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AN66.1-2 | Connective tissue histology — Gate Quiz
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On a H&E-stained slide, you see a tissue with closely packed parallel wavy eosinophilic fibres with flattened nuclei arranged in rows between the fibre bundles. The predominant cell type is fibroblast (fibrocyte). This tissue is most likely:
Correct. Dense regular connective tissue (tendon) has parallel collagen I bundles with fibroblasts (fibrocytes) in rows between them. The fibres are wavy in the relaxed state. This parallel arrangement allows the tendon to withstand unidirectional tensile stress during muscle contraction.
Dense CT types: Regular (tendon, ligament, aponeurosis) — parallel fibres + fibrocytes in rows; withstands tension in ONE direction. Irregular (dermis, organ capsules) — random criss-cross fibres; withstands multidirectional stress. Tendon injury (Achilles tendon rupture — "felt a pop in the heel" + inability to plantarflex on Thompson test) is common in athletes and elderly in India. Collagen I is the major fibre type in both.
Dense regular connective tissue is the answer. Key features: PARALLEL collagen fibres (uniform direction), fibrocytes in rows between fibres, very few cells and ground substance. Tendons are the classic example. Compare: Dense IRREGULAR (dermis) = thick collagen bundles in random criss-cross pattern; Loose CT (subcutaneous) = loosely arranged fibres with many cell types; Reticular CT = fine silver-staining fibrils, found in lymph nodes.
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During an immunology lecture, the professor explains that certain connective tissue cells release histamine and heparin from large granules when their surface IgE receptors are cross-linked by a specific antigen. The connective tissue cell being described is:
Correct. Mast cells are tissue-resident connective tissue cells with large basophilic/metachromatic granules (purple with toluidine blue, basic dyes). Surface FcεRI receptors bind IgE. Antigen cross-links IgE → degranulation → release of histamine (vasodilation, bronchospasm), heparin (anticoagulant), tryptase, and eosinophil chemotactic factor → type I hypersensitivity reaction.
Mast cell subtypes: (1) Connective tissue mast cell (CTMC) — in skin, peritoneum; contains heparin + tryptase + chymase. (2) Mucosal mast cell (MMC) — in gut and airway mucosa; contains chondroitin sulphate. Clinical: Anaphylaxis (systemic mast cell degranulation → shock, bronchospasm, urticaria) — treated with adrenaline. Urticaria (skin whealing). Systemic mastocytosis (excess mast cell proliferation → bone lesions, urticaria pigmentosa, organomegaly). Mast cells vs basophils (blood cells): similar granules, different origin and location.
Mast cells are the answer. IgE-mediated degranulation is the hallmark of mast cell function. Mast cells contain large metachromatic granules (stain purple/red with toluidine blue). Plasma cells produce antibodies (IgG, IgM, IgA, IgE) — they don't degranulate with antigens. Eosinophils are blood cells that migrate into tissue; their granules contain MBP (toxic to parasites). Macrophages phagocytose rather than degranulate histamine.
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To demonstrate reticular fibres in a lymph node section, the pathologist uses a special stain. The reticular fibres of lymph nodes and spleen are composed of type III collagen and are correctly demonstrated using:
Correct. Reticular fibres (type III collagen) are argyrophilic — they reduce silver salts → silver impregnation stain (Gomori's silver stain, Gordon-Sweet silver) stains them black against a pale background. H&E does not highlight them clearly. Elastic fibres use Orcein or Weigert stain. PAS stains carbohydrates (glycogen, mucins).
Special histological stains table (common exam question): H&E → routine. PAS (Periodic acid-Schiff) → glycogen, mucins, fungal cell walls, basement membranes (magenta). Masson's trichrome → collagen (green/blue), muscle (red). Silver stain (Gomori) → reticular fibres (black), also used for melanin, neurons (Bielschowsky). Orcein/Weigert → elastic fibres (brown/dark). Toluidine blue/Giemsa → mast cells (metachromatic — purple). Luxol fast blue → myelin. Congo red → amyloid (apple-green birefringence under polarised light).
Silver stain is correct for reticular fibres. Reticular fibres = type III collagen = fine fibrils forming mesh/scaffold in lymph nodes, spleen, bone marrow, liver sinusoids (space of Disse). Argyrophilic = reduces silver → black colour on silver stain. Compare: Elastic fibres → Orcein (brown/black), Weigert (purple). Collagen I (regular) → green on Masson's trichrome. Glycogen/mucin → magenta on PAS. Mast cells → purple/red on toluidine blue or Giemsa.
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An electron micrograph of an active fibroblast synthesising collagen shows a cell with the cytoplasm distended by prominent stacks of membrane-bound ribosomes arranged in parallel cisternae. This organelle, which is the site of procollagen synthesis, is:
Correct. The rough ER (RER) is characterised by ribosomes studded on the cytoplasmic face of the membrane cisternae (visible as parallel membrane stacks with attached ribosomes on EM). Procollagen (the precursor of collagen) is synthesised on ribosomes → enters the RER lumen → undergoes hydroxylation (requires Vit C) and initial glycosylation → moves to Golgi for further processing → secreted as procollagen → cleaved extracellularly to collagen.
Collagen synthesis pathway: (1) Transcription → translation of pro-alpha chains on RER ribosomes → (2) Hydroxylation of proline and lysine (requires Vit C and O2) in RER → procollagen triple helix → (3) Glycosylation in RER + Golgi → (4) Vesicle secretion → (5) Procollagen peptidase cleaves pro-peptides extracellularly → collagen → (6) Lysyl oxidase cross-links → fibres. Scurvy (Vit C deficiency): impaired hydroxylation → defective collagen → bleeding gums, poor wound healing, perifollicular haemorrhages. Rare in India but seen in alcoholism.
Rough ER (RER) is the answer. Key EM feature: parallel membrane cisternae with ribosomes (electron-dense dots) on the outer surface. Active fibroblasts = very prominent RER (synthesising lots of collagen, proteoglycans, fibronectin). Smooth ER (SER) = tubular membranes without ribosomes (lipid synthesis, steroid synthesis, drug detoxification — prominent in liver hepatocytes and steroid-secreting cells). Golgi = stacked flattened cisternae without ribosomes (post-translational modification, packaging).
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In pulmonary tuberculosis, Mycobacterium tuberculosis is inhaled, phagocytosed by alveolar macrophages, but is NOT killed. Instead, it survives within phagosomes by preventing fusion with lysosomes. The organelle that normally kills phagocytosed bacteria by fusing with the phagosome to form a phagolysosome is:
Correct. Lysosomes contain hydrolytic enzymes (acid hydrolases: acid phosphatase, cathepsins, elastase, DNase, lysozyme) at pH 4.5–5.0. In normal phagocytosis: phagosome + primary lysosome → phagolysosome → bacteria killed. M. tuberculosis escapes death by secreting proteins (ESAT-6, Rv3684) that prevent phagosome-lysosome fusion → survives intracellularly → tuberculosis persists.
Lysosome types: Primary lysosome (storage, inactive). Secondary lysosome (active; formed by fusion with phagosome or autophagic vacuole). Residual body (undigested material; lipofuscin = residual body of lipid oxidation products, accumulates in ageing). Lysosomal storage diseases (Gaucher = glucocerebrosidase deficiency; Niemann-Pick = sphingomyelinase deficiency; Tay-Sachs = hexosaminidase A deficiency — all from failed lysosomal digestion). India: Gaucher disease is most common lysosomal storage disease.
Lysosome is the key organelle. Macrophage phagocytosis: bacteria bound by PRRs (TLRs, macrophage mannose receptor) → engulfed in phagosome → primary lysosome (from Golgi) fuses → phagolysosome → low pH + hydrolytic enzymes → bacteria killed. M. tuberculosis escape mechanism = blocks phagolysosome formation (the "phagosome maturation block") → chronic granulomatous infection. Note: Mycobacteria are acid-fast (Ziehl-Neelsen stain, red bacilli on blue background) — their waxy cell wall (mycolic acid) also contributes to resistance.
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A histology slide from a post-mortem specimen of the interscapular region of a 2-week-old neonate shows cells with multiple small fat droplets in the cytoplasm, a central nucleus, and abundant mitochondria (giving the cytoplasm a granular appearance). This is brown adipose tissue. Which of the following correctly describes its function, distinguishing it from white adipose tissue (WAT)?
Correct. BAT = thermogenesis via uncoupling protein 1 (UCP1 = thermogenin) in the inner mitochondrial membrane. UCP1 dissipates the proton gradient → electron transport chain runs without producing ATP → heat generated instead. Critical in neonates (interscapular depot, perirenal, axillary) who cannot shiver. WAT = long-term energy storage as triglycerides (unilocular); also an endocrine organ (adiponectin, leptin).
Adipose tissue: White (WAT) = unilocular fat cell; peripheral nucleus; large single fat droplet; endocrine function (leptin signals satiety to hypothalamus; adiponectin increases insulin sensitivity; resistin causes insulin resistance). Brown (BAT) = multilocular; central nucleus; multiple small droplets; abundant mitochondria; UCP1 → thermogenesis. Metabolic syndrome (common in India, especially urban South Indians): excess visceral WAT (omental, mesenteric) → adipokine dysfunction → insulin resistance → T2DM + hypertension + dyslipidaemia.
BAT = thermogenesis (non-shivering, UCP1). WAT = energy storage. On slide: BAT = multilocular (multiple small droplets), central nucleus, packed mitochondria (granular cytoplasm). WAT = unilocular (single large droplet), peripheral nucleus (signet-ring appearance). UCP1 (uncoupling protein 1 = thermogenin) in the inner mitochondrial membrane bypasses ATP synthesis → proton gradient dissipated as heat. Neonatal significance: at birth, neonates are cold and cannot shiver → BAT thermogenesis essential. BAT is re-activated in some adults during prolonged cold exposure.
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On a histology section of chronically inflamed tissue, you identify a cell with an eccentrically placed round nucleus, coarse "clock-face" (cartwheel) chromatin pattern, and an intensely basophilic cytoplasm with a pale area (perinuclear hof) near the nucleus. This cell is:
Correct. Plasma cell classic features: (1) Eccentric nucleus; (2) Clock-face/cartwheel chromatin (heterochromatin clumped at periphery like spokes of a wheel); (3) Intensely basophilic cytoplasm (abundant RER for immunoglobulin synthesis); (4) Perinuclear hof = pale area next to nucleus = large Golgi apparatus (for Ig glycosylation). Plasma cells secrete antibodies (IgG, IgM, IgA, IgE, IgD).
Plasma cell histology: clock-face chromatin = condensed heterochromatin clumped at inner nuclear membrane margin (mimics clock numbers). Basophilic cytoplasm = abundant RER (ribosomes stain blue/basophilic on H&E; the more ribosomes, the more basophilic). Perinuclear hof (clear/pale zone) = large Golgi apparatus. Increased plasma cells in tissue = chronic inflammation (RA, syphilis, chronic osteomyelitis, multiple myeloma marrow). Russell bodies = intracytoplasmic Ig accumulations in plasma cells (spherical pink PAS-positive inclusions).
Plasma cell is the answer. Memory trick "CERE-Basophilic-Hof": Clock-face chromatin, Eccentric nucleus, RER-rich cytoplasm (basophilic), Hof (perinuclear pale Golgi). Multiple myeloma = malignant plasma cell tumour; M-protein spike on serum electrophoresis, Bence-Jones protein in urine, lytic bone lesions. Compare: Mast cell = ROUND nucleus, large metachromatic granules (purple); Macrophage = kidney-shaped nucleus, phagolysosomes; Eosinophil = bilobed nucleus, orange-red eosinophilic granules.
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A 22-year-old man with a tall, slim build, long limbs, arachnodactyly, lens dislocation (ectopia lentis), and a dilated ascending aorta (4.5 cm on echo) is diagnosed with Marfan syndrome. Genetic testing shows a mutation in the FBN1 gene (fibrillin-1). Fibrillin-1 is a key component of which extracellular matrix fibre?
Correct. Elastic fibres have two components: (1) Elastin core (amorphous protein with cross-linked desmosine residues; provides elasticity/recoil); (2) Fibrillin microfibrils (surrounds the elastin core; provides structural scaffold for elastin deposition; also in ciliary zonules of the lens). FBN1 mutation → defective fibrillin-1 → weakened elastic fibres → aortic aneurysm (ascending aorta, risk of dissection/rupture) + lens dislocation.
Elastic fibre components: Elastin (hydrophobic protein; cross-linked desmosine and isodesmosine residues → rubber-like elasticity) + Fibrillin microfibrils (scaffold). Locations: ligamentum nuchae, ligamentum flavum, aortic wall, lung parenchyma, vocal cords, skin dermis (elastic recoil). Disorders: Marfan syndrome (FBN1 → fibrillin-1) → aortic aneurysm; Cutis laxa (elastin mutation → sagging skin); Pseudoxanthoma elasticum (ABCC6 mutation → calcification of elastic fibres in skin, eye, arteries). Note: elastic fibres stain with Orcein (brown) or Weigert (dark purple); do NOT stain with H&E.
Elastic fibres are the answer. Fibrillin = a glycoprotein that forms the microfibrillar scaffold on which elastin is deposited during elastic fibre assembly. The mature elastic fibre = central amorphous elastin + peripheral fibrillin microfibrils. Marfan syndrome (FBN1 mutation → fibrillin-1 defect) → weakened elastic fibres in aortic wall + ciliary zonules → aortic root dilatation/dissection + ectopia lentis (lens subluxation). Also: tall stature, arachnodactyly, pectus excavatum, scoliosis. Abraham Lincoln suspected to have had Marfan syndrome.
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A 25-year-old man undergoes appendicectomy for acute appendicitis. Three weeks later, the surgical wound contains granulation tissue. The primary cell responsible for synthesising the collagen that fills the wound during the proliferative phase of healing is:
Correct. The fibroblast is the key cell of wound repair. Under the influence of growth factors (TGF-β, PDGF) secreted by macrophages and platelets, fibroblasts migrate into the wound bed, proliferate, and synthesise type III collagen (initially) → granulation tissue. Later, type III is remodelled to type I. Myofibroblasts (α-SMA-positive fibroblasts) contract the wound.
Granulation tissue composition: fibroblasts + new capillaries (angiogenesis, pink on H&E) + loose type III collagen + scattered inflammatory cells (macrophages, lymphocytes). "Granulation" = looks like pink granules at wound base due to new capillary loops. Impaired healing: diabetes (poor macrophage and neutrophil function), corticosteroids (suppress fibroblasts), vitamin C deficiency (impairs procollagen hydroxylation), zinc deficiency (enzyme cofactor for collagen crosslinking). India: keloid (excess collagen) is the most common wound healing complication in darker skin types.
Fibroblast is the answer. Wound healing cell sequence: (1) Platelets (immediate clot); (2) Neutrophils (first 24–48h, pus cells, phagocytose bacteria); (3) Macrophages (3–5 days, phagocytose debris, secrete TGF-β/PDGF); (4) Fibroblasts (3 days–3 weeks, collagen synthesis, granulation tissue); (5) Endothelial cells (angiogenesis, new capillaries); (6) Myofibroblasts (wound contraction). After 21 days: remodelling (type III → type I collagen, scar matures).
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The extracellular matrix of connective tissue contains a "ground substance" that fills the space between fibres and cells, acting as a molecular sieve and reservoir for growth factors. The most important structural component of ground substance is:
Correct. Glycosaminoglycans (GAGs) are long, unbranched polysaccharide chains that are strongly negatively charged (sulphate and carboxylate groups) → attract water → form a hydrated gel. Major GAGs: hyaluronic acid (largest, non-sulphated, forms a matrix for proteoglycans), chondroitin sulphate, dermatan sulphate, heparan sulphate (basement membranes), keratan sulphate, heparin (mast cells). Proteoglycans = GAGs covalently linked to a core protein (e.g., aggrecan in cartilage).
GAG types and locations: Hyaluronic acid (non-sulphated) → vitreous humor, synovial fluid, loose CT; Chondroitin sulphate → cartilage, bone, cornea; Dermatan sulphate → skin, blood vessels, heart valves; Heparan sulphate → basement membranes, cell surface; Keratan sulphate → cartilage, cornea; Heparin → mast cell granules (anticoagulant). Proteoglycans: core protein + many GAG chains (e.g., Aggrecan in cartilage = 100 chondroitin sulphate + 50 keratan sulphate chains → bottle-brush shape). Defective GAG degradation = mucopolysaccharidoses (Hurler, Hunter syndromes).
GAGs and proteoglycans are the main components of ground substance. They are highly hydrophilic (attract water), forming a gel that fills the ECM. Functions: (1) Molecular sieve — size-selective filtration (important in glomerular filtration); (2) Hydration — provide compressive resistance in cartilage (aggrecan); (3) Growth factor binding — heparan sulphate proteoglycans bind and store FGF, VEGF. Clinical: heparin (a GAG produced by mast cells) → anticoagulant used clinically. Hyaluronic acid → viscosupplement for osteoarthritis (intraarticular injections).
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