Chapter 2- Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death Flashcards
_______ is devoted to the study of structural, biochemical, and functional changes in cells, tissues, and organs that underlie disease
Pathology
Four aspects of a disease process that form the core of pathology
- Cause (etiology)
- Biochemical and molecular mechanisms of its development (pathogenesis)
- Structural alterations induced in the cells and organs of the body (morphologic changes)
- Functional consequences of these changes (clinical manifestations)
All disease etiologies can be classified into two classes
Genetic (inherited mutations, disease-associated gene variants, or polymorphisms)
Acquired (infectious, nutritional, chemical, physical)
Most common afflictions (i.e. cancer, atherosclerosis) are multifactorial, involving external triggers and a genetically susceptible individual
Pathogenesis definition
Sequence of cellular, biochemical, molecular events that follow the exposure of cells or tissues to an injurious agent
Morphologic change definition
Structural alterations in cells or tissues that are either characteristic of a disease or diagnostic of an etiologic process
Definition of cellular adaptation
Reversible functional and structural responses to changes in physiologic states and some nonlethal pathologic stimuli, allowing the cell to survive and continue functioning (e.g. hypertrophy, hyperplasia, atrophy, metaplasia)
When does cell injury occur?
When the limits of adaptive responses are exceeded, or if cells are exposed to injurious agents or stress, deprived of essential nutrients, or become compromised by mutations that affect essential cellular constituents
Cell injury is reversible up to a certain point, but if the stimulus persists or is severe enough from the beginning, the cell suffers irreversible injury and ultimately undergoes cell death.
Adaptation, reversible injury, and cell death may be stages of progressive impairment following different types of insults. (i.e. Increased hemodynamic loads –> cardiac hypertrophy –> reversible injury (fat accumulation, cellular swelling) –> irreversible injury / cell death)
Stages of the cellular response to stress and injurious stimuli.
Two principal pathways of cell death
Necrosis
Apoptosis
TABLE: Cellular responses to injury
Nutrient depletion triggers an adaptive cellular response which can lead to cell death called:
Autophagy
Another definition of adaptation
Reversible changes in size, number, phenotype, metabolic activity, or functions of cells in response to changes in their environment
Hypertrophy definition
Increase in size of cells resulting in an increase in organ size Examples: Physiologic uterine hypertrophy during pregnancy Pathologic hypertrophy of cardiac muscle
What is the most common stimulus for hypertrophy of muscle?
Increased work load (i.e. “pumping iron”)
In the heart, stimulus is usually chronic hemodynamic overload (e.g. hypertension, faulty valves)
Hypertrophy is the result of increased production of cellular ______ (very broad)
Proteins
Three basic steps in molecular pathogenesis of cardiac hypertrophy:
- Integrated actions of mechanical sensors, growth factors (TGF-b, IGF1, FGF), and vasoactive agents (a-adrenergic agonists, endothelin-1, angiotensin II)
- These signals result in complex transduction. Two important pathways involved are: - Phosphoinositide 3-kinase (PI3k)/AKT pathway (important in physiologic hypertrophy) - Downstream signaling of G-protein-coupled receptors (induced by GFs and vasoactive agents) - important in pathologic hypertrophy
- Activation of transcription factors such as GATA4, nuclear factor of activated T cells (NFAT), and myocyte enhancer factor 2 (MEF2). These work to increase synthesis of muscle proteins
Biochemical mechanisms of myocardial hypertrophy. The major known signaling pathways and their functional effects are shown. Mechanical sensors appear to be the major triggers for physiologic hypertrophy, and agonists and growth factors may be more important in pathologic states. ANF, Atrial natriuretic factor; GATA4, transcription factor that binds to DNA sequence GATA; IGF1, insulin-like growth factor; NFAT, nuclear factor activated T cells; MEF2, myocardial enhancing factor 2.
Hyperplasia definition
Increase in number of cells in an organ or tissue in response to a stimulus. This can only take place in tissue with cells capable of dividing, and frequently occurs together with hypertrophy as stimuli are similar
Physiologic hyperplasia due to the action of hormones or growth factors occurs in several circumstances: when there is a need to increase functional capacity of hormone sensitive organs; when there is need for compensatory increase after damage or resection. Give some examples
- Hormonal hyperplasia of the glandular epithelium in the female breast at puberty and during pregnancy (usually accompanied by hypertrophy)
- epatocyte hyperplasia secondary to partial hepatectomy -
- Marrow undergoes remarkable hyperplasia in response to peripheral cytopenias
Most forms of pathologic hyperplasia are caused by excessive or inappropriate actions of hormones or growth factors acting on target cells. Give examples
- Endometrial hyperplasia post-menstration, due to imbalances in estrogen, progesterone - common cause of abnormal menstrual bleeding
- Benign prostatic hyperplasia occurs in response to hormonal stimulation by androgens
In these instances, the process remains controlled and it regresses if hormonal stimulation is eliminated. However, pathologic hyperplasia provides a fertile soil in which cancer can arise.
Hyperplasia is a characteristic response to certain _____ infections
Viral
Papillomaviruses and others can cause warts and mucosal lesions of hyperplastic epithelium. Some of these are precursors to cancer
Hyperplasia is the result of growth factor-driven proliferation of mature cells, and in some cases, by increased output of new cells from tissue stem cells.
Got that, you little bitch?
Atrophy definition
Reduction in the size of an organ or tissue due to a decrease in cell size and number
Physiologic vs. pathologic atrophy
Physiologic atrophy is common during normal development. - Embryonic structures (notochord, thyroglossal duct) undergo atrophy during fetal development. - Decrease in uterine size post-parturition Pathologic atrophy has several causes and can be localized or generalized.
Common causes of pathologic atrophy
- Decreased workload (atrophy of disuse): With prolonged disuse, cells can decrease in size and eventually in number (due to apoptosis)
- Loss of innervation (denervation atrophy): normal metabolism and function of skeletal muscle are dependent on its nerve supply.
- Diminished blood supply: Ischemia results in atrophy. This is what happens with senile atrophy (brain atrophy due to atherosclerosis)
- Inadequate nutrition: Profound protein-calorie malnutrition (marasmus) –> skeletal muscle protein utilizaiton for energy –> cachexia
- Loss of endocrine stimulation: (i.e. loss of estrogen after menopause –> physiologic atrophy of endometrium vaginal epithelium breast)
- Pressure: Tissue compression (enlarging benign tumor, others) can cause atrophy
Fundamental cellular changes of atrophy
Initial decrease in cell and organelle size (may reduce needs enough to permit survival)
Early on, there is diminished function but minimal cell death.
It may progress to point where cells are dying by apoptosis
Atrophy results from decreased protein synthesis and increased protein degradation in cells due to reduced metabolic activity. What pathway is mainly responsible for degradation of cellular proteins?
Ubiquitin-proteasome pathway Nutrient deficiency / disuse –> activate ubiquitin ligases –> target proteins for proteasomal degradation
This is thought to be responsible for cancer cachexia.
Often accompanied by autophagy, marked by appearance of increased numbers of autophagic vacuoles. Starved cells eat their own components to decrease nutrient demand. Lipofuscin granules in hepatocytes are examples of residual bodies (autophagic debris that resisted digestion)
Metaplasia definition and examples
Reversible change in which one differentiated cell type (epithelial or mesenchymal) is replaced by another cell type.
Most common epithelial metaplasia is columnar to squamous (respiratory tract secondary to chronic irritation)
Squamous metaplasia of salivary, pancreatic, biliary ducts with stones
Squamous metaplasia of respiratory epithelium with Vitamin A deficiency
If influences that predispose to metaplasia are persistent, it can initiate malignant transformation in metaplastic epithelium
TRUE
Connective tissue metaplasia
Formation of cartilage, bone, adipose in tissues that normally do not contain these elements (e.g. myositis ossificans after intramuscular hemorrhage)
Metaplasia does not result from a change in the phenotype of an already differentiated cell type; instead it is the result of a reprogramming of stem cells that are known to exist in normal tissues, or of undifferentiated mesenchymal cells present in connective tissue
This was bolded
KEY CONCEPTS: Cellular adaptations to stress
What are the hallmarks of reversible cellular injury?
- Reduced oxidative phosphorylation –> depletion of energy (ATP) stores
- Cellular swelling due to [ion] changes and water influx - Various intracellular organelles (mitochondria, cytoskeleton) may show alterations
When continuing cellular damage becomes irreversible, the cell cannot recover and it dies. Give brief descriptions of the two principle types of cell death
- Necrosis - “accidental”, unregulated cell death due to cell membrane damage and loss of ion homeostasis. When membrane damage is severe, lysosomal enzymes enter cytoplasm and digest the cell. This causes cellular content leakage and inflammation. Seen with many common injuries (e.g. ischemia, toxins, infections, trauma)
- Apoptosis: When the cell’s DNA or proteins are damaged beyond repair, the cell kills itself. This is characterized by nuclear dissolution, fragmentation of cell without loss of membrane integrity, and rapid removal of cellular debris. No inflammatory response is expected
Necrosis is always a pathologic process. Is this true for apoptosis?
NO!
Apoptosis serves many normal functions and is not necessarily associated with cell injury.
Causes of cell injury
- Oxygen deprivation: Hypoxia –> decreased aerobic oxidative respiration. This can result from ischemia, cardiorespiratory failure, decreased O2-carrying capacity of the blood (anemia, metHgbemia). This can lead to adaptation, injury, or death, depending on severity of hypoxia.
- Physical agents: mechanical trauma, extreme temepratures, sudden atmospheric pressure changes, radiation, electric shock
- Chemical agents and drugs: Too many to list. This can be due to hypertonic concentrations of glucose or salt. Even very high [O2] is toxic. Trace amounts of poisons (arsenic, cyanide, etc.) can cause massive cell death. Environmental and air polluants, CO, asbestos, alcohol, and many therapeutic drugs all can cause cell injury/death
- Infectious agents: Viruses, rickettsiae, bacteria, fungi, parasites can cause cell injury and death in many ways
- Immunologic reactions: Injurious reactions to endogenous self antigens during autoimmune disease. May be due to external agents (viruses, environmental substances)
- Genetic derangements: This can be due to a variety of mechanisms (protein deficiencies, damaged DNA or misfolded protein accumulations. Common polymorphisms in DNA sequences can also influence susceptibility of cells to injury.
- Nutritional imbalances: Protein-calorie deficiencies cause high numbers of human deaths. Specific vitamin deficiencies and excesses are important causes of cell injury (e.g. high cholesterol -> atherosclerosis, obesity -> diabetes, cancer). Apparently obesity is rampant in the United States… I haven’t noticed that though, I think our plane seats and doorways are just too small.
Sequential development of biochemical and morphologic changes in cell injury. Cells may become rapidly nonfunctional after the onset of injury, although they may still be viable, with potentially reversible damage; a longer duration of injury may lead to irreversible injury and cell death. Note that irreversible biochemical alterations may cause cell death, and typically this precedes ultrastructural, light microscopic, and grossly visible morphologic changes.
TABLE: Features of apoptosis and necrosis
Schematic illustration of the morphologic changes in cell injury culminating in necrosis or apoptosis
What are a few morphologic changes that characterize reversible injury?
- Cellular and organellar swelling
- Blebbing of plasma membrane
- Detachment of ribosomes from the ER
- Clumping of nuclear chromatin
These changes are associated with decreased ATP generation, loss of membrane integrity, defects in protein synthesis, cytoskeletal damage, and DNA damage
What are two characteristic features typically associated with necrosis?
- Severe mitochondrial damage with ATP depletion
- Rupture of lysosomal and plasma membranes
Two features of reversible cell injury can be recognized under the light microscope:
- Cellular swelling (a.k.a. hydropic change, vacuolar degeneration): cells incapable of maintaining ionic and fluid homeostasis due to failure of energy-dependent ion pumps in plasma membrane
- Fatty change: hypoxic injury, various forms of toxic or metabolic injury cause this. Lipid vacuoles seen in cytoplasm, seen mostly in hepatocytes and myocardial cells (these are dependent on fat metabolism)
The morphologic appearance of necrosis as well as necroptosis is the result of _____________
Denaturation of intracellular proteins and enzymatic digestion of the lethally injured cell by the cell’s own lysosomal enzymes
Morphologic changes associated with necrotic tissue on H&E slides
- Increased eosinophilia (loss of cytoplasmicc RNA and increased denatured proteins)
- Glassy, homogeneous appearance (loss of glycogen particles)
- Vacuolated, moth-eaten cytoplasm (enzymes digesting organelles)
- Myelin figures - whorled phospholipid masses derived from damaged membranes
- Nuclear changes:
- Karyolysis (faded chromatin color): enzymatic degradation of DNA
- Pyknosis (nuclear shrinkage): Condensed chromatin into a solid mass
- Karyorrhexis (nuclear fragmentation)
What are six patterns of tissue necrosis?
- Coagulative necrosis
- Liquefactive necrosis
- Gangrenous necrosis
- Caseous necrosis
- Fat necrosis
- Fibrinoid necrosis
Coagulative necrosis
Archesticture of dead tissues is preserved for at least some days.
Affected tissues exhibit a firm texture
Eosinophilic, anucleate cells persist for days or weeks due to degradation of enzymes, blocking proteolysis
Leukocytes come in and digest the dead cells
Often due to ischemia from an obstructed vessel.
Localized area of coagulative necrosis: Infarct
Liquefactive necrosis
Characterized by digestion of dead cells into a liquid, viscous mass
Seen in focal bacterial or fungal infections (microbes stimulate leukocyte accumulation and enzyme liberation by the leuks)
Necrotic material is creamy yellow (pus)
Caseous necrosis
Most often seen in foci of tuberculous infection
Caseous (cheese-like) is used due to the friable appearance
Granuloma: necrotic area (structureless collection of lysed cells and amorphous debris) enclosed with a distinctive inflammatory border (macs and lymphs)
Fat necrosis
Pretty non-specific
Local areas of fat destruction (typically from release of activated pancreatic lipases - acute pancreatitis)
Fatty acids from lipase activity bind calcium to produce a chalky white area (saponification)
Fibrinoid necrosis
Usually seen in immune reactions involving blood vessels
Antigen-antibody complex deposition in artery walls combine with fibrin and result in a bright pink and amorphous appearance called fibrinoid
KEY CONCEPTS: Morphologic alterations in injured cells and tissues
The principal biochemical mechanisms and sites of damage in cell injury. ATP, Adenosine triphosphate; ROS, reactive oxygen species.
Mechanisms of cell injury: Depletion of ATP
- Reduction of ATP levels is a fundamental cause of necrotic cell death
- Two major routes of ATP production:
1. Oxidative phosphorylation of ADP in mitochondria
2. Glycolytic pathway (can be anaerobic)
Major causes of depletion of ATP are reduced supply of oxygen and nutrients, mitochondrial damage, and actions of some toxins (e.g. cyanide)
Mechanisms of cell injury: Mitochondrial damage
Mitochondria are critical players in cell injury and cell death by all pathways.
Mitochondria can be damaged by increased intracellular [Ca2+] and hypoxia, so are sensitive to all types of injurious stimuli (hypoxia, toxins)
Mutations of mitochondrial genes are causes of some inherited disorders.
Three major consequences of mitochondria damage:
- Formation of a high-conductance channel in mitochondrial membrane (mitochondrial permeability transition pore), –> loss of mitochondrial membrane potential –> failure of oxidative phosphorylation and progressive ATP depletion –> necrosis
* *Cyclophilin D** (targeted by cyclosporine) is a structural component of the transition pore - Abnormal oxidative phosphorylation –> reactive oxygen species formation –> many deleterious effects
- Protein sequestration between inner and outer membranes of mitochondria that can activate apoptosis pathways (e.g. cytochrome c, caspases. Increased permeability may result in leakage of these into cytosol and apoptosis
Mechanisms of cell injury: Influx of Calcium and loss of Calcium homeostasis
Calcium ions are important mediators of cell injury; depleting intracellular [Ca2+] protects agents harmful stimuli
Intracytoplasmic [Ca2+] is very low compared to extracellular levels; most is sequestered in mitochondria and ER
Ischemia, toxins can cause increased cytosolic [Ca2+] first due to intracellular Ca2+ release, and then due to influx across plasma membrane
Increased cytoplasmic [Ca2+] causes cell injury by several mechanisms:
- Ca2+ accumulation in mitochondria –> opening of mitochondrial permeability transition pore –> failure of ATP generation
- Increased cytosolic [Ca2+] activates phospholipases (membrane damage), proteases (cytoskeletal/membrane protein break down), endonucleases (DNA, chromatin fragmentation), and ATPases (hastening ATP depletion)
- Induction of apoptosis by direct activation of caspases and increasing mitochondrial permeability
Pathologic effects of free radicals
- Lipid peroxidation in membranes: In aerobic conditions, free radicals can cause peroxidation of lipids within plasma and organellar membranes. Double bonds in unsaturated fatty acids attacked by ROS (particularly *OH) releasing unstable peroxides and initiating propagation, an autocatalytic chain reaction causing membrane damage
- Oxidative modification of proteins: Free radicals promote oxidation of amino acid side chains, formation of covalent pr-pr cross links (e.g. disulfide bonds), and oxidation of protein backbone. This can disrupt conformation of structural proteins and enhance proteasomal degradation of misfolded proteins –> havoc inside the cell
- Lesions in DNA: free radicals single- and double- stranded breaks in DNA, cross-linking of DNA strands, and adduct formation. Oxidative DNA damage has been implicated in cell aging and malignant transformation of cells.
TABLE: Properties of the principal free radicals involved in cell injury
Mechanisms of membrane damage in cell injury. Decreased O2 and increased cytosolic Ca2+ are typically seen in ischemia but may accompany other forms of cell injury. Reactive oxygen species, which are often produced on reperfusion of ischemic tissues, also cause membrane damage (not shown).
KEY CONCEPTS: Mechanisms of cell injury
KEY CONCEPTS: ischemic and toxic injury
Morphologic features of apoptosis
Morphologic features of apoptosis.
A, Apoptosis of an epidermal cell in an immune reaction. The cell is reduced in size and contains brightly eosinophilic cytoplasm and a condensed nucleus.
B, This electron micrograph of cultured cells undergoing apoptosis shows some nuclei with peripheral crescents of compacted chromatin, and others that are uniformly dense or fragmented.
C, These images of cultured cells undergoing apoptosis show blebbing and formation of apoptotic bodies (left panel, phase contrast micrograph), a stain for DNA showing nuclear fragmentation (middle panel), and activation of caspase-3 (right panel, immunofluorescence stain with an antibody specific for the active form of caspase-3, revealed as red color).
The intrinsic (mitochondrial) pathway of apoptosis.
A, Cell viability is maintained by the induction of anti-apoptotic proteins such as BCL2 by survival signals. These proteins maintain the integrity of mitochondrial membranes and prevent leakage of mitochondrial proteins.
B, Loss of survival signals, DNA damage, and other insults activate sensors that antagonize the anti-apoptotic proteins and activate the pro-apoptotic proteins BAX and BAK, which form channels in the mitochondrial membrane. The subsequent leakage of cytochrome c (and other proteins, not shown) leads to caspase activation and apoptosis.
The extrinsic (death receptor initiated) pathway of apoptosis, illustrated by the events following Fas engagement. FADD, Fas-associated death domain; FasL, Fas ligand.
Mechanisms of apoptosis. The two pathways of apoptosis differ in their induction and regulation, and both culminate in the activation of caspases. In the mitochondrial pathway, proteins of the BCL2 family, which regulate mitochondrial permeability, become imbalanced and leakage of various substances from mitochondria leads to caspase activation. In death receptor pathway, signals from plasma membrane receptors lead to the assembly of adaptor proteins into a “death-including signaling complex,” which activates caspases, and the end result is the same.
The unfolded protein response and endoplasmic reticulum (ER) stress.
A, In healthy cells, newly synthesized proteins are folded with the help of chaperones and are then incorporated into the cell or secreted.
B, Various external stresses or mutations induce a state called ER stress, in which the cell is unable to cope with the load of misfolded proteins. Accumulation of these proteins in the ER triggers the unfolded protein response, which tries to restore protein homeostasis; if this response is inadequate, the cell dies by apoptosis.
KEY CONCEPTS: apoptosis
Molecular mechanism of TNF-mediated necroptosis. Cross-linking of TNFR1 by TNF causes recruitment of RIP1 and RIP3 along with caspase 8. Activation of the caspase leads to apoptosis as described in the text. Inhibition of caspase 8, as may occur in some viral infections, allows RIP1 and RIP3 to initiate signals that affect mitochondrial generation of ATP and ROS. This is followed by events typical of necrosis.
KEY CONCEPTS: necroptosis and pyroptosis
Autophagy. Cellular stresses, such as nutrient deprivation, activate an autophagy pathway that proceeds through several phases (initiation, nucleation, and elongation of isolation membrane) and eventually creates double-membrane-bound vacuoles (autophagosome) in which cytoplasmic materials including organelles are sequestered and then degraded following fusion of the vesicles with lysosomes. In the final stage, the digested materials are released for recycling of metabolites. See text for details.
KEY CONCEPTS: Autophagy
Mechanisms of intracellular accumulations discussed in the text
Intracellular lipid accumulations
- Steatosis (fatty change): These terms describe abnormal accumulations of triglycerides within parenchymal cells; it is often seen in the liver due to its involvement in fat metabolism, but also occurs in heart, muscle, and kidney. Causes: toxins, protein malnutrition, diabetes mellitus, obesity, anorexia
- Cholesterol and cholesterol ester accumulations:
(1) Atherosclerosis:In atherosclerotic plaques, smooth muscle cells and macrophages in intimal layer of aorta and large arteries are filled with lipid vacuoles made up of cholesterol and its esters.
(2) Xanthomas: Intracellular accumulation of cholesterol within macrophages is also characteristic of acquired and hereditary hyperlipidemic states. Clusters of foamy cells seen in skin and tendons
(3) Cholesterolosis: focal accumulations of cholesterol-laden macrophages in lamina propria of gallbladder.
(4) Niemann-Pick disease, type C: lysosomal storage disease caused by mutations in cholesterol-trafficking enzyme –> cholesterol accumulation in multiple organs
Intracellular accumulations of proteins
These usually appear as rounded, eosinophilic droplets, vacuoles, or aggregates in cytoplasm.
Different causes:
- Reabsorption droplets in proximal renal tubules: associated with significant proteinuria; increased PCT cell pinocytosis of proteins–> reversible protein accumulations in cells
- Normally secreted proteins may accumulate if produced in excessive amounts (e.g. plasma eclls in active Ig syntehsis (a.k.a. mott cells, russell bodies)
- Defective intracellular transport and secretion of critical proteins: in a1-antitrypsin deficiency, mutations in the gene –> slow protein folding –> buildup of partially folded intermediates in ER of liver
- Accumulation of cytoskeletal proteins: several microtubules, thin actin filaments, thick myosin filaments, and intermediate filaments. Accumulations of types of intermediate filaments (keratin filaments in epithelial cells, and neurofilaments in neurons) are asscoaited with cell injury.
- Aggregation of abnormal proteins (a.k.a. proteinopathies, protein-aggregation diseases): Abnormal/misfolded proteins may deposit in tissues and interfere with function. These can be intra- or extra-cellular or both (i.e. amyloidosis)
KEY CONCEPTS: Abnormal intracellular depositions and calcifications
Mechanisms that cause and counteract cellular aging. DNA damage, replicative senescence, and decreased and misfolded proteins are among the best described mechanisms of cellular aging. Nutrient sensing exemplified by calorie restriction, counteracts aging by activating various signaling pathways and transcription factors. IG, Insulin-like growth factor; TOR, target of rapamycin
KEY CONCEPTS: Cellular aging
