--- module: 083-02 language: en chapter: 83 title: "General Histology: Epithelium, Connective Tissue, Muscle, and Nerve" module_title: "Connective tissue, extracellular matrix, blood, cartilage, bone, and adipose tissue" source_sha256: d70d5a041e5953c4d630382b2d19d6e3dfa5c8e418e5ec697637ca706fc0f59a --- # Connective tissue, blood, cartilage, bone, and fat ## Connective tissue plan ### Support, bind, nourish, defend, store, transmit force ### Abundant extracellular matrix between cells ### Properties from fibres, ground substance, minerals, cells ### Develops from embryonic mesenchyme ### Fibroblasts produce matrix #### Active cells: more basophilic cytoplasm #### Become myofibroblasts after injury ## Fibres and ground substance ### Collagen resists tension #### Type one: thick fibres in dermis, tendon, bone, scar #### Type two: cartilage and vitreous body #### Type three: reticular fibres of cellular organs #### Type four: basement-membrane sheets ### Elastic fibres: elastin core, fibrillin microfibrils #### Arterial lamellae store energy in systole #### Fibrillin defects: skeleton, eye, aorta #### Elastin degradation: emphysema, arterial change ### Ground substance is a hydrated gel #### Negative glycosaminoglycans attract cations and water #### Hyaluronan occupies space, aids migration #### Fibronectin links cells, laminins organise membrane #### Poorly preserved, looks like empty space ## Connective tissue types ### Loose: diffusion and immune surveillance #### Readily develops oedema ### Dense irregular resists multidirectional stress ### Dense regular aligns collagen with force ### Tendon: parallel bundles, tenocytes, few vessels #### Tensile strength but slow healing #### Repetitive load: disorganisation, failed repair ### Ligaments join bones, may add elastic fibres ## Resident cells and adipose tissue ### Macrophages phagocytose, present antigen, remodel ### Mast cells release histamine and proteases ### Plasma cells: clock-face nucleus, pale Golgi ### White fat: one large droplet, signet ring #### Stores triglyceride, secretes adipokines #### Visceral and subcutaneous depots differ ### Brown fat: many droplets, abundant mitochondria #### Uncoupling protein one leaks protons #### Heat instead of adenosine triphosphate ## Blood ### Plasma is the fluid matrix ### Erythrocytes: anucleate biconcave discs #### Pale centre about one third #### Size, colour, shape, inclusions as clues ### Neutrophils dominate acute bacterial inflammation ### Eosinophils: parasite defence, allergy ### Monocytes become macrophage-like cells in tissue ### Platelets: fragments of megakaryocytes #### Adhere, activate, recruit, support coagulation #### In-vitro clumping mimics a low count ## Cartilage ### Chondrocytes in lacunae, avascular matrix #### Nutrients diffuse from perichondrium or synovial fluid #### Poor repair in adult articular cartilage ### Hyaline: fine type two fibrils #### Articular cartilage lacks perichondrium ### Elastic cartilage: pinna and epiglottis ### Fibrocartilage resists tension plus compression ## Bone structure and formation ### Mineralised support, calcium and phosphate store ### Osteoblasts secrete osteoid ### Osteocytes sense load through canaliculi ### Osteoclasts resorb in sealed acid compartments ### Osteons: lamellae around a central canal #### Interstitial lamellae are old osteon remnants ### Trabecular bone aligns with load ### Woven bone remodelled into lamellar bone ### Intramembranous or endochondral ossification ## Growth, remodelling, and repair ### Growth plate zones #### Columns divide, enlarge, die, are replaced #### Plate closure ends longitudinal growth ### Remodelling couples resorption and formation #### Ligand and decoy receptor steer osteoclasts #### Hormones, loading, inflammation shift balance ### Repair: haemostasis through remodelling #### Granulation tissue is not granulomatous #### Type three collagen replaced, strength rises ### Healing follows vascularity, cellularity, turnover