosphorylation and eventually deplete the ATP supply.  Membrane damage is a critical step in the development of lethal injury and increased intracellular calcium can turn on enzymes which can chew up proteins, cell membrane components, and DNA.

Irreversible cell injury hallmarks: 1) cannot reverse mitochondrial dysfunction after original injury is resolved (ie: restoration of blood flow)  2)  profound membrane function problems (key of irreversible cell injury) due to loss of phospholipids, cytoskeletal abnormalities, toxic oxygen radicals, and lipid breakdown.

Usually functional changes occur before morphological changes.  Reversible injury ultrastructural changes:  plasma membrane problems (blebbing, distortion of microvilli, loosening of intracellular attachments), mitochondrial changes (swelling and dense opacities of mitochondria), dilation of the ER (with ribosome detachment), and nuclear alterations.    Reversible injury starts with cell swelling (hydropic changes) and will see fatty change (from hypoxic/toxic/metabolite injury).    Irreversible cell injury: (necrosis)  morphologic changes that  follow cell death in living tissue.   Get mitochondrial and cell swelling.  DNA degeneration with pyknosis, karryohexis, karyolysis.   Dead cells increase in eosinophilia (pink dye) because of increase eosin binding.

M. (pg 25)  Understand definitions of karyolysis, karyorrhexis, pyknosis

Pyknosis:  nuclear shrinkage and increased basophilia (blue staining)
Karyorrhexis:  pyknotic nuclease fragments
Karylysis:  basophilia of the chromatin may fade secondary to DNAse activity

N. (pg 25)  Compare and contrast liquefactive, caseation, coagulative, fat, and gangrenous necrosis.   Understand what is happening at the cellular level in each of these processes, which organs tend to exhibit each of them, and recognize examples of diseases in which they play an important role

Liquefactive Necrosis:  enzymatic digestion is the primary pattern, due to bacteria or fungal infections.  WBCs accumulate.  Hypoxic death of cells within the CNS also exhibit this type of necrosis.  This necrosis completely digests dead cells.

Caseous Necrosis:  most often due to TB, it is a cheesy white gross appearance of the central necrotic area. Granular debris is enclosed in a ring of inflammation.  Cannot tell what type of tissue you are looking at (unlike coagulative).

Coagulative Necrosis:  denaturation is the primary pattern recognized, basic outer structure of the cells is preserved (ie: can tell what cell is dead) for days.  Classic characteristic of hypoxic death of cells in all tissues EXCEPT the brain.  See increasing acidosis and the injury causes denaturation of structural and enzyme proteins blocking cellular proteolysis.  Ex: myocardial infarction.

Fat Necrosis:  focal areas of fat destruction due to pancreatic injury secondary to the release of pancreatic enzymes.  Causes: pancreatitis, pancreas trauma.  Fat is liquefied by pancreatic enzymes.  This releases fatty acids which combine with calcium to produce chalky white aresas (fat supponification).  On histo slides, only shadowy outlines of necrotic fat cells may be seen with calcium deposits and surrounding inflammatory reaction occurring.

Gangrenous Necrosis: refers to ischemic coagulative necrosis (usually of a limb).  When there is a infection with it is called wet gangrene.

Most of the time, necrotic cells are phagocytized.  If not, these cells attract calcium salt and other minerals leading to dystrophic calcification.

O. (pg 26)  Understand the mechanisms and various causes of apoptosis at the subcellular level (Fig. 1.26)
Apoptosis: programmed cell death, cell suicide
Examples of apoptosis at work: destruction of cells in embryogenesis, hormone dependent involution (ie endometrial thickening during menstruation, pathologic atrophy in prostate after castration, decrease in breast size/lactation with weaning, rapid turnover cells (intestinal epithelium), deletion of autoreactive T-cells in thymus)  If failed to undergo apoptosis, would have tumors or autoimmune dz.  Usually involves single or clusters of cells that have eosinophillic cytoplasm. CELLS SHRINK and form buds with cytosol or organelles called apoptotic bodies.  NO INFLAMMATORY RESPONSE OCCURS.

Stimulus:  physiologic and pathologic factors
Histo appearance:  single cells, chromatin condensation, apoptotic bodies
DNA breakdown: internucleosomal
Mechanisms: gene activation, endonucleases, proteases
Tissue reaction: NO inflammation, phagocytosis of apoptotic bodies

Mechanisms of apoptosis:
Signaling: due to lack of growth factor, intrinsic programmed event, release of granzymes from cytotoxic T cells, radiated cells, etc.  TNF receptor shares the death domain that when oligimerized leads to activation of caspases and cascade leading to cell death
Control and Integration:  proteins that connect the original death signals to the final execution program  (not sure how indepth he is wanting us to go thru---review pg 27)

P. (pg 28-30)  Understand and provide the examples of events that occur in cell aging

 As we age, oxidative phosphorylation, synthesis of proteins, nutritional uptake, and the ability to repair chromosomal damage is reduced.  Morphological changes include: irregular nuclei, vacuolated mitochondria, diminished ER, and distorted Golgi, increased accumulation of lipofuscin, abnormally folded proteins and AGES (advanced glycosylated end products occur.  Involves intrinsic clock of aging, and extrinsic wear and tear. Intrinsic clock: fibroblast stop dividing after 50 doublings in adults (65 in neonates and 35 in patients with progeria who age prematurely)

Incomplete replication of chromosome ends (telomere shortening) and clock genes are apart of the explanations behind intrinsic clock.  

Cellular aging mechanisms involve both programmed cell events and the consequences of progressive environmental injury.  Programmed aging assumes a predetermined sequence of events including repression and derepression of specific genetic programs leading ultimately to senescence.