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.