Histology Lesson 3 (connective tissues)

• Connective tissue is composed of extracellular matrix (ECM), which includes collagen fibers, elastic fibers, reticular fibers, ground substance, proteoglycans, glycoproteins, and water.
• Connective tissues are present throughout the body as loose connective tissue in skin and dense irregular connective tissue in bones and tendons.
• Fibroblast is the principal cell of connective tissue, responsible for the synthesis of fibers (e.g. collagen) and ground matrix, derived from mesenchymal cells
• Fibers + Ground Substance

  Extracellular Matrix (ECM)
• Mesenchymal Cells are similar but smaller than fibroblasts, flattened in a stellate shape, also known as multipotential or pluripotential cells
• Connective Tissue


  Characterized by cells which function to elaborate and maintain a variety of extracellular matrix, originating from embryonic mesenchyme (from the mesoderm)
• Plasma Cells have a cartwheel or spokewheel appearance of the nucleus, synthesize antibodies found in the bloodstream, and are rare in most areas of the body
• Mononuclear Phagocytic System

  • Liver = Kupffer cells, Kidney = Mesanglial cells, Bones = Osteoclasts, Spleen = Splenic Macrophage, Lungs = Alveolar Macrophage, Placenta = Hoffbauer cells, Skin = Langerhans cells, Brain = Microglial cells
• Components of Connective Tissue

  • Tissue, cells, Protein fibers, Ground substance
• Fibrocyte is the mature fibroblast, spindle-shaped, and stellate-shaped, with acidophilic cytoplasm and a darker, elongated nucleus
• Types of Connective Tissue Cells

  • Fixed Cells, Wandering
• Fat cells (Adipocytes or Adipose Cells) are specialized for the storage of neutral fats or heat production, found singly or in groups with a "Signet ring" appearance
• Mast cells are large ovoid cells actively involved in anaphylactic sensitivity reactions, with abundant intensely staining basophilic granules containing Heparin and Histamine
• Macrophages or Histiocytes have the ability to take particulate materials with their cytoplasm and degrade the ingested substance with hydrolytic enzymes by phagocytosis
• Elastic Fibers


  Imparts a yellowish color due to the presence of a protein ELASTIN, can be stretched and return to original dimension, seen in lungs and blood vessels
• Collagen Fibers


  Most abundant protein in the body, colorless strands in fresh collagen fibers, great strength due to its molecular configuration, 28 collagen types in vertebrates
• Types of Connective Tissue

  • Collagenous Connective, Loose Connective Tissue, Dense Connective Tissue, Dense Irregular Connective Tissue
• Mast cells


  • Cytoplasm - abundant intensely staining basophilic granules
  • Nucleus - spherical and centrally located
  • Contents of Mast Cell Granules: Heparin - anticoagulant, Histamine - promotes increased vascular permeability and smooth muscle contraction, Serine proteases - activates inflammation, Chemotactic factors - attract leukocytes
• Synthesis of antibodies found in the bloodstream

  Precursor are the B-lymphocytes (blood)
• Connective Tissue Fibers

  • Collagen
  • Reticular
  • Elastic
• Mast cells are large ovoid cells actively involved in anaphylactic sensitivity reactions
• Reticular Fibers


  Principally Type III collagen, found in stroma of many glandular organs, basal lamina of most epithelia, supporting tissue in lymphoid and blood-forming organs
• Ground Substance


  Translucent matrix with no structural organization, contains acid (mucopolysaccharide or glycosaminoglycans), hyaluronic acid, chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparan sulfate
• Dense Regular Connective Tissue


  • Occurs in cordlike structure and as bands
  • Collagenous fibers are usually in bundles oriented parallel to one direction
  • Seen in tendons, ligaments, aponeurosis, and fascia
• Reticular Tissue


  • Contains "Argyrophilic fibers" with special staining affinity to silver salts
  • Forms the supporting framework of bone marrow and most lymphoid and hematopoietic organs
  • Found in close association with the basal lamina of most epithelia
• Subtypes of Connective Tissue

  • Collagenous Connective
  • Dense Connective Tissue
• Types of Adipose Tissue

  • White or Yellow adipose tissue
  • Brown Adipose Tissue
• Brown Adipose Tissue


  • Fetal fat or immature form with color ranging from tan to reddish brown
  • Cell contains multiple lipid droplets in the cytoplasm; multilocular in morphology
  • Found in interscapular and inguinal regions of newborn
• Dense Irregular Connective Tissue


  • Occurs in the form of bands, sheets, cords, or bundles
  • Bundles are randomly oriented or run in few directions
  • Found in the dermis of the skin, submucosa of esophagus, capsule of organs, periosteum, and perichondrium
• Locations of reticular fibers

  • Papillary layer of dermis, hypodermis of skin, pia mater of spinal cord, endoneurium, endomysium of muscle and nerve
• Elastic Tissue


  • Abundance of elastic fiber confers great elasticity
  • Takes the form of fibers and fenestrated lamellae or sheets
  • Found in the wall of organs, large artery, trachea, bronchi, and vocal chords
• White or Yellow Adipose Tissue


  • Comprises the bulk of the body
  • Adult fat or mature form with color ranging from white to dark yellow
  • Cell contains a single large fat droplet in the cytoplasm; unilocular in morphology
• Components of Mucous Connective Tissue

  • Abundance of ground matrix composed chiefly of hyaluronic acid
  • Jelly-like tissue containing collagen fibers and a few elastic or reticular fibers
  • Classic example is Wharton’s Jelly of umbilical cord
• Functions of Adipose Tissue

  • Storage of fats
  • Insulation against heat loss
  • Mechanical support in certain body regions
• Chemotactic factors – attract leukocytes to the site of infection.
• Heparin - anticoagulant
• Histamine – promotes increased vascular permeability and smooth muscle contraction
• Three types of Loose Connective Tissue

  • Areolar
  • Adipose
  • Reticular
• Areolar Connective Tissue

  • Complex collection of fibers and cells
• Adipose Connective Tissue

  • Unusual, Highly Cellular