Intracellular Accumulations

Transcript

1. Intracellular Accumulations Lipids, proteins, glycogen, pigments and lysosomal storage disorders
2. The defining principle Accumulations reflect imbalance between production, handling and clearance. Material may be an endogenous normal constituent, an abnormal product or an exogenous substance. It may be synthesized within the affected cell or delivered from elsewhere. Deposits may be temporary or persistent, innocuous or toxic. Localization may be cytoplasmic, lysosomal or, less often, nuclear.
3. How to reason from vacuoles and inclusions Appearance alone rarely identifies the accumulated substance. Ask whether the material is intracellular or extracellular. Assess size, color, staining characteristics and organ distribution. Correlate with systemic metabolic state, medication use and genetic history. Confirm the biochemical identity with a fit-forpurpose assay where necessary.
4. Four mechanistic patterns 01 02 03 04 Excess normal material Abnormal protein folding Missing degradative enzyme Indigestible exogenous matter Lipid synthesis exceeds export Retention in ER Lysosomal substrates persist Particles remain in phagocytes Different mechanisms can produce similar cytoplasmic vacuolation.
5. A useful pathologic classification Neutral lipids Cholesterol Proteins Triglyceride steatosis in liver and other organs. Macrophage foam cells in atheroma and xanthomas. Reabsorption droplets, immunoglobulins and damaged filaments. Glycogen Endogenous pigments Exogenous pigments Diabetes-related changes and inherited glycogenoses. Lipofuscin, melanin, hemosiderin and bilirubin. Inhaled carbon and tattoo pigment.
6. Steatosis: definition and sites Abnormal triglyceride accumulation within parenchymal cells. The liver is especially susceptible because it governs fatty acid metabolism and lipoprotein export. Steatosis may also involve heart, skeletal muscle and kidneys. Fatty change may accompany metabolic dysfunction, alcohol exposure or toxins. Severity and consequence depend on the cause and duration.
7. Lipid droplets are normal organelles Disease emerges when lipid storage exceeds controlled turnover. Triglycerides are stored in cytosolic droplets enclosed by a phospholipid monolayer. Perilipin-associated proteins regulate access by lipid-processing enzymes. Altered uptake, synthesis, oxidation and export can expand droplet volume. The pathological diagnosis depends on organ and clinical context.
8. Triglyceride balance in hepatocytes 01 02 03 04 05 Fatty acid uptake Mitochondria l oxidation Triglyceride synthesis VLDL assembly Steatosis Adipose / diet / lipogenesis Fatty acid disposal Esterification Apoproteindependent export Synthesis or delivery exceeds disposal Defects in oxidation or export and increased lipid delivery can coexist.
9. Causes of hepatic steatosis Several mechanisms converge on triglyceride retention. Metabolic dysfunction associated with obesity and insulin resistance. Excessive alcohol exposure. Drugs, toxins and altered fatty-acid oxidation. Hypoxia and nutritional protein deficiency in selected settings.
10. Metabolic steatotic liver disease Insulin resistance shifts hepatic fatty-acid flux. Adipose lipolysis raises fatty-acid delivery to liver. Hepatic de novo lipogenesis may increase. Mitochondrial oxidation and VLDL export may not match total input. Steatosis may coexist with inflammation, ballooning and fibrosis.
11. Alcohol-associated steatosis Ethanol metabolism perturbs hepatic redox balance. Alcohol metabolism raises the hepatic NADH/NAD⁺ ratio. O₂•− H₂O₂ This favors lipogenesis and reduces oxidation of fatty acids. Oxidative stress and acetaldehyde-related damage may accompany lipid retention. Steatosis can be reversible with removal of the injurious exposure. •OH
12. Gross morphology of steatotic liver Characteristic gross features The liver may become enlarged, yellow, soft and greasy. Gross change may be subtle in mild disease. A yellow liver is not diagnostic without correlation with histology and clinical context. Gross liver specimen from the supplied accumulations lecture.
13. Hepatic steatosis on H&E Microscopic recognition Large clear fat vacuoles often displace hepatocyte nuclei. Smaller droplets may be numerous in early microvesicular change. H&E removes most neutral lipid during routine processing. Representative photomicrographs from the supplied accumulations lecture.
14. Macrovesicular versus microvesicular steatosis Pattern Appearance Typical interpretation Macrovesicular Single / multiple large vacuoles Common with metabolic or alcohol-related disease Microvesicular Fine dispersed vacuoles Consider mitochondrial fattyacid oxidation impairment Microscopic pattern is suggestive, not by itself etiologic.
15. Staining fat versus glycogen Routine H&E vacuolation has a broad differential. Oil Red O and Sudan stains detect neutral fat on frozen sections. PAS detects glycogen and some glycoproteins; diastase digestion supports glycogen. Paraffin processing dissolves many neutral lipids. Use histochemistry only with appropriate clinical and morphologic context.
16. Cholesterol and cholesteryl ester Foam cells are macrophages or smooth muscle cells filled with lipid. Atherosclerotic intima contains lipid-laden macrophages and sometimes smooth muscle cells. Xanthomas are localized collections of foam cells, often associated with dyslipidemia. Cholesterol clefts appear as needle-shaped spaces in processed tissue. Some chronic inflammatory or necrotic foci also develop cholesterol deposits.
17. Cholesterol accumulation in an atheroma Histologic phenotype Foam-cell regions appear pale and vacuolated. Fibrous tissue and chronic inflammation may coexist. Cholesterol clefts are artifacts of crystal dissolution during tissue processing. Atherosclerotic lesion microscopy from the supplied lecture.
18. Foam cells in soft tissue Diagnostic framing Foamy histiocytes may accumulate in the dermis or tendon. Clinical morphology and distribution help distinguish xanthoma variants. Hyperlipidemia should be assessed where clinically appropriate. Cutaneous foamy macrophage histology from the supplied lecture.
19. Cholesterolosis of the gallbladder Lipid accumulation can form a characteristic mucosal pattern. Cholesterol-rich macrophages expand the gallbladder lamina propria. Grossly this may create a stippled yellow “strawberry” appearance. The process is not synonymous with cholesterol gallstones. Foam cells are best recognized histologically.
20. Gallbladder cholesterolosis: histology Where are the foam cells? Pale foamy macrophages expand lamina propria beneath the epithelium. The epithelial surface can remain relatively intact. Recognize intracellular lipid accumulation rather than epithelial vacuolation. Gallbladder photomicrograph from the supplied accumulations lecture.
21. Protein accumulation The phenotype depends on which protein accumulates. Proximal renal tubular epithelial cells may contain protein reabsorption droplets in heavy proteinuria. Immunoglobulins can accumulate in plasma cells as Russell bodies. Misfolded α1-antitrypsin may form hepatocellular cytoplasmic globules. Damaged intermediate filaments may assemble into inclusion bodies.
22. α1-antitrypsin deficiency A folding defect traps protein inside hepatocytes. Certain SERPINA1 alleles encode a protein prone to polymerization and ER retention. Cytoplasmic PAS-positive, diastase-resistant globules support the diagnosis. The retained protein may cause liver injury, while low circulating inhibitor favors pulmonary emphysema. Not all genotypes confer the same liver and lung risk.
23. Protein inclusion examples Reabsorption droplets Russell bodies Endocytosed proteins in proximal tubular epithelium. Immunoglobulin retained in dilated rough ER of plasma cells. α1-antitrypsin globules Mallory–Denk bodies Misfolded protein retained in hepatocyte ER; PAS-positive, diastase-resistant. Aggregates of keratin intermediate filaments and associated proteins.
24. Hyaline change is descriptive “Hyaline” does not define one chemical substance. On H&E, hyaline material appears homogeneous, glassy and eosinophilic. Intracellular examples: protein droplets, Russell bodies and Mallory–Denk bodies. Extracellular hyaline may reflect plasma proteins, scar collagen or other material. Use an etiologic diagnosis rather than “hyaline degeneration” alone.
25. Mallory–Denk bodies Cytoskeletal inclusion Irregular eosinophilic ropy cytoplasmic inclusions may occur in ballooned hepatocytes. They contain aggregated keratins and associated proteins. They are common in steatohepatitis but not specific for alcohol use. Liver histology image from the supplied accumulations lecture.
26. Accumulation of glycogen Excess glycogen may be acquired or inherited. Poorly controlled diabetes can cause glycogenrich tubular epithelium or hepatocytes. Glycogen storage diseases arise from defects in synthesis or degradation pathways. Glycogen often creates clear cytoplasm in routine sections. PAS positivity abolished by diastase digestion supports glycogen.
27. Lysosomal storage diseases: mechanism A genetic degradative defect leads to substrate retention. The defect may be in an acid hydrolase, activator, transporter or processing protein. Undegraded macromolecules accumulate within membrane-bound lysosomes. Affected tissues depend on substrate distribution, production and turnover. Progressive dysfunction reflects both storage and secondary cellular injury.
28. Storage disease: compartment and substrate Category Stored substrate Representative disorder Sphingolipidosis Sphingolipids Gaucher / Niemann–Pick Gangliosidosis Gangliosides Tay–Sachs Glycogenosis Glycogen Pompe Mucopolysaccharidosis Glycosaminoglycans Hurler / Hunter These are broad mechanistic classes, with marked phenotypic variation.
29. Lysosomal storage: pathogenesis 01 02 03 04 05 Gene / protein defect Substrate persists Lysosomes enlarge Cell dysfunction Organ injury Enzyme or accessory pathway impaired Incomplete lysosomal breakdown Material accumulates Trafficking, inflammation, autophagy affected Often chronic and multisystem Phenotypes range widely; not every disorder is uniformly fatal in childhood.
30. Selected storage disease examples Gaucher disease Tay–Sachs disease Glucocerebrosidase deficiency; lipidladen macrophages. Hexosaminidase A deficiency; GM2 ganglioside accumulation. Niemann–Pick A/B Pompe disease Acid sphingomyelinase deficiency; sphingomyelin storage. Acid α-glucosidase deficiency; lysosomal glycogen accumulation.
31. Neural tissue is especially vulnerable Long-lived cells accumulate poorly cleared substrates. Neurons may develop progressive storagerelated dysfunction. Disturbed membrane trafficking and autophagy can contribute to neural injury. Lysosomal storage disorders may involve liver, spleen, marrow, heart or skeletal muscle. Age of onset and severity vary by residual enzyme activity and genotype.
32. Lipofuscin: pigment of cellular wear A lipid-containing residual product of lysosomal digestion. Golden-brown finely granular pigment accumulates in long-lived cells. Common in aging myocardium and hepatocytes and in tissue atrophy. Represents incompletely degraded lipid and protein material. Usually harmless and does not by itself demonstrate active cell death.
33. Hemosiderin and iron overload Hemosiderin contains aggregated iron storage complexes. Localized deposition may follow hemorrhage and red blood cell breakdown. O₂•− H₂O₂ Systemic iron overload may follow excessive absorption or repeated transfusion. Iron can catalyze oxidative damage in overloaded tissues. Prussian blue highlights ferric iron in tissue sections. •OH
34. Local hemosiderin versus systemic siderosis Distribution identifies a possible source of iron. Old hemorrhage produces localized hemosiderin-laden macrophages. O₂•− H₂O₂ Chronic pulmonary congestion may yield hemosiderin-laden alveolar macrophages. Systemic iron loading can injure liver, endocrine organs and myocardium. Iron staining confirms pigment; systemic disease needs laboratory correlation. •OH
35. Melanin, bilirubin and exogenous pigments Pigment identification relies on site and context. Melanin is produced in melanocytes and transferred to adjacent epithelial cells. Bilirubin pigment may be evident in cholestatic hepatocytes or bile canaliculi. Inhaled carbon remains within pulmonary macrophages and lymph nodes. Tattoo pigments can persist in dermal macrophages.
36. Carbon and tattoo pigment Persistent exogenous particles are usually handled by macrophages. Inhaled carbon accumulates in lung macrophages and draining nodes. Anthracotic pigment is commonly black and usually reflects environmental exposure. Tattoo pigment remains in dermal macrophages and fibroblastic tissue. Distribution and exposure history prevent confusion with endogenous pigment.
37. Differentiate common brown pigments Pigment Typical clue Special study Lipofuscin Fine perinuclear granules; aging Usually H&E context Hemosiderin Coarse golden granules; hemorrhage / iron Prussian blue positive Melanin Skin melanocytes / pigment transfer Fontana–Masson may support Bile pigment Cholestatic canalicular plugs Correlate with cholestasis Pigment chemistry should not be inferred solely from brown color.
38. Applied reasoning: vacuolated hepatocyte A liver biopsy shows clear hepatocytic vacuoles on routine H&E. The clinician asks whether these represent triglyceride. What is established morphologically, and which stain directly demonstrates neutral fat?
39. Applied reasoning: hepatocyte globules Hepatocytes contain rounded PAS-positive inclusions that resist diastase digestion in a patient with early cirrhosis. Which inherited protein-handling disorder should be considered?
40. A practical diagnostic approach Classify the material Localize Lipid, protein, glycogen, pigment or exogenous particle. Cytosol, ER, lysosome or nucleus. Choose appropriate technique Assess consequence H&E, special stains, immunohistochemistry, enzymology or genetics. Harmless finding, metabolic dysfunction or progressive storage injury.
41. When accumulated material becomes toxic Deposition and injury should be assessed separately. Amount of storage does not always correlate perfectly with functional loss. ROS production, impaired lysosomal flux and secondary inflammation may drive damage. Tissue vulnerability explains the different phenotypes of systemic storage disorders. Morphologic deposition requires clinicopathologic interpretation.
42. Suggested reading Robbins, Cotran & Kumar Pathologic Basis of Disease · Chapter: Cell Injury, Cell Death, and Adaptations For further study: correlate the molecular mechanisms with gross and microscopic pathology. Illustrative pathways are original schematics; selected histology images are retained from the supplied teaching material.