Cellular Responses: Accumulations, Pigments, Aging, and Injury

Lecturer Introduction and Objectives

  • Instructor Note on Video Format: The lecturer prefers not to display their live picture in the corner of recordings for three reasons:

    • It is personally distracting to see one's own image.

    • It causes stress regarding the appearance of aging.

    • The lecturer utilizes the entire screen to display detailed images and descriptions for educational purposes.

  • Lecture Scope: This session covers cellular responses, focusing on intracellular accumulations, pigments, calcification, cellular aging, and mechanisms of cell injury.

Intracellular Accumulations: Glycogen

  • Etiology: Increased intracellular glycogen results from abnormalities in glucose or glycogen metabolism.

  • Rare Inborn Errors of Metabolism: These conditions affect various organs depending on the specific enzymatic defect.

    • Pompe's Disease: Glycogen accumulation is prominently seen in the heart.

    • Von Gierke Disease (Glycogen Storage Disease Type 1): Glycogen builds up significantly in the liver.

  • Morphological Appearance: Under standard microscopy, cells with glycogen accumulation demonstrate a clear cytoplasm.

  • Histological Differentiation (Glycogen vs. Fat):

    • PAS Stain (Periodic Acid-Schiff): This special stain is used to identify glycogen. If positive, cells light up a dark pink color.

    • Diastase Digestion: To confirm that PAS-positive material is glycogen (and not another substance or organism), the enzyme diastase is used. Diastase hydrolyzes (digests) glycogen.

    • Validation Process: If a tissue section is PAS-positive (dark pink) and the pink color disappears after treatment with diastase, the presence of glycogen is confirmed.

    • Anatomical Landmarks in Liver: Identification of liver tissue is often confirmed by the presence of a bile duct.

Exogenous Pigments

  • Carbon (Anthracosis):

    • Most Common Exogenous Pigment: Carbon is a ubiquitous urban air pollutant (coal dust).

    • Pathophysiology: Small particles (approximately 2.5 μm2.5\,\mu\text{m}) reach the alveolar spaces. Lung macrophages engulf the dust but cannot digest it.

    • Clinical Presentation: Grossly, the lung shows black discoloration. Microscopically, black pigment is seen within macrophages. The body attempts to wall off this foreign material through pink fibrosis.

    • Lymphatic Involvement: Macrophages may transport the carbon to the hilar lymph nodes.

  • Lead (Plumbism):

    • Exposure Routes: Respiratory tract, gastrointestinal (GI) tract, and skin.

    • Sources: Battery plants, painting, mining, construction, gun ranges, old lead pipes (drinking water), moonshine, and certain herbal supplements.

    • Mechanism of Injury: Lead interferes with the normal breakdown of ribosomes.

    • Hematologic Sign: Basophilic stippling in red blood cells (RBCs). This represents the clumping of ribosomes that have not been degraded. While not pathognomonic, it is a significant indicator of lead poisoning.

    • Clinical Manifestations:

      • Neurological: Headaches, memory loss, demyelination leading to peripheral neuropathy.

      • Abdominal: Pain.

      • Skeletal: Radiodense deposits in the epiphyses of children.

      • Oral: A purple "lead line" on the gums.

      • Renal/Heme: Hemolytic anemias and renal tubular necrosis.

  • Tattoo Inks:

    • The second most common exogenous pigment.

    • Pigment ink is stored within dermal macrophages.

Endogenous Pigments

  • Lipofuscin:

    • Known as the "wear and tear" pigment.

    • Origin: Derived from lipid peroxidation (a sign of free radical injury).

    • Context: Part of normal aging, but also seen in severe malnutrition and cancer cachexia.

    • Appearance: A golden-brown pigment found in the heart, liver, and kidneys.

  • Melanin:

    • A brown-black pigment produced by melanocytes.

    • Locations: Skin, eyes, central nervous system (CNS), and mucosal surfaces.

    • Clinical Significance: Because melanocytes are present in mucosal surfaces (including the GI tract), malignant melanoma can be diagnosed via GI biopsy.

    • Immunohistochemical Stains: Used to differentiate melanin from other brown pigments (like iron). Common markers include Melan-A and MART-1. A labeled antibody binds to proteins in melanin, turning the target cells brown upon washing.

  • Iron (Hemosiderin):

    • Iron Management: Unbound iron is highly reactive and forms free radicals. It is stored complexed to macromolecules.

    • Storage Forms:

      • Ferritin: Can store 1,0001,000 or more iron molecules.

      • Hemosiderin: A yellow-brown, granular pigment derived from hemoglobin (broken down red cells).

    • Local and Systemic Excess: Seen in hemorrhage, repeated blood transfusions, and Hemochromatosis (a disease of increased iron absorption and lack of excretion).

    • Heart Failure Cells: Macrophages in lung alveolar spaces containing hemosiderin. They are associated with pulmonary congestion and left-sided heart failure (capillaries leak blood into alveoli, which macrophages consume).

    • Histological Differentiation: On H&E stain, iron appears brown. Prussian Blue stain is used for confirmation; it turns iron dark blue.

  • Copper:

    • Wilson's Disease (Hepatolenticular Degeneration): A rare autosomal recessive disorder characterized by abnormal copper accumulation in the brain, liver, and cornea.

    • Genetics: Defect on Chromosome 13 affecting the copper-transporting ATP gene in the liver.

    • Clinical Sign: Kayser-Fleischer rings (golden rings around the cornea).

  • Bilirubin:

    • A byproduct of red cell breakdown processed by the liver.

    • Jaundice: Buildup of bilirubin leads to yellow discoloration of the skin and sclera (icteric skin and sclera).

    • Kernicterus: Bilirubin encephalopathy caused by unconjugated bilirubin buildup in the brain.

Pathologic Calcification

  • Definition: Abnormal tissue deposition of calcium salts. They appear dark purple under the microscope.

  • Dystrophic Calcification:

    • Deposited in damaged tissue or areas of necrosis (e.g., atherosclerosis, aging heart valves).

    • Serum calcium levels and calcium metabolism are normal.

    • Psammoma Bodies: Concentric, laminated calcifications often found in Meningioma, Thyroid cancer, and Ovarian cancer. They form around a necrotic cell acting as a "seed."

  • Metastatic Calcification:

    • Deposition in normal tissue due to hypercalcemia (increased serum calcium).

    • Common in alkaline environments: Gastric mucosa, lungs, and kidneys.

    • Causes of Hypercalcemia:

      • Hyperparathyroidism (Primary via parathyroid tumors or Ectopic PTH-related protein).

      • Bone Resorption: Primary bone tumors (leukemia, myeloma), diffuse skeletal metastasis (breast cancer), Paget's disease, or long-term immobilization.

      • Vitamin D Disorders: Sarcoidosis and vitamin D intoxication.

      • Renal Failure: Retention of phosphate leads to secondary hyperparathyroidism.

Cellular Aging

  • Definition: Progressive decline in cellular function caused by genetic abnormalities and accumulated molecular damage (specifically from ROS).

  • Mechanisms of Aging:

    • DNA Damage: Cumulative damage from carcinogens or replication errors.

    • Cellular Senescence: Normal cells have a fixed number of divisions (the Hayflick limit).

    • Telomere Shortening: Somatic cells lose telomere length with each division. Eventually, they exit the cell cycle (replicative senescence).

    • Telomerase: An enzyme that replenishes telomeres. It is present in stem cells and germ cells. Re-activation in somatic cells can lead to cancer (immortalized cells).

    • Defective Protein Homeostasis: Impaired folding and degradation of proteins lead to accumulation and apoptosis.

  • Progeroid Syndromes:

    • Werner Syndrome: Autosomal recessive; defect in DNA helicase protein (needed for repair and telomere maintenance). Results in premature aging.

    • Hutchinson-Gilford Progeria Syndrome: Autosomal dominant; mutation in Lamin A (inner nuclear membrane protein). Causes nuclear instability and premature aging.

  • Nutrient Sensing and Longevity:

    • Caloric restriction is shown to increase longevity through two pathways:

      • IGF-1 Pathway: Reduction in insulin-like growth factor 1 signaling lessens metabolism and cell growth (reducing wear and tear).

      • Sirtuins: Upregulated proteins that promote DNA repair, protein folding, and counteract ROS while inhibiting apoptosis.

Cell Injury Mechanisms

  • Transition from Reversible to Irreversible:

    • Biochemical changes occur within minutes.

    • Ultrastructural changes occur within hours.

    • Light microscopic changes occur within hours to days.

  • Reversible Cell Injury:

    • Cell Swelling: The earliest manifestation.

    • Mechanism: Hypoxia leads to decreased ATP production. The ATP-dependent sodium-potassium pump fails. Sodium and water influx into the cell, causing swelling of the cell and its organelles.

    • Other signs: Ribosome detachment (dropping protein synthesis), surface blebbing, nuclear clumping.

  • Irreversible Cell Injury (Cell Death):

    • Marked by fragmentation of plasma membranes and organelles.

    • Rupture of lysosomes (autolysis).

    • Large mitochondrial densities and calcifications.

    • Profound mitochondrial dysfunction.

  • Causes of Cell Injury: Hypoxia (deficiency of oxygen due to ischemia, anemia, or CO poisoning), physical agents, chemical agents, infectious organisms, and genetic/nutritional imbalances.

Free Radicals and Oxidative Stress

  • Definition: Chemical species with a single unpaired electron in the outer orbit. Termed Oxidative Stress when in excess.

  • Species:

    • Superoxide (O2−O_2^-)

    • Hydrogen Peroxide (H2O2H_2O_2)

    • Hydroxyl Radical (⋅OH\cdot OH): The most reactive oxygen-derived free radical; responsible for damaging lipids, proteins, and DNA.

    • Peroxynitrite.

  • Formation Factors: Radiant energy (ionizing radiation), metabolism (1-3% of oxygen reduction intermediates leak), transitions metals (unsequestered iron/copper), and exogenous chemicals (e.g., CCl4CCl_4 converted to CCl3⋅CCl_3 \cdot in the liver).

  • Removal Systems:

    • Antioxidants: Vitamins A, E, C, and glutathione.

    • Iron/Copper Sequestration: Binding to transferrin, ferritin, or lactoferrin.

    • Enzymes: Catalase, Superoxide Dismutase (SOD), and Glutathione Peroxidase.

Questions & Discussion

  • Case Study: A 78-year-old man with advanced esophageal cancer dies. Autopsy shows heart size within normal limits and no significant atherosclerosis. Microscopy of myocardial cells shows prominent yellow intracytoplasmic granules.

  • Question: Which of the following most likely accounts for the observed microscopic changes?

    • A. Exogenous pigment endocytosis

    • B. Glucose polymerization

    • C. Iron overload

    • D. Lipid peroxidation

    • E. Protein accumulation

  • Explanation: The pigment is Lipofuscin. Lipofuscin is the result of lipid peroxidation (wear and tear). Iron overload (Hemosiderin) would require a Prussian Blue stain to distinguish from lipofuscin definitively, but the context of aging and cachexia (from cancer) strongly points to lipofuscin.