General Pathology (PATH 303) Lecture # 8

APOPTOSIS

Apoptosis or programmed cell death (apoptosis –a falling away from)

It is a genetically controlled homeostatic mechanism to remove unwanted cells. Death genes give rise to proteins which direct executioner molecules to:

1. Break chromosomes 2. Depolymerize the cytoskeleton 3. Cause mitochondria to release cytochrome-c and 4. Cytoplasmic blebs and pyknotic nuclei are shed from

cell as apoptotic bodies

Significance

Critical processes : Programmed cell death (PCD) is essential for: Gene - directed cell deletion in

embryogenesis. Physiological involution such as during

menstrual cycle in humans Removal of neoplastic cells with lethal

mutations. Deletion of cells damaged by toxins and

infectious agents.

Caspases (cystein –containing aspartate specific proteases).

Most of cellular changes in PCD are brought about by protein cleaving enzymes known as caspases. There are two groups:

Initiator caspases: Caspase 8 and 10 respond to death signals and activate the second group i.e.

Execution caspases: caspases 3, 6 and 7 enzymes that make specific cuts in key proteins that are required for cell survival.

Mechanism:

Initiation: Signals that initiate the process of PCD may be extra- or intracellular. In embryogenesis, atrophy and neoplasia, the initiating

signals arise from within the cell. Toxins, drugs, cytokines and steroid hormones can all initiate

PCD via specific signals that occur at the cell surface. Execution: Caspase cascade: The death and survival of cell depends on the ratio of

two genes:a) BAX- a gene that lead to death of the cell.b) BCL-2 genes that are death repressor genes Mitochondrial shutdown: Mitochondria play a critical

role in caspase cascade pathway to PCD.

Cytochrome-c released from mitochondria binds to a critical protein apoptotic protease activating factor- 1 (Apaf -1) which in turn binds to and activates caspse-9

Caspase 9 activation

Cytochrome -c binds to Apaf-1

Mitochondrial release of cytochrome- c

Caspase 3 activation

DNA cleavageCytoskeletal

depolymerization

Mitochondrial breakdown

Comparison between Coagulative necrosis and apoptosis

Coagulative necrosis Apoptosis

1. Stimuli Hypoxia , toxins Physiological and Pathological factors

2. Microscopic appearance

Cellular swelling, Coagulative necrosis , disruption of organelles

Single cell, chromatin condensation, apoptotic bodies

3. DNA breakdown Random ,diffuse Inter-nucleosomal

4. Mechanisms ATP depletion, Membrane injury, free radical damage

Gene activation, endonucleases, proteases

5. Tissue reaction Inflammation No inflammation, phagocytosis of apoptotic bodies.

Gangrene

It is invasion and putrefaction of necrotic tissues by saprophytic bacteria

Types

Gangrene may be dry or moist depending upon the moisture (blood circulation) and temperature in the tissues

1-Dry Gangrene

Observed in the extremities, legs, ears, tail, comb, wattle etc.

Causes

1. Certain drugs like ergot and molds growing on grasses and wheat straw as in Deg Nala disease in Pakistan

2. Freezing and subsequent invasion by saprophytes

In the extremities, blood circulation is limited and the temperature is also lower, therefore, the invasion and spread of bacteria is slow.

Gross Appearance

The affected part is dry, shriveled, mummified as a result of dehydration.

The color may be light or dark grey according to the amount of iron sulphide (pseudomelanosis)

A sharp line of inflammation (redness) separates the affected area from healthy tissues

Microscopic Appearance

The dead tissue appears homogenous without cellular details

Saprophytic bacteria are usually presentA few gas bubbles may be present as

clear spacesAcute inflammatory reaction may be

present at the junction of dead and live tissues

2-Moist Gangrene

It occurs in internal organs where moisture and temperature are favorable for bacterial growth

Death occurs rapidly from septicemia, toxemia and shock

Causes

In lungs: By faulty drenching and wrong insertion of

stomach tube Irritating medicines cause necrosis (drenching

pneumonia) and gangreneIn intestine: Gangrene occurs due to malpositions e.g.

torsion, volvulus, hernia and intussusceptions Necrosis is caused due to venous obstruction and

congestion Presence of bacteria in the intestinal ingesta allows

rapid spread of moist gangrene

Gross Appearance

The gangrenous part is moist, red, green or black as a result of iron sulphide formation.

The intestine is usually distended with gas formation.

There is bad odour from hydrogen sulphide (rotten eggs) and putrefaction.

Microscopic Appearance

Same as autolysis, with many gas bubbles and saprophytic bacilli

Gas gangrene, malignant edema and black quarter disease

These are fatal disease conditions caused by different species of spore forming bacteria- Clostridium (C. septicum, C. perfringens and C. chauvei)

These organisms are anaerobic, spore forming, soil inhabitant and cause diseases as wound infections.

Under anaerobic conditions the organisms multiply, produce toxins causing tissue digestion like lecithinase and collagenase

The organisms produce edema and gas in the affected tissues and spread to surrounding tissues and cause death of the animal

Results of necrosis and gangrene

Necrosis may terminate in several ways:1. Liquefaction and removal by neutrophils,

lymph or blood- (small areas)2. Liquefaction and cyst formation- (large

areas). Fibrous capsule may be formed3. Liquefaction , abcessation and discharge-

(invasion by pyogenic bacteria)4. Encapsulation without liquefaction-

(coagulation and caseous necrosis)5. Sloughing and desquamation- (on external

surfaces)

Results of necrosis and gangrene

6.Organization of necrotic tissue

7.Dystrophic calcification

8-Death of animal – usually in case of moist gangrene

Post- mortem changes

Postmortem changes may be distinguished from lesions of the disease

Factors affecting the onset of postmortem changes

1. Environmental temperature

2. Size, insulation and nutritional status of the animal

Autolysis:

Digestion of tissues by their own cellular enzymes

Putrefaction

Decomposition of tissues by enzymes of saprophytic bacteria

Rigor mortis

It is the stiffening and immobilization of body due to the muscular contraction after death

Rigor mortis begins in the anterior part and progresses towards the posterior direction ( head, neck, trunk and limbs) and disappears in the same order.

It appears 1 to 8 hours after death and disappears 20-30 hours after death

Rigor mortis appears earlier when there is high external temperature or violent exercise (racing, fighting etc.) and it is retarded by low temperature and emaciation

Postmortem clotting of blood

Endothelial cells in the blood vessels release thromboplastin which causes clotting of blood in the heart and blood vessels

Clotting fails in anthrax due to fibrinolysin produced by B. anthracis and in sweet clover poisoning due to inhibition of prothrombin activity

Imbibition with hemoglobin

Erythrocytes are hemolyzed after death and hemoglobin diffuses into the surrounding tissues staining them red

Hypostatic congestion

Blood accumulates in the ventral parts of organs and the carcass due to gravity

Pseudomelanosis

It is the appearance of grey, green or black pigment in tissues after death. Hydrogen sulphide produced by putrefaction combines with iron to form iron sulphide – a black pigment

Imbibition of bile

This is yellow pigmentation of tissue around the gallbladder due to the diffusion of bile after death

Postmortem emphysema

Accumulation of gas in the tissues as a result of bacterial fermentation. Bloat may occur in ruminants after death

Rupture and displacement

Accumulation of gas due to postmortem fermentation may cause rupture of stomach or intestine. Intussusception (telescoping) can also occur in terminal condition. There are no circulatory changes in such cases