The Collagen Family

• Collagen is the generic name for a family of proteins whose members can be found throughout the body. Type I collagen is the primary component of ligaments and tendons, while Type II is a component of cartilage. Type IV collagen forms the basal laminae, the thin layer of tissue that underlies and supports the skin. Some collagen types seem to play helper roles for other types, and a few collagen types don't form typical fibers. The structures and functions have not been established for all types of collagen, but they share certain structural similarities, such as a helical structure, and they are subject to similar metabolic processes.
  
  

Structure

• Collagen is made inside cells called fibroblasts, which are found in tendons, ligaments and other connective tissues. When first produced, the proteins are tagged with extra pieces that help them associate into groups of three and make the triple helix that is their functional shape. After assembly into the triple helix, the new structure, called procollagen, is ejected from the fibroblast through the Golgi apparatus. Once outside the cell, special proteins called enzymes cut off the extra pieces, leaving a collagen triple helix molecule that is essentially rod-shaped. This unit is called tropocollagen, and it is the smallest complete collagen molecule.
  
  

Fibers

• Once the concentration of tropocollagen is sufficiently high, the molecules will begin to aggregate in a very specific way. The tropocollagen rods have "sticky ends" that link together to make long strands, and the rods also stack laterally like bundles of wood. The final result is called a collagen fiber. These fibers make up the collagenous tissues of our bodies.
  
  

Regeneration

• Like all tissues of the body, collageneous tissues are subject to wear and tear. Unlike some proteins, such as elastin, collagen can be synthesized throughout the body's lifetime. This is why bones (which are built on a collagen template) can heal naturally even after adulthood. A number of disease states (such as rheumatoid arthritis, cancer, lupus and bacterial infection) can interfere with the normal balance of collagen generation and degradation.
  
  

Breakdown

• Collagen degradation is less well-understood than collagen formation. It is, however, known that collagen is relatively resistant to being broken down. Its tightly packed triple helical structure and fibrous nature offers few weak points for protein-snipping enzymes called collagenases to exploit. It is much easier for the enzymes to cut unraveled, or denatured, collagen, such as that found in injured tissue. Whereas healthy collagen initially self-assembles spontaneously as described above, breakdown of collagen must in many tissues be assisted by nearby cells that partially digest the protein.