Biology: Types of Connective Tissue

Connective Tissue

• Connective tissue is mesodermal in origin.
• It supports, binds, and connects various tissues and organs together.
• It is formed of connective tissue cells, fibers, and ground substance (matrix).
• It is rich in blood vessels and nerves.

General Types of Connective Tissue

• Embryonic connective tissue:
  • Mucous connective tissue
  • Mesenchymal connective tissue
• Connective tissue proper:
  • Loose areolar connective tissue
  • Adipose connective tissue
  • White collagenous connective tissue
  • Yellow elastic connective tissue
  • Reticular connective tissue

Mucous Connective Tissue

• Formed of mesenchymal cells, fine collagen fibers, and mucoid matrix.
• Matrix is soft, jelly-like, and rich in mucin.

Connective Tissue Cells

• Fixed cells:
  • Fibroblasts
  • Histocytes (fixed macrophages)
  • Fat cells (adipocytes)
  • Mesenchymal cells
  • Pericytes
  • Reticular cells
• Free cells:
  • Free macrophages
  • Plasma cells
  • Mast cells
  • Leucocytes

Fibroblasts

• Irregular or fusiform in shape.
• Basophilic cytoplasm rich in RER, Golgi complex, mitochondria, and secretory granules.
• Pale oval nucleus.
• Functions:
  • Formation of connective tissue fibers and ground substance.
  • Helps in healing of wounds.

Reticular Cells

• Star-shaped cell with long processes and central rounded nuclei.
• Functions:
  • Form the reticular stroma (meshwork) of organs like spleen, lymph node, and thymus.
  • Can change into phagocytic cells when stimulated by antigens.

Mesenchymal Cells

• Embryonic, undifferentiated cell.
• Pale basophilic cytoplasm with central oval nucleus and multiple cytoplasmic processes.
• Function: Can differentiate into other types of connective tissue cells.

Mast Cell

• Oval cell with basophilic granular cytoplasm and central rounded nucleus.
• Cytoplasmic granules rich in heparin and histamine, stain purple with methylene blue stain.
• Functions:
  • Secretion of heparin (anticoagulant).
  • Secretion of histamine (antiallergic).

Pigment Cell (Melanophore)

• Large cell with multiple processes and large nucleus.
• Cytoplasm rich in melanin granules called melanosomes.
• Seen in the dermis of the skin and can be stained with vital stains.
• Function: Carries melanin pigment.

Leucocytes

• Blood leucocytes that migrate from blood to connective tissue.
• Include neutrophils, eosinophils, basophils, monocytes, and lymphocytes.
• Increase in cases of infection to phagocytose microorganisms.

Free Macrophages

• Originate from blood monocytes that migrate to connective tissue and change into free macrophages.
• Irregular surface with protrusions and indentations, and oval eccentric nuclei.
• Cytoplasm rich in lysosomes and multiple residual bodies.
• Function: Phagocytic cells that share in immune defense of the body.

Connective Tissue Fibers

• White collagen fibers:
  • Appear white in fresh state.
  • Formed of tropocollagen molecules.
  • Types:
    • Collagen type I: Found in tendon of muscle and bone.
    • Collagen type II: Found in cartilage.
    • Collagen type III: Forms reticular fibers.
    • Collagen type IV: Present in basement membranes.
    • Collagen type V: Found in fetal membranes, blood vessels, and around muscles.
• Yellow elastic fibers:
  • Thin, branching, and slender fibers.
  • Appear yellow in great amount.
  • Formed of protein called elastin.
  • Formed by fibroblast cell and smooth muscle cell.
• Reticular fibers:
  • Fine, delicate fibers that form the stroma of glands like liver and salivary glands.
  • Appear black with silver stain, red with PAS.
  • Formed of collagen type III, proteoglycan, and glycoproteins.
  • Formed by fibroblasts and smooth muscle.
  • Function: Form the reticular meshwork of the stroma of organs.

Connective Tissue Matrix (Ground Substance)

• Jelly-like substance formed of acid mucopolysaccharides.
• Formed by fibroblast cells.
• Binds connective tissue cells and fibers together.
• Allows diffusion of tissue fluids, nutrients, and waste products between components and the tissues of the body.