What is collagen, what is not.

Collagen represents a series of proteins that are broadly related in terms of chemical features, structure, and function. This presentation focuses on current information relevant to the function, chemistry, and structure of collagen in order to more precisely define the parameters within which a protein may be recognized as collagen. It is noted that ultrastructural investigation of collagen based on transmission and scanning electron microscopy alone is of limited value, but that chemical analyses of tissue specimens and the use of immunohistochemical techniques are essential in evaluating collagenous proteins in normal and pathologic tissues. In addition to the specific types of collagen (I-VI), the role of collagen in the biology and pathophysiology of connective tissue is discussed.