Cell Injury: Necrosis and Apoptosis

Question 1

Morphologic features of necrosis- cytoplasmic changes

Answer 1

Early phase: cytoplasm becomes homogenous pink in HE section

increased eosinophilia due to loss of RNA. (nb: RNA is responsible for cytoplasmic basophilia) and consolidation of cytoplasmic components upon cell collapse; degradation of cytoplasmic proteins–> ghost like appearance of cell.

Necrotic cells “individualize”- they lose adherence to basement membrane and adjacent cells- they’re found free in tubules, alveoli, follicles and other lumen surfaces

Late phase: cell rupture with loss of integrity and release of cell contents.

Question 2

Morphologic features of necrosis: nuclear changes

Answer 2

Pyknosis: nucleus is shrunken, dark, homogenous and round

Karyorrhexis: nuclear membrane is ruptured and dark fragments of the nucleus are released in the cytoplasm

Karyolysis: nuclear outline is extremely pale due to dissolution of chromatin (caused by action of DNAses)

Question 3

Types of oncotic necrosis

Answer 3

Oncotic necrosis- typical feature of cell injury is swelling

Coagulation necrosis: ischemic or toxin-induced in liver, heart and kidneys

Liquefactive necrosis: ischemic or toxin-induced necrosis in CNS

Caseous necrosis: associated with mycobacterial infections (TB)

Gangrenous necrosis: dry/moist/gas gangrene caused by bacterial toxins, other toxic agents, ischemia

Enzymatic necrosis: typically necrosis of adipose tissue caused by leakage of pancreatic enzymes (lipases) subsequent to exocrine acinar tissue injury.

Question 4

Coagulation necrosis

Answer 4

preservation of the basic outline of necrotic ceels

cytoplasm: homogenous eosinophilc appearance due to coagulation of cellular proteins

Injury or subsequent cellular acidosis denatures both structural proteins and enzymes

Nuclei: pyknotic/karyhorrhectic/karyolitic or absent

Occurs in any tissue, except brain parenchyma,

Question 5

Common causes of coagulation necrosis

Answer 5

hypoxia–>ischemia–>infarction

chemical toxins

bacterial toxins

Question 6

Gross and histo appearance of coagulation necrosis

Answer 6

example: bovine renal infarct
gross: pale area of central necrosis, hemorrhagic periphery

Histo: presence of detached necrotic cells within tubules (“individualized”); v. dense shrunken hyperbasophilic nuclei (pyknosis) and karyorrhexis; tubular necrosis with increased cytoplasmic eosinophilia; no nuclei detected in some necrotic cells.

Question 7

Coagulation necrosis can be caused by nephrotoxic substances

Answer 7

Plants: oak, acorn, oxalates (cows); easter lily (cats); red maple (horses); raisins/grapes (dogs)

Heavy metals: mercury and lead

Chemicals: ethylene glycol

Therapeutic drugs: ABX (gentamycin and cephalosporins), some chemotherapeutics

Fungal toxins

Pigments: hemoglobin and myoglobin

Question 8

Caseous necrosis

Answer 8

Necrotic cells and tissues transformed into a granular, friable material grossly resembling cottage cheese

Necrotic focus= coagulum of nuclear and cytoplasmic debris

Typical of TB and cornyebacterium pseudotuberculosis (small ruminants)

Any tissues affected; necrotic debris is mainly dead leukocytes

dystrophic calcification commonly occurs at later stages

Question 9

Comparison of coagulation necrosis with caseating necrosis

Answer 9

Caseous necrosis is an older (chronic) lesion often associated with poorly degradable lipid substances of bacterial origin.

Delayed degradation of bacterial wall components–>formation of a focal caseous necrosis–> surrounded by granulomatous inflammation and a peripheral fibrous capsule.

Question 10

Bovine TB lesions

Answer 10

LN has multifocal granulomas with central caseation necrosis

Mycobacterium bovis gets inahled–>bacilli within alveolar spaces in the lung–>phagocytosed by alveolar macrophages. Either 1) bacteria is killed and infection is stopped or 2) macrophage bactericidal activity is inhibited and macrophages get killed and the bacteria spread.

Question 11

Sheep/goat caseous lymphadenitis

Answer 11

Chronic suppurative (pus) lymphadenitis

Intracellular bacterium cornyebacterium pseudotuberculosis (c. ovis)

Bacteria enters through shearing wounds, arthropod bites

Spread by ruptured abscesses and oral and nasal secretions

Incubation period of 3 months

Ill-thrift, carcass condemnation.

inspissated (thickened/congealed) pus centrally with a pale, peripheral fibrous capsule.

Question 12

Liquefactive necrosis

Answer 12

Usual type of necrosis in CNS

Individual neurons initially show coagulation necrosis, followed by a liquefactive process affecting neuroparenchyma

Hypoxia or toxin induced neuronal necrosis–> enzymatic dissolution of the neuropil (brain parenchyma)

There’s little to no fibrous connective tissue in the CNS–>lack of support to necrotic tissue, no fibrotic reaction to replace tissue necrosis and loss.

The resulting cavity is filled with fluid and debris of neuronal membrane lipids–> debris cleared up by macrophages (gitter cells)

Question 13

Liquefactive necrosis in tissues other than CNS

Answer 13

pyogenic bacteria cause liquefactive necrosis–> recruitment of inflammatory cells (neutrophils)–>release of lytic enzymes–> destruction of bacteria + degeneration and necrosis of neutrophil–> abscess (pus-filled cavity) can be considered to be a type of liquefactive necrosis.

With dehydration–> pus inspissates–> caseous necrosis

Question 14

Example of liquefactive necrosis

Answer 14

Spinal cord compression:

histo features: “malacia” (grey and white matter- generalized softening of tissue)- ischemic neurons, necrosis, hemorhage, edema, liquefaction, pallor (rarefaction of brain parenchyma)

Fibrocartilagenous embolic myelopathy: infarction due to blockage of spinal cord vessels. Multifocal extensive areas of reddish-brown discoloration, softening and cavitation representing hemorrhage, “malacia” (necrosis) and loss of substance

Question 15

Gangrenous necrosis

Answer 15

initial lesion is coagulation necrosis which progresses with specific mechanisms and morphologic patterns

Question 16

Moist gangrene

Answer 16

area of necrotic tissue (coagulation necrosis) further degraded by saprophytic bacteria (microorganisms living on dead organic matter)–> causes putrefaction

typically observed with: ischemic necrosis of extremities (tight bandage); penetrating injury to arterial blood supply; lung necrosis caused by aspiration.

Question 17

Gross and microscopic appearance of moist gangrene

Answer 17

Necrotic tissue becomes soft, moist, reddish-brown to black.

Saprophytic bacteria produce gas–> gas bubbles and putrid smell from resulting H2S, ammonia and mercaptans

Tissue liquefaction caused by saprophytic bacteria and infiltrating neutrophils

Question 18

Dry gangrene

Answer 18

Coagulation necrosis secondary to ischemia/infarction followed by mummification.

Limited putrefaction and bacteria fail to survive

Observed on lower portions of an extremity i.e. leg, tail, ears, udder

Caused by: 1) ingested toxins (ergot and fescue)–>peripheral arteriolar vasoconstriction and damage to capillaries–> thrombosis and infarction

2) cold (frost bite): direct cell freezing and disruption by intra/extra-ceullar ice crystal formation, vascular damage leading to ischemia and infarction

Gross appearance: shriveled, dry, brown-black tissue.

Question 19

Gas gangrene

Answer 19

anaerobic bacteria proliferating and producing toxins in necrotic tissue (Clostridium perfringens and clostridium septicum)

bacteria introduced by penetrating wounds into muscles or subcutis

necrotic tissue–>anaerobic medium fro growth of the clostridia

Clostridium chauvoei (black leg): bacteria NOT introduced by penetrating wound, but from spores spread hematogenously from intestine and lodged in muscle. Once trauma/necrosis occurs and anaerobic conditions predominate, spores germinate and bacteria proliferate.

Question 20

Gross and histo appearance of gas gangrene

Answer 20

Gross: tissues are dark red to black with gas bubbles and a fluid exudate that may contain blood; lesions are characterized by coagulation necrosis of muscle with a sero-hemorrhagic exudate and gas bubble formation.

Question 21

Enzymatic necrosis

Answer 21

Saponification caused by enzymatic leakage due to pancreatitis.

Fat necrosis (type of enzymatic necrosis due to lipase): traumatic: crushed fat- pelvic fat in dystocia, sternal fat in recumbent animals

Abdominal fat necrosis of cattle: necrotic fat in mesentery, omentum and retroperitoneum

Question 22

Important disease conditions characterized by necrosis

Answer 22

Infectious bovine rhinotracheitis:

Canine parvovirus enteritis

Canine infectious hepatitis/canine adenovirus

Question 23

Infectious bovine rhinotracheitis

Answer 23

diffuse necrosis of tracheal mucosa

• transient, acute febrile illness with severe hypermia and focal necrosis of nasal, pharyngeal, laryngeal, tracheal (+/- bronchial) mucosae
• thick plaques of fibrinonecrotic exudate (diphtheritic membranes) cover the laryngeal and tracheal mucosae (due to secondary bacterial infection), necrosis and exfoliation of ciliated epithelial cells followed by repair.

Question 24

Canine parvovirus enteritis

Answer 24

multifocal/segmental necrosis

Flaccid, dilated, segmentally reddened intestine with serositis, granular necrotic appearance

Initial multiplication in lymphoid tissue–> viremia–> villous atrophy results from inability to replace enterocytes from crypts. Necrosis of crypt epithelial cells lead to crypt dilatation.

Question 25

Canine infectious hepatitis/canine adenovirus

Answer 25

Liver enlarged and friable, will often see fibrin on capsular surface, granular appearance of serosal surface; fibrin tags on liver lobs; gall bladder wall thickened by oedema; hepatocyte necrosis and loss; intranucelar inclusions in hepatocytes.

Question 26

Sequelae to oncotic necrosis

Answer 26

Inflammatory reaction within viable tissue: band of leukocytes, hyperemia

Digestion and liquefaction of necrotic tissue: phagocytosis by macrophages; diffusion by blood or lymphatics

Regeneration of normal tissue or fibrous scarring.

Question 27

Apoptosis

Answer 27

highly coordinated and active process/sequence leading to programmed cell death

Physiological: involution of tissues during embryonic development, age-related involution/atrophy of thymus

Pathological: irreversible cell injury with different underlying causes–infectious agents, ionizing radiation, chemicals, etc.

Apoptosis due to action of specific enzymes: Caspases (Cys-Asp cleaving proteins) and nucleases

Question 28

Apoptosis appearance

Answer 28

Reduced cell size, fragmentation of nucleus into nucleosome-size fragments, intact plasma membrane with altered structure, intact cellular contents (may be released in apoptotic bodies), no adjacent inflammation; often physiologic.

Question 29

Apoptosis mechanisms

Answer 29

Intrinsic (mito pathway): withdrawal of growth factors, hormones. Lack of survival signals or irradiation (DNA damage)–> activation of sensors–> activation of Bax/Bak channel–> leakage of cytochrome C, other proteins–> cytochrome C + APAF1 “Apoptosome”–> activation of caspase 9–> apoptosis

Extrinsic (death-receptor mediated): ligand activated fas or tnfr 1–> activate Bid

Possible connection btwn intrinsic and extrinsic pathway via caspase 8- mediated activation of the pro-apoptotic factor Bid.