Connective Tissue

Intro to CT - Body's Framework

• Connective Tissue (CT): Body's framework, providing structural & metabolic support to organs and other tissues.
• Origin: Predominantly mesodermal.
• Key Functions:
  • Binding & Supporting (e.g., tendons, ligaments)
  • Protecting (e.g., bone, cartilage)
  • Insulating (e.g., adipose tissue)
  • Transporting substances (e.g., blood)
• Two Main Components:
  • Cells: Diverse population (e.g., fibroblasts, adipocytes, chondrocytes, osteocytes, blood cells).
  • Extracellular Matrix (ECM): Non-living material surrounding cells, produced by them.
    • Fibers: Collagen (tensile strength), Elastic (recoil), Reticular (delicate framework).
    • Ground Substance: Hydrated, amorphous gel (proteoglycans, glycosaminoglycans (GAGs), adhesive glycoproteins like fibronectin).

⭐ The Extracellular Matrix (ECM), comprising fibers and ground substance, dictates the specialized physical properties of each connective tissue type (e.g., rigidity of bone, resilience of cartilage).

CT Components: Cells - The Living Units

• Fixed Cells (Resident): 📌 FAMM
  • Fibroblasts/Fibrocytes: Most common. Synthesize ECM (collagen, elastin). Fibroblast (active, RER) vs. Fibrocyte (inactive). Myofibroblasts: wound contraction.
  • Adipocytes: Store triglycerides. Unilocular (white fat): single large droplet, signet ring. Multilocular (brown fat): multiple droplets, thermogenesis.
  • Mast Cells: Oval; granules (histamine, heparin). IgE receptors. Allergic reactions. Metachromasia.
  • Macrophages (Histiocytes): Phagocytic; antigen presentation. From monocytes. Kidney-shaped nucleus. (e.g., Kupffer).
• Wandering Cells (Transient):
  • Plasma Cells: From B-lymphocytes. Antibody (Ig) synthesis. Eccentric "clock-face" nucleus, basophilic cytoplasm, Golgi halo.
  • Leukocytes: Neutrophils, Lymphocytes. Immune response via blood migration.

⭐ Mast cell granules contain preformed mediators like histamine (vasodilator, ↑ vascular permeability) and heparin (anticoagulant).

CT Fibers & Matrix - Matrix Marvels

Fibers: The Framework

• Collagen Fibers: Most abundant protein; tensile strength.
  • Type I: Bone, Skin, Tendon, Dentin, Cornea. (Strongest)
  • Type II: Cartilage, Vitreous body (📌 Cartwolage)
  • Type III: Reticular fibers (skin, vessels) (📌 R3ticular)
  • Type IV: Basement membrane (📌 Type 4 on the floor)
  • Vit C needed for synthesis; deficiency → Scurvy.
• Elastic Fibers: Elastin + Fibrillin; stretch & recoil.
  • Locations: Large arteries, lungs, skin.
  • Fibrillin-1 defect → Marfan Syndrome.
• Reticular Fibers: Type III collagen; delicate meshwork.
  • Locations: Liver, spleen, lymph nodes. Argyrophilic.

Ground Substance: The Gel

• Amorphous, hydrated gel. Components:
  • Glycosaminoglycans (GAGs): Long unbranched polysaccharides.
    • Hyaluronic acid: Largest, non-sulfated. Synovial fluid, vitreous.
    • Sulfated GAGs: Chondroitin, Dermatan, Heparan, Keratan sulfates.
  • Proteoglycans: Core protein + GAGs (e.g., Aggrecan).
  • Multiadhesive Glycoproteins: Link cells to ECM (e.g., Fibronectin, Laminin).

⭐ Osteogenesis Imperfecta ("Brittle Bone Disease") is primarily due to defects in Type I collagen synthesis, leading to fragile bones, blue sclerae, and hearing loss.

Classification of CT - The Tissue Tapestry

Connective tissues are diverse, classified by cell type, fiber density, and matrix.

• Embryonic CT:
  • Mesenchyme: Undifferentiated; gives rise to all other CT types.
  • Mucoid CT: Gelatinous; e.g., Wharton's jelly (umbilical cord).
• CT Proper:
  • Loose CT: Fewer fibers, more ground substance. (Areolar, Adipose, Reticular)
  • Dense CT: More fibers, less ground substance. (Regular, Irregular, Elastic)
• Specialized CT:
  • Cartilage: Avascular; (Hyaline, Elastic, Fibrocartilage).
  • Bone: Mineralized matrix; (Compact, Spongy).
  • Blood: Fluid matrix (plasma).

⭐ Wharton's jelly, a type of mucoid connective tissue, is rich in hyaluronic acid and proteoglycans, providing cushioning to umbilical cord vessels.

High‑Yield Points - ⚡ Biggest Takeaways

• Connective tissue, derived from mesoderm, is the most abundant in the body.
• Collagen Type I is the most common, found in bone, skin, and tendon.
• Elastic fibers (elastin, fibrillin) provide recoil; Marfan syndrome involves fibrillin-1 defect.
• Reticular fibers (Collagen Type III) form supportive networks in organs like liver, spleen.
• Ground substance consists of GAGs (e.g., hyaluronic acid), proteoglycans, and glycoproteins.
• Fibroblasts are the principal cells, responsible for ECM synthesis.
• Mast cells contain histamine and heparin, mediating allergic and inflammatory responses.