In contrast to epithelium, which contains no or little extracellular material, supporting tissues consists mainly of extracellular matrix (ECM) in which a few cells are imbedded. The extracellular matrix consists of at least 19 different types of collagens, the related fibrous proteins known as elastin and fibrillin, a series of glycosaminoglycans (GAGs) and extracellular structural glycoproteins.

• Coetzee et al p67
• Stevens and Lowe p49-57
• Wheater p65-67

• Glycosaminoglycans consist of large, unbranched polysaccharide chains of repeating disaccharide units
• Divided into four groups: hyaluronic acid, chondroitin sulphate and dermatan sulphate, heparin and heparan sulphate and keratan sulphate
• Large open coil confirmation: strongly hydrophilic
• High negative charge causes retention of water and positive ions

• Fibronectin: mediates adhesion between cells and extracellular matrix components
• Fibrillin: main component of microfibrils
• Elastin: hydrophobic protein which is the main component of elastic fibres
• Collagen: More than 19 different collagens have been identified. The fibrillar collagens are found in tissues as long, highly ordered fibrils with a characteristic banding pattern by electron microscopy.

  Type I collagen, the most abundant, is found as cross-striated fibrils in a large number of tissues.

  It is composed of two identical alfa-chains called alfa-1(I) and one alfa-2(I) chain.

  Type II collagen, a fibrillar collagen of cartilage, is composed of three identical alfa-chains called alfa-1(II).

  Type III collagen is found in small amounts in many tissues and in large amounts in large blood vessels.

  It is composed of three identical chains called alfa-1(III).

  Reticular fibres (reticulin), thin fibrils of type III collagen, form a loose mesh in many support tissue and are particularly prominent just below the basement membrane. They form a network to support the cells in parenchymal organs like the liver and kidney.

  The nonfibrillar collagens are similar to the fibrillar collagens, but they also contain large globular domains. Self-assembly of most of the nonfibrillar collagens usually involves binding between the globular domains to form networks. For example, the type IV collagen in basement membranes self-assembles into a complex three-dimensional network that provides a diffusion barrier in the renal glomerulus and pulmonary alveolus and provides support for epithelial and endothelial cells in these tissues and in skin, the gastrointestinal tract, and blood vessels.

• Laminin: major component of basement membranes
• Entactin: binds laminin to type iv collagen
• Tenascin: role in cell migration

• Sheet-like arrangement of ECM proteins and GAGs between parenchymal cells and support tissue
• Five major components: Type IV collagen, laminin, heparan sulphate, entactin and fibronectin
• LM: Thin PAS positive line
• EM: Lamina lucida, lamina densa, lamina fibroreticularis
• Functions: Cell adhesion, diffusion barrier, regulation of cell growth

• Hemidesmosome - anchor intermediate filament cytoskeleton to basement membrane (BM)
• Focal contacts - anchor actin filament cytoskeleton to BM
• Laminin receptor - anchor cells to laminin in BM

• Osteogenesis imperfecta - defective collagen formation due to genetic abnormalities
• Ehlers-Danlos syndromes - mutations in genes for type I collagen
• Marfan's syndrome - absence of fibrillin
• Chondrodysplasia - mutations in genes for type II collagen
• Alport syndrome - glomerular basement thickening
• Systemic sclerosis - overproduction and accumulation of collagen and other extracellular matrix proteins in skin and other organs

• Fibroblasts - most common support cell, secrete and maintain ECM components
• Adipocytes (fat cells) - store and metabolise fat
• Defense and immune cells: mast cells, macrophages, plasma cells, white blood cells

• Tendons and ligaments consist primarily of type I collagen fibrils and small amounts of other components that help organize the type I fibrils into fibres and fibre bundles.
• Cartilage consists primarily of fibrils of type II collagen in the form of arcades that are distended by highly charged GAGs.
• The extracellular matrix of the aorta contains collagens that provide tensile strength and elastin that provides elasticity.

Differences among the connective tissues also depend on the three-dimensional organization of the molecular components. The type I collagen fibrils in tendon are packed into thick, parallel bundles of fibres, whereas type I collagen fibrils in skin are randomly oriented. In cortical bone, helical arrays of type I collagen fibrils are deposited around haversian canals.

• White adipose tissue
  • Normal body fat
  • Very large spherical cells which may become polyhedral due to deformation
  • Contain single fat droplet with cytoplasm reduced to thin rim net to plasma membrane
  • Nucleus displayed to one side of cell and flattened by accumulated fat
  • Routine histological techniques extract the fat, leaving a large unstained space
  • Adipose tissue may be portioned by connective tissue septa visible to the naked eye

• Brown adipose tissue
  • Prominent in newborn of all mammals
  • In adults most conspicuous in species which hibernate
  • Present in man throughout life
  • Cells are smaller than those of white adipose tissue
  • Cytoplasm relative abundant and contains lipid droplets of varying size
  • Cytoplasm contains extraordinary large number of mitochondria
  • Brown fat has a lobular organization
  • Highly vascular with blood vessel distribution similar to that of a gland
  • Numerous small unmyelinated nerves with axons ending on cell surface

• Ehlers-Danlos syndrome - is characterized by hyperelasticity of the skin and hypermobile joints.
• Marfan syndrome - is characterized by a triad of features:
  1. long, thin extremities frequently associated with other skeletal changes;
  2. reduced vision as the result of dislocations of the lenses (ectopia lentis); and
  3. aortic aneurysms that typically begin at the base of the aorta.