Cellular Pathology

(VPM 152)

Lecture 3

(Web)

Paul Hanna Jan 2018

a) Direct Interaction

• some chemicals act directly by damaging particular organelles or critical cell

molecules

• eg’s cyanide

fluoroacetate

CHEMICAL INJURY

Mechanisms of Chemical Injury

CHEMICAL INJURY

Mechanisms of Chemical Injury

b) Conversion to reactive toxic metabolites (indirect action)

• toxic metabolites are usually produced by cytochrome P-450 (MFO) in the SER

of the liver

• therefore liver is particularly susceptible to drug / toxin-induced injury

• toxic metabolites can be free radicals or electrophiles forming adducts

[N-acetyl-p-benzo-quinone imine]

Acetaminophen Toxicity

[N-acetyl-p-benzo-quinone imine]

In humans & dogs most acetaminophen is detoxified in liver to

glucuronide and sulfate conjugates, which make them more

water soluble, and then they are excreted in the urine.

- only small amounts converted to highly reactive metabolite

(NAPQI) by P450 MFO’s.

Cats are relatively deficient in glucuronyl transferase (which

catalyzes the glucuronide conjugate) and a larger percentage

of acetaminophen is metabolized to NAPQI.

- cell stores of glutathione are then rapidly depleted in

“neutralizing” NAPQI.

- when NAPQI builds up, it is highly electrophilic, binding to

molecules in hepatocytes (adduct formation) causing

molecular damage & resultant necrosis; also with the

depletion of reduced glutathione (GSH) there is oxidative

stress.

Acetaminophen Toxicity

Additionally cats and dogs appear to produce relatively more

(compared to humans) of the metabolite para-aminophenol,

which is released from the liver cells and results in oxidative

damage to hemoglobin in rbc’s , ie oxidizes the iron in oxyHb

from the reduced Fe2+ state to the oxided Fe3+ state forming

methemoglobin (note, metHb cannot bind oxygen).

Normal blood (left) compared to blood with methemoglobinemia (right), which is bluish to

chocolate-brown in color. The NADH-dependent enzyme methemoglobin reductase (=

cytochrome b5 reductase)) converts metHb back to Hb.

(= methemoglobin reductase)

Carbon Tetrachloride (CCl4) Toxicity

Acute carbon tetrachloride toxicity

This rat liver has been damaged (showing fatty

change) due to carbon tetrachloride exposure

Fig 1-23 (Robbins 8th ed) Sequence of events leading to fatty change and cell necrosis in carbon tetrachloride

(CCl4) toxicity in liver cells. Early / low dose changes on the left and later / high dose changes on the right.

• cell death - irreversible injury; eg within 20 to 60 min of coronary artery occlusion

- biochemical changes / release of enzymes & EM changes within 2 hrs

- LM changes in 4 to 12 hrs

- gross changes in 12 to 24 hrs

MORPHOLOGY OF CELL INJURY

• grossly visible changes indicative of injury usually require hrs to develop

- eg necrosis not grossly evident until many hrs after coronary artery thrombosis

- however, affected heart muscle stops contracting within 60 sec

• cell swelling - reversible injury that can start within minutes of the initial insult

Fig 2-7 (Robbins) 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

Reversible vs Irreversible Ischemic Injury

• irreversible injury occurs if injurious stimulus persists (or is severe from start)

• critical transition point not known

• 2 features characterize irreversibility (the “point of no return”)

- inability to reverse mitochondrial dysfunction

- profound disturbances of membrane function

MORPHOLOGY OF CELL INJURY

1) Cellular Swelling

I. REVERSIBLE CELL INJURY (CELL DEGENERATION)

2) Fatty Change

1) Necrosis

II. IRREVERSIBLE CELL INJURY

2) Apoptosis

b) Gross Appearance of Cellular Swelling

• organ swollen with rounded edges

• cut surface: tissue bulges and wet / heavy

I. REVERSIBLE CELL INJURY (CELL DEGENERATION)

a) Etiology / Pathogenesis

• early, universal manifestation of cell injury

• loss of ion and fluid homeostasis → net intracellular H2O

1) Cellular Swelling

Normal equine kidney – note color of cortex & medulla Equine kidney, nephrosis; note slightly pale, wet-looking

cortical and medullary tissue.

Figure 01-11A (McGavin & Zachary). Acute cell swelling, liver, mouse. A, Hepatic swelling in a mouse exposed to

chloroform 24 hours previously. The accentuated lobular pattern and slight pallor in the liver on the right are the result of

acute cell swelling (hydropic degeneration) and necrosis of centrilobular hepatocytes. The liver on the left is normal.

c) Histologic Appearance of Cellular Swelling

• cells enlarged, pale cytoplasm, nucleus in normal position

1) Cellular Swelling

• altered cytoplasmic staining:

- if mild (slightly washed out appearance) → “cloudy swelling”

- if continues many variable sized vacuoles appear in the cytoplasm

moderate → “hydropic (vacuolar) degeneration”

severe → “ballooning degeneration”

Cloudy Swelling

(Guinea pig, liver, CCl4 toxicity)

200X

400X

Note, the cells in the center of the field are slighly

swollen with pale, “washed out” appearance of

the cytoplasm (ie cloudy swelling)

Hydropic Degeneration

(Dog, liver, endotoxemia)

200X

400X

Note with hydropic degeneration the cell swelling

is due to cytoplasmic vacuolation which is poorly

delineated (ie “moth-eaten” appearance) and the

nucleus is not displaced.

Ballooning degeneration, oral epithelium, Goat. Oral epithelium of a goat showing hydropic and ballooning degeneration due to

infection with Orf virus (a parapoxvirus infection also known as “contagious viral pustular dermatitis” or “contagious ecthema”)

e) Cell Swelling is reversible

• eg affected heart muscle stops contracting in 60 sec of coronary occlusion, however myocardial

cell not irreversibly injured for 20 - 60 min (ie changes are reversible if O2 is restored)

d) Ultrastructural Appearance of Cellular Swelling / Reversible Injury

Membrane

blebs

Blunting / loss of microvilli

Normal renal tubular epithelial cell

N

L = tubular lumen

mv = microvilli

N = nucleus

note, mitochondria often elongated

Fig 2-10 (Robbins) Ultrastructural features

of reversible & irreversible cell injury

(necrosis) in a rabbit kidney.

A, Electron micrograph of a normal

epithelial cell of the proximal kidney

tubule. Note abundant microvilli (mv) lining

the luminal surface (L)

Renal tubular epithelium – reversible ischemic injury

Loss of microvilli

Surface blebbing

Slight swelling of mitochondria

Clumping of nuclear chromatin

Fig 2-10 (Robbins) B, Epithelial cell of the proximal

tubule showing early cell injury resulting from

reperfusion following ischemia. The microvilli are lost

and have been incorporated in apical cytoplasm; blebs

have formed and are extruded in the lumen.

a) Etiology / Pathogenesis

• occurs in various forms of injury (hypoxic, toxic, metabolic)

• abnormal accumulation of lipids within the cell

• mainly in cells highly involved in fat metabolism;

esp. LIVER (also renal tubular epithelium & myocardium)

• seen in abnormalities of fat uptake, utilization &/or mobilization

• may be preceded or accompanied by cell swelling

2) Fatty Change

Increased dietary fat

Starvation (-ve energy balance)

- mobilized fat

- protein synthesis

Hypoxia

- oxidation of FA

- protein synthesis

Toxins

- protein synthesis

UPTAKE

EXPORT

METABOLISM

2) Fatty Change

Figure 1-30 (Robbins) Fatty liver. A, Schematic diagram of the possible mechanisms leading to accumulation of triglycerides in

fatty liver. Defects in any of the steps of uptake, catabolism, or secretion can result in lipid accumulation.

Normal cat liver Severe diffuse hepatic lipidosis

Severe diffuse hepatic lipidosis when severe, fatty liver will float!

Normal liver (above) and fatty liver (right) with

hepatocytes swollen due to well-delineated

cytoplasmic vacuoles (lipid filled). Note, lipid

vacuoles are well-delinated while the cyoplasmic

vacuolation with water that occurs in hydropic

degeneration is poorly-delineated.

Hepatic liposis, higher magnification. The well-delineated lipid-filled cytoplasmic vacuoles causing swelling of the

hepatocytes, usually pushing nucleus to the periphery of the cell. Note, how these well-delineated vacuoles can

be single and large (macrovesicular) or multiple and small (microvesicular).

Note patchy distribution of pallor (fatty

change) of ventricular myocardium Normal heart – note color of myocardium

Clear well-delineated vacuoles in myocardial fibers. Vacuoles stain positive for lipid with oil Red O