The Role of Connective Tissue

Connective tissue connects, supports, binds, and separates organs and tissues, forming a framework to support body tissues and organs, for structural and metabolic purposes. In connective tissue, cells are few and dispersed — they are not in close contact, as in epithelial tissue. Most connective tissues are vascularized (except cartilage). The extracellular spaces (space outside of cells) in connective tissue are referred to as the extracellular matrix.

Connective tissue, therefore, is made up of cells and extracellular matrix. The extracellular matrix is composed of glycosaminoglycans and proteoglycans. It is variations in the composition of the extracellular matrix that determine properties of the connective tissue.

Connective tissue is made up of:

• Fibrous components (collagen and elastin)
• Glycosaminoglycans or GAGs (long chains of repeating disaccharide units; the main role is to support collagen)
• Proteoglycans (GAGs attached to a core protein)

Classification of Connective Tissue

Connective tissue proper is classified as either loose irregular connective tissue or dense irregular connective tissue.

• Loose irregular connective tissue contains numerous cells and a loose fiber arrangement in a moderately viscous fluid matrix.
• The dense irregular connective tissue has a dense woven network of collagen and elastic fibers in a viscous matrix. Dense connective tissue is found in joint capsules, muscle fascia, and the dermis layer of skin.

Specialized connective tissue includes:

• Dense regular connective tissue (found in tendons and ligaments)
• Cartilage (a type of supporting connective tissue that consists of chondrocyte cells, collagen fibers, and elastic fibers; semi-solid or flexible matrix; includes hyaline cartilage, fibrocartilage, and elastic cartilage)
• Adipose tissue (a type of supporting connective tissue that cushions, stores excess fat and energy; contains reticular cells and reticular fibers)
• Hemopoietic or lymphatic tissue (a fluid connective tissue involved in blood cell production; contains leukocytes and fibers of soluble liquid proteins formed during clotting; extracellular portion is plasma) 
• Blood (contains erythrocytes, leukocytes, thrombocytes; fibers are soluble proteins; extracellular substance is plasma)
• Bone (a type of supporting connective tissue contains osteoblasts or osteocytes; consists of collagen fibers and is rigid or calcified)

Under normal circumstances, the fibers, proteoglycan, and GAGs are regulated and controlled by a balance between synthesis and degradation. The balance is maintained by cytokines, growth factors, and degradative MMPs (matrix metalloproteinases). If there is an imbalance, connective tissue diseases can develop. For example, there is a net degradation in conditions such as osteoarthritis, rheumatoid arthritis, and osteoporosis. A net increase in synthesis can lead to scleroderma or interstitial pulmonary fibrosis.

There are more than 200 diseases and conditions that affect connective tissue. Some connective tissue diseases are consequences of infection, autoimmune diseases, injury, or genetic abnormalities. The cause of some connective tissue diseases remains unknown.