Connective tissues are composed of stable cells (e.g., fibrocytes, adipocytes) and transient cells (e.g., leukocytes) plus intercellular fibres, fibrils, filaments or microfibrils immersed in an amorphous ground substance of variable viscosity and density. The amorphous material of low density is usually extracted during the preparation of histological sections, leaving empty-looking spaces. In other locations such as the cartilages and bones, the intercellular matrix is composed of fibrils and a dense amorphous substance. In the case of bone the connective tissue is calcified and forms the skeleton supporting the softer tissues and organs of the body. Cartilages and bone are described in Chapter 8.

Connective tissue cells and fibres are major components of all vessels and are associated with muscle cells and nerves. They are thus widely distributed throughout the body. In certain instances connective tissue cells and fibres are arranged into well-defined organs (lymph nodes, spleen, tonsils, etc.) that house leukocytes of various types involved in the defence of the organism. These lymphatic organs will be examined in Chapter 6. Lastly, connective tissues support the epithelia lining the skin and the mucosae of various systems (e.g., digestive, respiratory and reproductive systems) and also encase the epithelial cells of the exocrine and endocrine glands.