Hyaluronan in skin: aspects of aging and its pharmacologic modulation.

Hyaluronan is a glycosaminoglycan polymer prominent in embryogenesis and in tissues undergoing repair. It is responsible for the water content of skin, where half the hyaluronan of the body is present. As in other tissues, it undergoes rapid turnover. Its biology is vastly different between dermis and epidermis. Levels do not diminish with age but instead become increasingly associated with tissues and resistant to extraction in vitro. Hyaluronan-binding proteins are involved, most of which remain unidentified. Hyaluronan size is critical for its various functions. High molecular size reflects intact tissues and antiangiogenic and immunosuppressive state, whereas smaller polymers are distress signals and potent inducers of inflammation and angiogenesis.