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Cellular Communities and Cell Cycle - Connective Tissue Overview

  • Types of Animal Tissue

    • Four Major Types:
    • Nervous
    • Muscle
    • Epithelial
    • Connective
    • Nervous, muscle, and epithelial tissues have little extracellular matrix (ECM).

  • Connective Tissue Characteristics

    • Large volume consists of extracellular matrix (ECM).
    • ECM carries mechanical load.
    • Types of Connective Tissue:
    • Tough and flexible (e.g., tendons, skin)
    • Hard and dense (e.g., bones)
    • Resilient and shock-absorbing (e.g., cartilage)
    • Soft and transparent (e.g., vitreous humor of the eye)

  • Extracellular Matrix (ECM) Components

    • Most of connective tissue is ECM with embedded cells that secrete it.
    • Bone image shows:
    • Black spots = cells
    • Bands = collagen fibrils and calcium phosphate crystals (resistant to compression and tension).

  • Collagen in Connective Tissue

    • Major fibrous protein providing tensile strength.
    • About 20 different collagen genes exist.
    • Properties depend on the type of collagen and associated proteins.
    • Collagen structure:
    • Long, stiff, triple-stranded helix made up of three polypeptides.
    • Bundled into fibrils, then into fibers.
    • Example: Connective tissue in chick embryo shows varied orientations of collagen fibrils.

  • Cells in Connective Tissue

    • Fibroblasts:
    • Synthesize and secrete the ECM in skin, tendons, etc.
    • Osteoblasts:
    • Specialized for bone, synthesize collagen and other macromolecules.
    • Collagen is secreted as procollagen to prevent premature assembly.
    • Procollagen is activated by the removal of peptide extensions, enabling collagen assembly.

  • Matrix Remodeling

    • Matrix Proteases:
    • Cleave ECM proteins for growth, repair, and migration of cells like macrophages.
    • Fibroblasts organize collagen into functional patterns:
    • Skin: woven pattern for flexibility.
    • Tendons: parallel bundles for tensile strength along the direction of force.

  • Cell Attachment to ECM

    • Fibronectin:
    • Links ECM to cells, with collagen binding and cell attachment sites.
    • Integrin:
    • Connects fibronectin to the cell, linking to actin cytoskeleton.
    • Requires adapter proteins for cell movement along ECM.
    • Mechanism of action depends on binding affinity shifts upon ligand binding.
    • Mutations in integrin genes can affect cell migration and clotting (e.g., bleeding disorders due to improper platelet function).

  • Glycosaminoglycans (GAGs)

    • Resist compression, composed of repeating disaccharide units (one being an amino sugar).
    • Example: Hyaluronin, comprising 25,000 disaccharide repeats.
    • Form a gel-like hydrophilic environment due to negative charge attracting ions and water.
    • Proteoglycans:
    • Resulting aggregates from GAGs; associated with proteins.
    • Functions:
      • Filter formation
      • Binding growth factors
      • Regulating cell migration

  • Conclusion

    • Connective tissue’s unique properties arise from its composition of collagen and glucosaminoglycans, leading to strength and adaptability in various physiological contexts.