Overview of Cellular Responses to Stress and Noxious Stimuli

Slide 1 · Overview of Cellular Responses

to Stress and Noxious Stimuli

Homeostasis · adaptation · injury · cell death

An integrated introduction to cellular pathology

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Define cellular homeostasis and adaptation.

Distinguish adaptive change, reversible injury, and irreversible injury.

Classify major causes of cellular injury.

Use myocardial stress and ischemia to explain cell fate.

Connect cellular events to tissue injury and clinical disease.

Slide 3 · From etiology to clinical manifestations

Etiology

Initiating cause

Pathogenesis

Cellular and molecular events

Morphology

Structural changes

Clinical effects

Functional consequences

Slide 4 · Pathology begins at the cellular level

Cellular basis

Most diseases begin with molecular or structural alterations in cells.

Organ-level expression

Cell and extracellular matrix injury produce tissue dysfunction and manifestations.

Pathology connects mechanism to clinically detectable disease.

Slide 5 · A normal cell operates within defined limits

Metabolic state

Energy production and substrate availability

Differentiation

Specialized structure and function

Local environment

Neighboring cells and tissue context

Functional demand

Physiologic work imposed on the cell

Slide 6 · Homeostasis is a healthy steady state

Homeostasis

Physiologic demands

Cellular capacity

Structure and function remain compatible with survival.

Slide 7 · Steady state does not mean inactivity

Changing inputs

Metabolic demand and external conditions fluctuate.

Regulated outputs

The healthy cell maintains essential functions within tolerable limits.

Homeostasis is dynamic control, not a fixed metabolic rate.

Slide 8 · Three principal responses to stress

Normal cell

Adaptation

Survive in altered steady state

Reversible injury

Potential for recovery

Irreversible injury

Progression to cell death

Slide 9 · Outcome depends on the nature of the insult

Type

Hypoxia, chemical, physical, immune, or other stimulus

Intensity

Demand and damage relative to cellular reserve

Persistence

Brief versus continuing exposure

Context

Cell type, baseline state, and available resources

Slide 10 · Adaptation creates a different viable state

Stress

Altered functional demand

Adaptive response

Change in size, number, or phenotype

New steady state

Cell continues to function

Original schematic

Slide 11 · Adaptations can be physiologic or pathologic

Physiologic stimulus

Change in normal demand; E.g., pregnancy

Pathologic stimulus

Response to an abnormal burden; E.g., raised cardiac workload

Both are initially reversible strategies for cell survival.

Slide 12 · Four fundamental adaptive patterns

Hypertrophy

↑ cell size

Hyperplasia

↑ cell number

Atrophy

↓ cell size / activity

Metaplasia

Changed differentiated phenotype

Slide 13 · Hypertrophy means increased cell size

Usual-sized cells

Enlarged cells

Increased workload may be met by greater cellular mass.

Slide 14 · Cardiac workload illustrates hypertrophy

Raised hemodynamic load

Greater mechanical demand

Cardiomyocyte hypertrophy

Larger muscle cells

Ventricular wall thickening

Adapted organ morphology

Original schematic

Slide 15 · Adaptation can carry a metabolic cost

Immediate benefit

Increased cardiac muscle mass helps meet workload.

Potential vulnerability

Greater metabolic demand can make the heart more susceptible to injury.

An adaptive response is not necessarily risk-free.

Slide 16 · Hyperplasia means increased cell number

Baseline cell population

Expanded cell population

Adaptation occurs by an increase in the number of cells.

Slide 17 · Atrophy reduces cell size and activity

Baseline size

Smaller cells

Reduced size is accompanied by decreased metabolic activity.

Slide 18 · Gradual arterial narrowing may favor atrophy

Reduced arterial supply

Chronic diminution in substrate delivery

Lower tissue demand

Adaptive reduction in size

Atrophy

Tissue remains viable under constraint

Original schematic

Slide 19 · Metaplasia changes the cellular phenotype

Original differentiated cell type

Alternative differentiated phenotype

An altered phenotype may be more suited to persistent stress.

Slide 20 · Adaptations differ in what changes

Response

Cell size

Cell number

Phenotype

Hypertrophy

Increases

Not defining

Retained

Hyperplasia

Not defining

Increases

Retained

Atrophy

Decreases

Not defining

Retained

Metaplasia

Not defining

Not defining

Changes

Slide 21 · Remove the stress: Adaptation may regress

Original state

Physiologic homeostasis

Adapted state

Altered yet viable equilibrium

Stress removed

Return toward original state

Original schematic

Slide 22 · The limit of adaptation defines a key threshold

Stress persists or overwhelms capacity

Successful adaptation

New steady state

Failed adaptation

Cell injury begins

Direct severe insult

Cell injury without adaptation

Slide 23 · Cell injury follows failure or direct damage

The insult may exceed the limits of adaptive capacity.

A cell may be deprived of critical nutrients or oxygen.

Toxic exposures may damage essential cellular structures.

Mutations can compromise functions required for survival.

Slide 24 · Reversible injury preserves recovery potential

Mild or transient insult

Function disturbed

Reversible injury

Cell is viable but altered

Insult resolves

Recovery is possible

Original schematic

Slide 25 · Irreversible injury leads to cell death

Severe or persistent insult

Critical damage accumulates

Irreversible injury

Viability cannot be restored

Cell death

Loss of injured cells

Original schematic

Slide 26 · Severity and duration jointly influence cell fate

Tolerable stress

Adaptation

Mild / transient

Reversible injury

Severe / sustained

Irreversible injury

Qualitative continuum — not a numerical injury threshold

Slide 27 · Progression is not an obligatory sequence

Gradually increasing workload

Adaptation may precede injury.

Abrupt major coronary occlusion

Severe ischemia can cause irreversible injury without prior adaptation.

Cell fate reflects the actual insult, not a required sequence of stages.

Slide 28 · Restoration of blood flow can be decisive

Is perfusion restored before injury becomes irreversible?

YES

NO

Recovery

Injured cells may recover

Cell death

Continuing ischemia can produce necrosis

Slide 29 · Cell death has normal physiologic functions

Embryogenesis

Removes cells as structures develop

Organ development

Supports tissue remodeling

Adult homeostasis

Eliminates aged or unnecessary cells

Slide 30 · Excess cell death causes disease

Injurious exposure

Ischemia, infection, or toxins

Pathologic cell loss

Exceeds useful homeostatic removal

Tissue dysfunction

Disease develops or progresses

Original schematic

Slide 31 · Main pathways of cell death: Orientation

Necrosis

Major pathway of death in many forms of pathologic injury.

Apoptosis

Major pathway; Also supports physiologic removal of cells.

The full molecular pathways belong to subsequent chapter topics.

Slide 32 · Cell death mechanisms extend beyond two labels

Necroptosis

Additional regulated death pathway

Pyroptosis

Additional regulated death pathway

Ferroptosis

Additional regulated death pathway

Slide 33 · Not all stress responses are adaptation or death

Metabolic derangement or chronic injury

Intracellular accumulation

Proteins, lipids, carbohydrates

Pathologic calcification

Calcium at sites of dead tissue

Cellular aging

Characteristic structural and functional change

Slide 34 · Intracellular accumulations indicate disturbed handling

Proteins

Abnormal protein content may accumulate

Lipids

Fat or other lipids can build up

Carbohydrates

Stored carbohydrates may become excessive

Slide 35 · Calcification may follow tissue death

Cellular death

Tissue damage

Calcium deposition

Dystrophic-type context

Visible lesion

Pathologic mineral accumulation

Original schematic

Slide 36 · Cellular aging also changes structure and function

Structural dimension

Characteristic morphologic alterations occur as cells age.

Functional dimension

Aging is accompanied by changed cell performance.

Aging belongs to the wider framework of altered cellular states.

Slide 37 · Myocardium demonstrates the complete response model

A single organ can show adaptation, reversible injury, or necrosis under different stresses.

Baseline

Adapted

Schematic

Slide 38 · Healthy myocardium: Baseline state

Baseline myocardium

Wall structure and myocyte function meet prevailing workload.

Original schematic

Slide 39 · Pressure loading produces adapted myocardium

Concentric wall thickening (schematic)

Larger myocytes are associated with a thicker left ventricular wall.

Original schematic

Slide 40 · Compromised perfusion changes viable myocytes

Functional myocyte

Reversibly injured myocyte

Early reversible injury may show swelling or lipid accumulation.

Slide 41 · Irreversible ischemia culminates in infarction

Regional loss of viability (schematic)

A region of myocardium may become necrotic after loss of blood flow.

Original schematic

Slide 42 · No gross change is required for reversible injury

Functional alteration

Cellular injury may disrupt function before overt gross changes appear.

Microscopic change

Swelling and fatty change may be detectable when present.

Absence of obvious gross abnormality does not prove normal cellular function.

Slide 43 · Viability staining: Interpret the principle

Viable myocardium

Infarcted region unstained

Triphenyltetrazolium chloride stains viable myocardium brick-red; Nonviable infarcted areas remain unstained.

Slide 44 · Cardiac enlargement is not the same as infarction

Hypertrophy

Adaptive increase in wall thickness after increased workload.

Infarction

Death of myocardium due to inadequate blood flow.

The two findings describe different processes and may coexist.

Slide 45 · Two myocardial stresses, two distinct mechanisms

Increased pressure

Greater work

Myocyte enlargement

Hypertrophy

Reduced perfusion

Oxygen shortage

Myocyte injury

Possible necrosis

Slide 46 · Sudden arterial occlusion tests the exception

Clinical situation

A major coronary artery becomes abruptly and completely occluded.

Question

Must the myocytes hypertrophy before they become irreversibly injured?

Interpretation

No. Severe acute ischemia can bypass an adaptive stage.

Slide 47 · Etiologic categories organize the diagnostic search

Oxygen deprivation

Physical agents

Chemicals / drugs

Infectious agents

Immunologic reactions

Genetic abnormalities

Nutritional imbalance

Cell injury

Response depends on insult and cell state

Cause taxonomy

Slide 48 · Oxygen deprivation impairs aerobic respiration

Hypoxia

Insufficient oxygen availability

Oxidative respiration falls

Aerobic energy production affected

Cell response

Adaptation, injury, or death

Original schematic

Slide 49 · Three routes to tissue hypoxia

Ischemia

Reduced blood flow

Poor oxygenation

Cardiorespiratory failure

Low oxygen capacity

Anemia, CO poisoning, severe blood loss

Slide 50 · Ischemia is a flow problem, not a synonym for hypoxia

Hypoxia

Insufficient oxygen supply to the cell.

Ischemia

Reduced tissue blood flow; One important cause of hypoxia.

Causes of hypoxia also include impaired oxygenation or carrying capacity.

Slide 51 · Physical agents can injure cells directly

Mechanical

Trauma and tissue disruption

Thermal

Burns and extreme cold

Pressure

Sudden atmospheric changes

Other

Radiation and electric shock

Slide 52 · Chemical injury spans ordinary and unusual exposures

Common solutes

Hypertonic glucose or salt

Environmental

Pollutants and occupational toxins

Poisonings

Arsenic, cyanide, mercury

Drugs

Therapeutic adverse effects or substance use

Slide 53 · Dose and context can turn normal substances toxic

Concentration matters

Hypertonic salt or glucose deranges cell fluid/electrolyte balance.

Essential is not always harmless

Oxygen can be toxic at high concentrations.

A substance's normal physiologic role does not ensure safety at every exposure.

Slide 54 · Infectious agents cause diverse patterns of damage

Viruses

Submicroscopic infectious agents

Bacteria and rickettsiae

Different pathogen groups

Fungi and parasites

Additional biologic causes

Slide 55 · Immune defense can itself produce tissue injury

Protective role

Immune responses remove pathogens and other threats.

Injurious role

Responses to self-antigens or external agents can damage cells.

Autoimmune disease demonstrates injury driven by recognition of self.

Slide 56 · Genetic changes can threaten essential functions

Protein function

Absent or defective enzyme function

Genome integrity

Unrepaired DNA damage

Protein quality

Accumulation of misfolded proteins

Susceptibility

Variants modify vulnerability to exposures

Slide 57 · Nutritional deficiency is an injurious exposure

Global deficit

Protein–calorie malnutrition

Micronutrient deficit

Deficiency of specific vitamins

Different origins

Food shortage, poor diet, or restricted intake

Slide 58 · Nutritional excess can also injure tissues

Overnutrition

Obesity-related disease risk

Lipid composition

Dietary patterns influence circulating cholesterol

Tissue consequences

Metabolic disease and atherosclerosis risks

Slide 59 · Cause and cellular susceptibility interact

Etiologic agent

Environmental or genetic insult

Cellular susceptibility

Existing functional and genetic state

Biologic outcome

Adaptation, injury, or death

Original schematic

Slide 60 · Stress pathways can initially protect cells

Integrated stress response

Modulates synthesis of stress-response proteins

Unfolded protein response

Response to ER protein-folding stress

Autophagy

Cellular material recycling under stress

Slide 61 · Protective responses may be insufficient

Cellular stress

Homeostatic imbalance

Stress-response pathways

Attempt to preserve viability

Unresolved severe damage

Death programs may be activated

Original schematic

Slide 62 · Clinicopathologic reasoning: A pressure-loaded heart

Clinical situation

A patient has longstanding increased hemodynamic load and a thickened left ventricular wall.

Question

What cellular adaptation best explains the wall thickening?

Interpretation

Hypertrophy: Increased cardiomyocyte size in response to workload.

Slide 63 · Clinicopathologic reasoning: An acutely ischemic heart

Clinical situation

A patient develops abrupt prolonged obstruction of a major coronary artery.

Question

What determines whether affected myocytes recover or die?

Interpretation

Severity and persistence of ischemic injury; Irreversible injury causes cell death.

Slide 64 · Decision map for any cellular stress

Cell experiences increased demand or damaging stimulus

Can adaptation sustain essential function?

Yes

No

Sustained viability

Is the injury reversible if the insult resolves?

Yes

No

Recovery possible

Irreversible injury → cell death

Slide 65 · Synthesis: The cellular response continuum

Homeostasis is the reference state.

Adaptation preserves survival under altered demand.

Mild/transient damage may be reversible.

Persistent/severe damage may produce cell death.

The exact cause and tissue context shape the result.

Slide 66 · Primary source and scope

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th ed. (2026). Chapter 2, pp. 29–34.

Primary teaching section: “Overview of Cellular Responses to Stress and Noxious Stimuli,” pp. 30–31.

Related contextual sections: “Causes of Cell Injury,” pp. 31–32, and introductory “Pathways of Cellular Response to Stress and Injury,” p. 32.

All illustrations in this lecture are original editable schematics, not reproduced textbook figures or diagnostic specimens.