Lecture 5: Cell injury- Cellular accumulations

What are the three main mechanisms leading to intracellular and extracellular accumulations?

1. Excessive production (or normal production) exceeding removal rate - accumulation faster than degradation/excretion (e.g., fatty change).

2. Inability to metabolize, package, or excrete substances - due to genetic or acquired defects (e.g., lysosomal storage diseases).

3. Accumulation of indigestible exogenous substances - lack of enzymatic machinery to degrade or transport material (e.g., carbon).

Differentiate between intracellular and extracellular accumulations.

Intracellular accumulations: Build-up of substances within the cell cytoplasm or nucleus.

Extracellular accumulations: Deposits occur outside the cell in the interstitium or extracellular space.

Provide an example of a metabolic abnormality that leads to intracellular accumulation.

Fatty change in hepatocytes due to excessive lipid production or impaired metabolism.

Give an example of a genetic abnormality causing accumulation and its mechanism.

Lysosomal storage disease - defective lysosomal enzymes prevent breakdown of macromolecules, causing excessive storage within the cell.

Why can some indigestible substances accumulate in cells? Give an example.

Cells may lack the enzymatic machinery to degrade the material or transport it out; example: carbon particles in pulmonary macrophages.

How can intracellular or extracellular accumulations impact cell health?

Some are harmless in small quantities, but excessive accumulation can cause cellular injury and death.

What is lipidosis (steatosis) and which organ is most notably affected?

Lipidosis is the accumulation of lipid within parenchymal cells; most notably affects the liver, due to its central role in lipid metabolism.

What are the three main causes of hepatic lipidosis?

1. Increased mobilization of free fatty acids.

2. Abnormal hepatocellular metabolism of lipids.

3. Impaired release of lipoproteins.

Describe the gross appearance of a liver with hepatic lipidosis.

Swollen with rounded edges, yellow to yellow-brown color, accentuated centrolobular pattern, greasy texture, and often floats in formalin.

Describe the histologic appearance of hepatic lipidosis.

Hepatocytes contain one or more colorless punctate cytoplasmic vacuoles that may displace the nucleus; in severe cases, can lead to degeneration and necrosis. Vacuoles appear clear because lipid is dissolved during alcohol processing, leaving empty spaces.

How can lipid vacuoles in hepatic lipidosis be confirmed?

Histochemical stains: Sudan Black or Oil Red O.

How can hepatic lipidosis be distinguished grossly from other causes of hepatomegaly?

The combination of greasy texture, yellow discoloration, and liver floating in formalin is characteristic of lipid accumulation.

Where is glycogen normally stored in the body under homeostasis?

In the liver and skeletal muscle cells.

In what situations are glycogen stores depleted?

In sick and starving animals.

List two main categories of conditions that cause glycogen accumulation.

Glycogen storage diseases (inherited metabolic disorders).

Endocrine diseases - e.g., canine hyperadrenocorticism (Cushing's disease) and diabetes mellitus.

Describe the gross appearance of a liver with glycogen accumulation.

Enlarged with rounded edges, pale to light brown, and may appear mottled.

Describe the histologic appearance of hepatocytes with glycogen accumulation.

Enlarged, swollen hepatocytes with intracytoplasmic fine lacy vacuolation.

Why can glycogen accumulation be confused with hydropic degeneration?

Both present as swollen hepatocytes with cytoplasmic clearing on H&E.

Which special stain technique can distinguish glycogen from hydropic degeneration?

PAS stain with diastase digestion:

- PAS stains carbohydrates pink.

- Diastase digests glycogen.

- If PAS-positive cytoplasm disappears after diastase treatment → confirms glycogen.

What are the three main reasons proteins can accumulate intracellularly?

- Increased resorption.

- Increased storage.

- Increased production.

How do proteins typically stain on H&E?

Pink to orange to red.

Describe crystalline protein inclusions.

Rhomboidal inclusions in renal and hepatocyte epithelial cells of dogs, more common in older dogs; significance unknown.

How can viral inclusions be classified by location? Give examples.

- Intranuclear only: herpesviruses, adenoviruses, parvoviruses.

- Cytoplasmic only: poxviruses, rabies virus.

- Both intranuclear & cytoplasmic: canine distemper virus.

How do viral inclusions appear on H&E?

Round, staining eosinophilic, basophilic, or amphophilic, depending on the viral agent.

What type of inclusion does lead poisoning cause?

Intranuclear inclusions composed of lead and protein; pale, eosinophilic, and acid-fast positive.

What is amyloidosis?

A biochemically diverse group of protein misfolding disorders where proteins convert into insoluble, non-functional aggregates rich in β-pleated sheets.

Where does amyloid most frequently deposit in tissues?

Blood vessels, basement membranes (especially in liver and kidney), and spleen.

List five mechanisms of amyloidosis.

1. Propagation of misfolded proteins serving as templates for self-replication.

2. Accumulation of misfolded precursor peptides with failure to degrade them.

3. Genetic mutations promoting protein misfolding.

4. Protein overproduction due to abnormalities or proliferation in synthesizing cells.

5. Loss of chaperone molecules or other essential components of protein assembly.

What are the main types of amyloidosis and their causes?

AL amyloid - secreted in B-cell proliferative disorders (plasma cell tumors, multiple myeloma).

AA amyloid - derived from serum amyloid A (SAA) produced during inflammation; includes hereditary forms in Shar Peis and Abyssinian cats.

β-amyloid - associated with cognitive dysfunction in dogs and Alzheimer's disease.

Describe the gross appearance of amyloid deposits.

Organs may be enlarged with a yellow, waxy appearance, nodular or diffuse deposits, and rounded edges; texture is firm and waxy.

What gross stain can be used to detect amyloid in fresh tissue?

Iodine stain - binds carbohydrate moieties in amyloid, producing a black color.

Describe the histologic appearance of amyloid on H&E.

Pale eosinophilic, hyalinized deposits, often thickening basement membranes and mesangium.

What is the classic histochemical stain for amyloid, and what is its special property under polarized light?

Congo red stain - stains amyloid red, and under polarized light shows apple-green birefringence.

Why does amyloid bind iodine?

Amyloid is not pure protein; it contains carbohydrate moieties to which iodine binds.

What is gout?

Extracellular deposition of sodium urate crystals (tophi) in tissues.

What are the two forms of gout, and how are they named?

Visceral gout - uric acid accumulates on surfaces of viscera.

Articular gout - uric acid accumulates in and around joints.

Describe the gross appearance of gout.

Chalky white deposits over joints (articular) or on visceral surfaces (visceral).

In which species is gout most common?

Birds and reptiles.

What are common causes of gout?

Dehydration, renal failure, and excess dietary protein.

Define pathologic calcification.

Deposition of calcium salts (phosphates and carbonates) in soft tissues where it would not occur in healthy states.

What is dystrophic mineralization, and what is its mechanism?

Occurs in dying or necrotic cells.

Calcium precipitates when released from sequestered sites within necrotic cells or into the extracellular space.

Give an example of dystrophic mineralization.

Calcification of necrotic myocardium following myocardial infarction.

Q: What is metastatic mineralization, and what is its mechanism?

Occurs due to hypercalcemia from a calcium/phosphorus imbalance.

Calcium deposits in specific sites: tunica intima and media of blood vessels, lungs, pleura, kidneys, and stomach.

List causes of metastatic mineralization.

Vitamin D toxicosis, hyperparathyroidism, chronic kidney disease, excessive calcium supplementation, and PTH-rp-producing neoplasms (e.g., AGASACA, lymphoma, certain carcinomas).

What is melanin and what cell type produces it?

A brown-black intracellular pigment produced by melanocytes.

What enzyme is required for melanin production, and what does it act on?

Tyrosinase - oxidizes tyrosine; requires copper.

What is albinism and its biochemical cause?

Congenital lack of pigmentation due to absence of tyrosinase activity.

List three causes of depigmentation.

1. Copper deficiency - fading coat color (leukotrichia), wiry hair, hair loss.

2. Chediak-Higashi syndrome - LYST mutation affecting lysosomal trafficking → partial albinism.

3. Various disease processes causing pigment loss.

What are two causes of hyperpigmentation?

Chronic inflammatory states and neoplasia.

What is congenital melanosis, and is it pathologic?

Pigment present at birth in certain tissues; non-pathologic. Seen in black-faced sheep, some cattle breeds, chow chows, etc.

Differentiate between lipofuscin and ceroid pigment in terms of pathologic significance.

Lipofuscin - "wear and tear" pigment, non-pathologic, age-associated.

Ceroid - pathologic pigment associated with disease states.

What is the cellular origin and mechanism of lipofuscin accumulation?

Intracellular lipoprotein pigment resulting from autophagocytosis of cell components, accumulating in lysosomes of long-lived post-mitotic cells (neurons, cardiac myocytes).

What are histologic staining characteristics of lipofuscin?

PAS-positive and positive with lipid stains (Sudan Black, Oil Red O).

List three causes of ceroid accumulation.

Cachexia, vitamin E deficiency (brown smooth muscle in gut), and lysosomal storage diseases.

What are carotenoids, and what is their biological role?

Fat-soluble plant pigments, precursors of vitamin A (β-carotene), and function as antioxidants.

In which dietary group are carotenoids more commonly seen, and why?

Herbivores, because they consume carotenoid-rich plants.

How can carotenoid pigmentation be mistaken for another condition grossly?

It can resemble icterus due to the yellow discoloration of tissues.

What is anthracosis?

Accumulation of carbon pigment in tissues.

What are common sites of carbon pigment deposition?

Skin (tattooing), lungs (inhalation of smoke or dust), and tracheobronchial lymph nodes (macrophage drainage from lung).

Describe the gross appearance of pulmonary anthracosis.

Fine black-grey stippling visible on the pleural surface.

Describe the histologic appearance of carbon pigment in the lung.

Fine black granular pigment, often with refractive crystalline material (silica), located in peribronchiolar macrophages.

How does tetracycline cause pigmentation?

Binds to calcium phosphate in developing teeth and bones, leading to permanent discoloration if given during mineralization.

What is the color change of tetracycline staining over time?

Initially yellow, turning brown after light exposure and oxidation.

What is a unique diagnostic feature of tetracycline pigmentation?

Fluoresces bright yellow under UV light.

What is the normal function and appearance of hemoglobin?

Red pigment in RBCs that carries oxygen; gives a pink tinge to well-perfused tissues.

What is methemoglobin, how is it formed, and what is its color?

Hemoglobin in which the ferrous (Fe²⁺) ion is oxidized to ferric (Fe³⁺); caused by nitrates or chlorates; produces brown-colored blood.

What causes cyanosis and what pigment change is involved?

Poorly perfused or hypoxic tissues contain deoxygenated hemoglobin, giving a blue color.

What is hemosiderin and how is it formed?

Iron-storage pigment formed from aggregates of ferritin; ferritin bound to iron precipitates into golden-yellow to light brown refractive granules.

Where is hemosiderin commonly found?

Spleen, liver, "heart failure cells" in lungs, and sites of bruising.

Why is iron stored bound to ferritin rather than free?

Free iron is toxic because it catalyzes formation of reactive oxygen species.

What is hematoiden and how does it differ from hemosiderin?

Bright yellow-brown to orange-red crystalline pigment derived from hemosiderin; contains no iron; resembles bilirubin.

What is bilirubin and how is it produced?

Yellow pigment produced from breakdown of heme in macrophages; accumulation in tissues causes icterus (jaundice).

What is porphyria, its biochemical cause, and its gross features?

Inherited metabolic defect in heme synthesis due to uroporphyrinogen III cosynthetase deficiency; causes pink to red-brown discoloration of dentin and bone.

What is the unique diagnostic feature of porphyria under UV light?

Affected tissues fluoresce red under UV light.