3. Connective Tissues

Connective Tissues

Overview

  • Connective tissues (CT) bind, support, and strengthen other body tissues.
  • They form a major transport system (blood).
  • They serve as a major site for stored energy reserves (fat).

Features of Connective Tissues

  • Unlike epithelia, CT is not found on body surfaces.
  • CT can be highly vascular, except for cartilage (avascular) and tendons (little blood supply).
  • Like epithelia, CT is supplied by nerves, except for cartilage.

Composition of Connective Tissue

  • Connective tissue (CT) is composed of:
    • Extracellular Matrix (ECM)
    • Cells
  • CT=ECM+CellsCT = ECM + Cells

Extracellular Matrix (ECM)

  • The ECM is composed of:
    • Ground Substance (GS)
    • Protein Fibres
  • The protein fibres of the ECM are secreted by the cells within the ECM/CT.
  • The structure of the ECM largely dictates the connective tissue qualities.
    • Cartilage: ECM is firm and rubbery.
    • Bone: ECM is hard and inflexible.
  • ECM=GS+FibresECM = GS + Fibres
  • CT=ECM+CellsCT = ECM + Cells
Ground Substance (GS)
  • Composed of:
    • Water
    • Proteins (like gelatin)
    • Polysaccharides (sugars)
  • GS=H2O+proteins+polysaccharidesGS = H_2O + proteins + polysaccharides
  • The sugars are Glycosaminoglycans (GAGS) or mucopolysaccharides.
  • GAGS join with core proteins to form proteoglycans.
Terminology
  • Glycosaminoglycans (GAGS) or mucopolysaccharides are long, unbranched polysaccharides with repeating disaccharide units.
    • Amino sugar (e.g., N-acetylglucosamine)
    • Uronic sugar (e.g., glucuronic acid)
Proteoglycans
  • Core Protein + Glycosaminoglycan (GAG)
Glycosaminoglycans (GAGS) Examples
  • Sulphated GAGs:
    • Dermatan sulphate
    • Heparan sulphate
    • Keratan sulphate
    • Chondroitin sulphate
    • These bind to core proteins to form proteoglycans (PGs).
  • Non-sulphated GAG:
    • Hyaluronic acid
      • Does not bind directly to a protein backbone but is joined to various PGs.
Hyaluronic Acid
  • The glycosaminoglycan hyaluronic acid is unusual in that it is not sulphated or covalently bound to a core protein.
  • Highly polar and attracts water, making the ground substance more jelly-like.
  • Composed of Glucuronic acid and N-acetylglucosamine (Dissacharide in hyaluronic acid).
Applications of GAGS
  • Hyaluronidase:
    • Produced by white blood cells, sperm, and some bacteria.
    • Makes ground substance more liquid for easier movement or access to the egg.
  • Hyaluronic Acid:
    • Viscous, slippery substance that binds cells together, lubricates joints, and maintains the shape of the eyeball.
  • Keratan Sulphate: Found in bone, cartilage, cornea of the eye.
  • Dermatan Sulphate: Found in skin, tendons, blood vessels, heart valves.
  • Chondroitin Sulphate: supports and provides the adhesive features of cartilage, bone, skin, blood vessels.
Abnormal ECM Ground Substance
  • Periorbital ECM deposition and thyroid disease (Exophthalmos).
  • The deposition of glycosaminoglycans and the influx of water increase the orbital contents.
    • Most common in younger women.
    • Goitre (swollen thyroid gland).
    • Autoimmune over-activation of thyroid (goitre).
    • Autoimmune action on fibroblasts in ECM of eye (exophthalmos).
Protein Fibres
  • Three different major types of protein fibres exist in different proportions.
    *RECALL: ECM = GS + Fibres
    *CT = ECM + Cells
Three Types of Connective Tissue Fibres
  • Collagen fibres (thick):
    • Very strong but flexible to resist pulling forces.
    • Feature variations.
    • Collagen is 25% of your body and is the most abundant protein!
    • Common in bone, cartilages, tendons, and ligaments.
    • Parallel bundles.
  • Reticular fibres:
    • Composed of collagen with a coating of glycoprotein.
    • Collagen in fine bundles with coating of glycoprotein (more protein than sugar).
    • Made by fibroblasts.
    • Provide strength and support.
    • Form part of the basement membrane.
    • Thinner, branching - spreads through tissue.
    • Form networks in vessels and through tissues, especially adipose tissue, nerve fibres, smooth muscle tissues.
  • Elastic fibres:
    • Thinner than collagen fibres.
    • Fibrous network.
    • Consist of the protein elastin surrounded by the glycoprotein fibrillin.
    • Can be stretched 150% without breaking.
    • Found in skin, blood vessels, and lung.
Marfan Syndrome
  • A hereditary defect in elastic fibres, usually resulting from a dominant mutation in a gene on chromosome 15, which codes for fibrillin.
  • Fibrillin is a large glycoprotein (350kDa350 kDa) that contributes to a structural scaffold for elastin.
  • The body produces growth factor Transforming Growth Factor beta (TGFb) – increases growth because it doesn't bind normally to fibrillin to keep it inactive (theory).
  • Individuals with Marfan Syndrome are usually tall, long-limbed, and often with a chest deformity (e.g., protruding or collapsed sternum).
  • Normal life span but need medical vigilance to control BP, etc.
  • They may have weakened heart valves and arterial walls, which can be life-threatening.
  • Occurs in about 1 in 20,000 live births.

Connective Tissue Cells

  • CT=ECM+CellsCT = ECM + Cells
Two Common Connective Tissue Cell Types
  • Fibroblasts:
    • Location: Widely distributed in connective tissues; migratory
    • Function: Secrete components of the matrix (fibres and ground substance)
  • Adipocytes (fat cells):
    • Location: Under skin and around organs
    • Function: Store fat (triglycerides)
Other Cells Found in Solid CT
  • Macrophages (Histiocytes):
    • Phagocytic cells
    • Fixed and wandering forms in connective tissues (CT)
      • Fixed: “Dust cells” (lung); “Kupffer cells” (liver); “Langerhan’s cells” (skin)
      • Wandering in CT: sites of infection/inflammation/ injury
  • Plasma cells:
    • From B-lymphocyte
    • These produce antibodies
      • Many CT sites but especially in gut and lung, salivary glands, lymph nodes, spleen, red bone marrow
  • Mast cells:
    • Produce histamine that dilates vessels
      • Alongside blood vessels
  • Leucocytes:
    • White blood cells
      • E.g., neutrophils, eosinophils
      • Migrate out from blood

Classification of Connective Tissue

  • Embryonic OR Mature
Embryonic Connective Tissues
  • Mesenchyme:
    • Gives rise to all other connective tissues.
    • Consists of connective tissue cells (mesenchymal cells) in a semi-fluid ground substance containing reticular fibres.
  • Mucous:
    • Mucous CT has widely scattered fibroblasts embedded in jelly-like ground substance.
    • Supports Umbilical cord of foetus.
Mature Connective Tissues
  • Connective Tissue Proper
    • Loose
      • areolar tissue
      • adipose tissue
      • reticular tissue
    • Dense
      • dense regular
      • dense irregular
      • elastic
  • Fluid Connective Tissues
    • Blood
    • Lymph
  • Supporting Connective Tissues
    • Cartilage
      • hyaline cartilage
      • elastic cartilage
      • fibrous cartilage
    • Bone
Loose Connective Tissue
  • Areolar Connective Tissue (most common form of CT)
    • Three types of fibres are present (collagen, reticular, and elastic).
    • Widely distributed around almost every structure; like a “packing material”.
    • Strength, Elasticity, Support.
  • Adipose Connective Tissue
    • Adipocytes dominant.
    • Central triglyceride droplet.
    • Found with areolar connective tissue (incl. fibroblasts).
    • White adipose (energy storage) and Brown adipose (heat production).
    • Insulation, Energy source, Temperature control
      *Buttocks, flanks, abdomen, orbit of eye
Dense Connective Tissue
  • Regular
    • Tendons (muscle to bone); Ligaments (bone to bone); aponeuroses (muscle to muscle).
    • Regularly arranged collagen (e.g., tendons pulling along the fibre axis).
    • Shiny white colour.
    • Collagen fibres are not a living tissue.
    • Slow healing
    • Attachment.
Supporting CT: Cartilage
  • Hyaline Cartilage
    • Abundant.
    • Relatively weak, resilient gel in which fibres are present but not obvious.
    • e.g., anterior ends of ribs; respiratory cartilage – i.e., nose, trachea, bronchi
      *Nasal septum, ends of long bones
      *Flexibility and movement.
Supporting CT: Bone or osseous tissue
  • Bones are organs composed of several connective tissue types, including bone tissue (which is either compact or spongy (BM)).
  • Compact bone
    • Outer layer of bone and forms the shaft of long bones.
    • Composed of many rod-shaped units known as either Osteons or Haversian systems.
  • Spongy bone
    • Porous inner bone tissue that lies underneath compact bone.
    • Lacks osteons, stores triglycerides (yellow marrow) and produces blood cells (red marrow).
Bone Cells
  • Four cell types are found in bone.
    • Osteogenic cells:
      • Mesenchymal stem cells that develop, start to lay down collagen; become trapped and become osteoblasts.
    • Osteoblasts:
      • Bone-forming cells. Lay down more collagen, mineralization process starts.
    • Osteocytes:
      • Mature bone cells derived from osteoblasts trapped within the extracellular matrix
      • Maintain bone tissue. Involved in the exchange of nutrients and wastes
        *Have gap junctions
    • Osteoclasts:
      • Large, multinucleated cells
      • Formed from the fusion of blood monocytes
      • Break down bone
The Structure of Osteons (Haversian Systems)
  • The basic unit of compact bone is the osteon, with four parts:
    1. Lamellae:
      • Concentric rings of mineral salts for hardness (e.g., calcium phosphate and calcium hydroxide, which together form hydroxyapatite) and collagen (for tensile strength).
    2. Lacunae:
      • Small spaces between lamellae that contain mature bone cells (osteocytes)
    3. Canaliculi:
      • “Minute canals” (containing EC fluid and minute osteocytic processes) that radiate from lacunae and provide routes for oxygen, nutrients and waste.
    4. Central (Haversian) canal:
      • Blood, lymph, and nerves.
  • Osteons are aligned along lines of stress (e.g., long axis of bone shaft).
Bone Remodeling and Fracture Healing
  • Osteoclasts reabsorb dead bone.
  • Osteoblasts lay down new bone.
  • Osteoclasts remodel new bone.
  • Chondroblasts lay down hyaline cartilage callus.
Liquid Connective Tissue: Blood
  • Consists of blood plasma (a liquid extracellular matrix) and formed elements (red cells, white cells and platelets) (RBC no nucleus).
Formed Elements of Blood
  • Erythrocytes: transport oxygen and carbon dioxide.
  • Leukocytes:
    • combat disease
      • Neutrophils and monocytes (macrophages) are phagocytic, engulfing bacteria.
      • Basophils (mobile) and Mast cells (immature circulate; mature are fixed in tissues) release substances (e.g. histamine) that intensify the inflammatory reaction.
      • Eosinophils are effective against certain parasitic worms and in acute allergic response.
      • Lymphocytes are involved in the immune response.
  • Platelets:
    • (from megakaryocytes in red marrow): clotting, secrete Abs.