2. Cellular response to stress

Question 1

Cellular response to environmental stimuli

Answer 1

• Stress / increased demand:
  Normal cell (homeostasis) → Adaptation
• Injurious stimulus:
  Normal cell (homeostasis) → Cell injury → Cell death
• Inability to adapt:
  Adaptation → Cell injury → Cell death

Question 2

What is homeostasis?

Answer 2

Homeostasis is the ability of cells & tissues to maintain a steady state & handle physiological demands.

Question 3

What are adaptations?

Answer 3

Adaptations are reversible changes in the number, size, phenotype, metabolic activity, or functions of cells in response to changes in their environment.

Question 4

What is cellular adaptation?

Answer 4

Cellular adaptation occurs in response to a certain stimulus & stops once the need for adaptation has stopped (stimulus removed).

Question 5

What is physiologic adaptation?

Answer 5

Physiologic adaptations represent responses of cells to normal stimulation (physiological stressors, such as hormones or endogenous chemicals mediators.

Question 6

What are pathological adaptations?

Answer 6

Pathologic adaptations are responses to stress that induce cells to change their structure & function to escape injury & preserve viability & function.

Question 7

Hypertrophy

Answer 7

• Increase in the size of the cell leads to increase in the size of the organ.
• Caused by:
  
  1. increased functional demand
  2. growth factors
  3. hormonal stimulation
• Adaptive response in cells with limited capacity to divide ( skeletal muscle & heart).

Question 8

Hypertrophy examples

Answer 8

1. Skeletal muscles
2. Heart
3. Uterus

Question 9

Pathological hypertrophy

Answer 9

• Pathological hypertrophy:
• Cardiac hypertrophy involving the left ventricle of a patient with systemic hypertension.
• The size of myocardial fibers increase in response to the increased workload leading to the marked thickening of the left ventricle.
• The left ventricle of a healthy human adult is normally less than his 1.5 cm.

Question 10

Hyperplasia

Answer 10

• Hyperplasia:

1. Is the number of cells in a tissue or organ (bone marrow after blood loss).
2. Is an adaptive response in cells capable of replication.
3. Example: Hyperplasia in the female breast (glandular epithelium) at puberty & during pregnancy & lactation additional hormones).
4. (Note: The breast also increase in size by hypertrophy)

Question 11

Pathological Hyperplasia

Answer 11

• Thyroid goiter:
• insufficient available dietary iodine
• cannot make enough thyroid hormone
• extra demand by pituitary (TSH)
• thyroid gets huge.

Question 12

Atrophy

Answer 12

• Reduction in cell size by loss of cell substance caused by decreased protein synthesis & increased protein degradation in cells. It happens due to:
• Decreased workload (atrophy of disuse). Immobilization causes skeletal muscle atrophy.
• Loss of innervation (paralysis)
• Diminished blood supply (ischemia) as a result of arterial occlusive disease or arteriosclerosis.
• Inadequate nutrition: profound protein-calorie malnutrition results in marked wasting (cachexia).
• Loss of endocrine stimulation: The loss of estrogen stimulation after menopause results in physiologic atrophy of the endometrium & breast. The uterus decreases in size shortly after parturition.
• Aging (senile atrophy): particularly the brain & heart.

Question 13

Metaplasia

Answer 13

• Adaptive substitution of one type of differentiated cell for another type of cell. It usually occurs in response to chronic irritation.

Question 14

Stages in the cellular response to stress & injurious stimuli

Answer 14

Question 15

When does cell injury happens?

Answer 15

• Cell injury happens when cells are no longer able to adapt in response to damaging agents or intrinsic abnormalities.
• Injury may progress through a reversible stage or may lead to cell death.

Question 16

What does the cellular response to injurious stimuli depends on?

Answer 16

• Nature, duration, & severity of the injury
• Type & adaptability of the injured cell

Question 17

Define reversible cell injury

Answer 17

cellular injury is mild & sublethal

Question 18

Define irreversible cell injury or cell death

Answer 18

Injury is severe & lethal

Question 19

Internal causes of cell injury

Answer 19

1. Oxygen deprivation: hypoxia is the most common cause of cell injury.
   Causes of hypoxia include:
   
   • reduced blood flow (ischemia)
   • inadequate oxygenation of blood ( hypoxemia, cardiorespiratory failure, & anemia).
   • ???
2. Genetics defects: causing deficiencies of functional properties.
3. Immune-mediated mechanisms: autoimmune diseases
4. Aging

Question 20

External causes of cell injury

Answer 20

• Physical or mechanical trauma:
  
  1. Extreme temperatures
  2. UV lights
  3. Radiation
• Chemicals & toxins:
  
  1. Drugs
  2. Alcohol
  3. Environmental & occupational hazards
• Microbial agents:
  
  1. Bacteria
  2. Viruses
• Nutritional:
  
  1. Deficiency of proteins or vitamins
  2. Excess cholesterol

Question 21

Mechanisms of cell injury

What are the cell injury damages key cellular functions?​​

Answer 21

1. Mitochondrial damage:
   
   • ​​↓ ATP → Multiple downstream effects
   • ↑ ROS → Damage of lipids, proteins, & DNA
2. Entry of Ca²⁺:
   
   • ​​↑ Mitochondrial permeability
   • Activation of multiple cellular enzymes
3. Membrane damage:
   
   • ​​Plasma membrane → Loss of cellular components
   • Lysosomal membrane → Enzymatic digestion of cellular components
4. Protein misfolding & DNA damage:
   
   • ​​Activation of pro-apoptotic proteins

Question 22

What is the most common injury stimulus?

Answer 22

Ischemia

Question 23

Mitochondrial Damage

What are the functional & morphological consequences of decreased intracellular ATP during cell injury?

Answer 23

1. ↓ Oxidative phosphorylation
2. ↓ ATP:
   
   • ​↓ ​Na⁺ pump
   • Anaerobic glycolysis
   • Detachment of ribosomes

Question 24

Mitochondrial Damage Consequences

↓ ATP

Answer 24

• ↓ ATP:​​
  
  1. ↓ ​Na⁺ pump:
     
     • ​​Influx of Ca²⁺, H₂0, & Na⁺
     • Efflux of K⁺
     • → Cellular Swelling
  2. ↑ Anaerobic glycolysis:
     
     • ​​↓ Glycogen
     • ↑ Lactic acid → ↓ pH → clumping of nuclear chromatin
  3. Detachment of ribosomes → ↓ protein synthesis

Question 25

* Mitochondrial Damage Consequences: ↑ **ROS** * **Role of reactive oxygen species (ROS) in cell injury**

Answer 25

* Damage of lipids, protein, & DNA * Production of ROS 1. → Removal of free radicals 2. → Pathological Effects: * Lipid peroxidation → Membrane damage * Protein modifications → Breakdown & Misfolding * DNA damage → Mutations

Question 26

ROS - Induced Injury

Answer 26

* In physiological state, there is a balance between the level of ROS formation & degradation **keeping ROS at low level (ROS homeostasis)**. ROS accumulation is counterbalanced by specialized enzymes ( SOD, Glutathione peroxides, Catalaze) & antioxidants (vitamins E, A, C ). * The production of ROS is increased by many injurious stimuli (radiation, reperfusion, toxins). **_Oxidative stress_ is a transient or persistent increase of ROS level that disturb cellular function & signaling pathways.** * **_Oxidative modification_ of lipids, proteins, & DNA results in cell injury** & may culminate in cell death via necrosis or apoptosis.

Question 27

What are the 3 mechanisms that **ROS damage cells**?

Answer 27

1. **Lipid peroxidation** damages the double bonds in membrane lipids. 2. **DNA fragmentation** through reacting with **_Thymine_** in nuclear & mitochondrial DNA to produce single strand breaks. 3. **Protein cross-linking**: Free radical promote sulfhydryl-mediated protein cross-linking, resulting in increased degradation or loss of protein activity.

Question 28

Cell injury - Increased cytosolic calcium

Answer 28

Question 29

What contributes to **membrane damage**?

Answer 29

1. **ROS**: Reactive Oxygen Species 2. **Phospholipid reacetylation / synthesis** 3. **Phospholipidase activation** by Ca²⁺ 4. **Protease activation**

Question 30

Mechanisms of **membrane damage** in cell injury

Answer 30

* Damage of cellular membranes may affect **plasma, lysosomal, & mitochondrial** membranes, which lead to necrosis. * **Decreased** O₂ & **increased** cytosolic Ca²⁺ typically are seen in **ischemia** but may accompany other forms of cell injury. * **Note: ROS produced on _repercussion_ of ischemic tissues can cause membrane damage & cell death.**

Question 31

What is the **first manifestation of cell injury**?

Answer 31

Swelling

Question 32

**Morphologic** changes in cell **injury**: **Swelling**

Answer 32

* **Swelling**: 1. is the first manifestation of cell **injury** 2. is a **reversible** alteration that may appear at the level of the whole organ. At the level of the organ it: * causes **pallor** (due to compression of capillaries) * increase in weight of the organ

Question 33

**Ultra**-structural changes of **reversible** cell injury

Answer 33

1. Plasma membrane alterations with **blebbing & loss of microvilli** 2. **Mitochondrial swelling** 3. Dilation of ER & detachment of ribosomes 4. Nuclear alterations with **clumping of chromatin** 5. The cytoplasm may contain phospholipid masses, called **myelin figures**.

Question 34

Hydropic change

Answer 34

* Accumulation of water in the cell. * Reversible injury in kidneys.

Question 35

Fatty change

Answer 35

* **Fatty change** is manifested by the appearance of **lipid vacuoles in the cytoplasm**. * It is principally encountered in _cells_ participating in **fat metabolism:** 1. **​​**Hepatocytes 2. Myocardial cells * It is caused by **defective transport of lipids** (defects in synthesis of transport proteins). * It is also **reversible**. * _Graph description_: **Fatty change** in the **liver**. Imposed lipoprotein transport due to injury (alcoholism) leads to accumulation of lipids in the cytoplasm of **hepatocytes**, which are **active in fat metabolism**.

Question 36

**Severe** cell injury is **irreversible** - Cell **death**

Answer 36

Question 37

What are two characteristics of **reversible** cell **injury**?

Answer 37

1. Hydropic change 2. Fatty change

Question 38

* True or False: * **In cell injury, there is less protein synthesis.**

Answer 38

* True. * There is **less protein synthesis** in cell injury.

Question 39

What are very important in determining reversible or irreversible cell injury?

Answer 39

Membranes

Question 40

Irreversible cell injury

Answer 40

* Injury beyond cell ability to adapt or maintain survival leads to **irreversible cell injury**. * Persistent or excessive injury causes cells to pass the **“point of no return"** into irreversible injury / cell death.

Question 41

What are the **phenomena's** that characterize **irreversibility**?

Answer 41

1. The inability to correct **mitochondrial dysfunction** 2. Profound disturbances in **membrane** function 3. Injury to **lysosomal** membranes 4. **Necrosis**

Question 42

Define **Necrosis**

Answer 42

* **Necrosis**: 1. Cell death due to **irreversible** cell injury 2. (Cells die in **groups**)

Question 43

Morphology of cell death - Necrosis What is **pyknosis**?

Answer 43

* Nuclear shrinkage & increased basophilia. DNA condenses into a solid shrunken mass.

Question 44

Morphology of cell death - Necrosis ## Footnote **Karyorrhexis**

Answer 44

* Karyo: Nucleus * **Karyorrhexis: Nucleus undergoes fragmentation.**

Question 45

Morphology of cell death - Necrosis ## Footnote **Karyolysis**

Answer 45

* In 1 to 2 days, the **nucleus** in a dead cell may **completely disappear**. * Deoxyribonuclease (**DNase**) activity _causes nuclear dissolution_. * **Karyolysis: Empty cell in necrotic tissue.**

Question 46

Why do necrotic cells show increased **eosinophilia** (pink staining from the eosin dye)?

Answer 46

* Increased eosinophilia in (H & E) stains, attributable in part to the **loss of cytoplasmic RNA** (which binds the blue dye, hematoxylin) and in part to **denatured cytoplasmic proteins** (which bind the red dye, eosin). * Increased eosinophilia? 1. **_Loss_ of cytoplasmic RNA** → binds **hematotoxylin / blue** dye 2. **_Denatured_ cytoplasmic proteins** → binds **eosin / red** dye

Question 47

What happens when **enzymes have digested cytoplasmic organelles**?

Answer 47

The **cytoplasm** becomes **vacuolated**.

Question 48

How are **necrotic** cells characterized?

Answer 48

1. **Discontinuities** in plasma & organelle membranes 2. **Dilation** of mitochondria 3. **Disruption** of lysosomes 4. Intracytoplasmic myelin figures

Question 49

Describe graph **A**

Answer 49

* Normal kidney tubules with viable epithelial cells * Normal **nucleus** & **cytoplasm**

Question 50

Describe graph **B**

Answer 50

* Early (reversible) ischemic injury showing: 1. **surface blebs** 2. **increased eosinophilia of cytoplasm** 3. **swelling of occasional cells** * **​**More compact * More red → eosinophilia → denaturation of protein

Question 51

Describe graph **C**

Answer 51

* Necrotic (irreversible) injury of epithelial cells, with: 1. Loss of nuclei 2. Fragmentation of cells 3. Leakage of contents * **No** known organization * Fragmented * **Nucleus** = compact + **blue** * **No** nucleus in cells

Question 52

Define **Coagulate Necrosis**

Answer 52

* **Coagulate necrosis** is a form of necrosis where the **underlying tissue architecture is preserved** for at least several days. * The injured tissue stays firm since the injury denatures not only structural proteins but also proteolytic enzymes, thereby **blocking the proteolysis of the dead cells**. * **Leukocytes are recruited to the site of necrosis,** and the dead cells are digested by the action of lysosomal enzymes from the leukocytes. The cellular debris is then removed by **phagocytosis**. * **Coagulate necrosis** is characteristic of infarction (ischemic necrosis) in all solid organs _except the brain_. * **Description of the graph**: **Coagulate necrosis in the kidney**. Within the infarcts cells are red (**eosinophilic**), nuclei are lysed (**karyolysis**) but tissue architecture is preserved. It is the result of proteins being denatured.

Question 53

Define **Liquefactive Necrosis**

Answer 53

* **Liquefactive necrosis is seen in focal bacteria or fungal infections**, bcz microbes stimulate the accumulation of inflammatory cells & the enzymes of leukocytes digest (liquefy) the tissue. * **Hypoxia death of cells within the CNS** (brain) appear as liquefactive necrosis (unknown reason). * **Dead cells are completely digested**, transforming the tissue into a liquid viscous mass. * The digested tissue is later removed by **phagocytes**. If bacterial infection is the cause of cell death, the material is creamy yellow & is called **pus**. * **_Description of the graph_**: cerebral infarction demonstrates liquefactive necrosis.

Question 54

Define **Caseous Necrosis**

Answer 54

* **Caseous** = **cheese-like**. * **Caseous necrosis** is found in loci of **TB** infection. * The tissue architecture is completely erased & **cellular outlines cannot be seen**. * The area of caseous necrosis is usually surrounded by an inflammatory border **(focus of inflammation called _granuloma_)**. * On microscopic examination the necrotic focus appears as a collection of **fragmented or lysed cells** with an amorphous granular appearance. * **_Description of the graph_**: * *TB** of the lung with an area of **caseous** necrosis.

Question 55

Define **Fat Necrosis**

Answer 55

* Fat necrosis occurs in acute pancreatitis where **fat destruction results from release of pancreatic enzymes** into the pancreas & the peritoneal cavity. * Pancreatic enzymes leak out of the damaged cells & ducts & **liquefy the membranes of fat cells in The peritoneum. Lipase split the triglyceride esters** contained within fat cells. * **The released fatty acids combine with calcium** to produce grossly visible **chalky white areas (fat saponification).**

Question 56

The relationship among normal, adapted, reversible injured, and dead myocardial cells.

Answer 56

* **Adaptation** → hyper**trophy** * **Reversible** injury → ischemia * ****_Ir_**reversible** injury → ischemic **coagulate** **necrosis**

Question 57

What is **apoptosis**?

Answer 57

* **Apoptosis = programmed cell death** * **Apoptosis** is a regulated mechanism that serves to eliminate unwanted & irreparably damaged cells to maintain homeostasis (**single** cell death)

Question 58

Physiological conditions of **apoptosis**

Answer 58

* During development (embroyogenesis) for removal of excess cells. * To maintain cell population in tissues with high turnover of cell, such as skin & bowels. * Hormone-dependent involution: 1. Endometrium 2. Ovaries 3. Breasts

Question 59

**Pathological** conditions of **apoptosis**

Answer 59

* **DNA damage** from: 1. ionizing radiation 2. toxic chemicals * **Stress conditions** ,e.g., starvation. * Excessive accumulation of **misfolded proteins**. * **To remove cells** damaged by viruses. * **Cell death** in autoimmune diseases.

Question 60

Apoptosis

Answer 60

Question 61

**Mechanisms of apoptosis**

Answer 61

* The two pathways of apoptosis differ in their induction but both lead to **activation of caspases**. * In the mitochondrial pathway, proteins of the **bcl-2 family & cytochrome C** activate caspases. * In the death receptor pathway, signals from plasma membrane receptors cause the assembly of adaptor proteins into a **"death-inducing signaling complex"**, which activates caspases. * **Caspases activation** leads to DNA fragmentation & cytoskeleton breakdown with intact plasma membranes (**apoptotic bodies**). * **Apoptotic bodies** express new ligands for binding & uptake by phagocytes. They are removed by **phagocytosis** without inducing inflammatory reactions.

Question 62

Mechanisms of Apoptosis (graph)

Answer 62

Question 63

**Apoptosis mitochondrial pathway** [IMPORTANT]

Answer 63

* Induction of apoptosis by the mitochondrial pathway is dependent on **a balance between pro- & anti-apoptotic proteins of the Bcl family**. * **Injurious stimuli activate cytoplasmic sensors** & lead to reduced production anti-apoptotic proteins & increased amounts of pro-apoptotic proteins. * **The proapoptotic proteins sense DNA & protein damage** & activate effectors that insert in the mitochondrial membrane & **promote leakage of mitochondrial proteins**. * In a viable cell, **anti-apoptotic members of the Bcl-2 family prevent leakage of mitochondrial proteins**. * **The mitochondrial proteins** that leak out activate 9 series of **caspases** (first the initiators & then the executioners). * These enzymes cause **fragmentation of the nucleus & the cell**.

Question 64

The unfolded protein response

Answer 64

* In heathy cells, newly synthesized proteins are folded with the help of **chaperones** & are then _incorporated_ into the cell or _secreted_. * **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: 1. tries to restore protein homeostasis 2. **activates signaling pathways that increase production of chaperones** 3. **reduce protein translation**, thus reducing the level of misfolded proteins. * If this response is inadequate, **the cell dies by apoptosis**.

Question 65

The unfolded protein response

Answer 65

Question 66

Apoptosis / Necrosis

Answer 66

Question 67

Features of necrosis & apoptosis

Answer 67