What is Hyaluronic Acid?

Canonically, hyaluronan is recognized for its role in the maintenance of joint health.  Hyaluronan is naturally synthesized locally by synoviocytes within the joint and once produced, it binds to collagen and elastin to form articular cartilage.  It is the presence of hyaluronan that makes cartilage strong enough to handle compressive forces within the joint (Seog, 2002).  Hyaluronan is also found in unbound form in the synovial fluid, where it provides the major source of lubrication that allows for smooth fluid movements in joints (Sabaratnam, 2005).

Hyaluronan within the synovial capsule is critical for smooth joint movement: the articular cartilage encapsulates the ends of the bones forming a smooth while synovial fluid forms a film of lubrication over the articular cartilage during movement.  Combined, these structures protect the bones from frictional grinding (Walker, 1968).

Within bone itself, the presence of hyaluronan is primarily linked to its roles in bone modeling and remodeling processes.  Hyaluronan has been shown to regulate bone remodeling by stimulating osteoblasts and osteocytes as well as inhibiting osteoclasts (Bastow, 2008; Prince, 2004).  Intriguingly, hyaluronan taken orally has been shown to reduce urinary markers of bone resorption and ovariectomy-induced bone loss (Gerdin, 1997), indicating that hyaluronan may suppress bone resorption.

One of the primary functions of hyaluronan is maintaining tissue hydration.  Interestingly, hyaluronan is so hydrophilic it can absorb, retain, and deliver over one-thousand times its weight in water (Wand, 2007).  The ubiquitous nature of the molecule ensures that hydrophilic delivery takes place throughout all tissue.

Hyaluronan is found most prevalently in the skin; approximately half of total body hyaluronan is located within the dermal and epidermal layers.  Primary functions of hyaluronan in the skin include moisturization and hydration (Meyer, 1941).  Skin turgor is the result of the ability of hyaluronan to absorb, retain, and deliver water (Wang, 2007).  With age, there is a distinct decrease in the percent composition of hyaluronan in epidermal tissue likely correlating with the increase in wrinkles and aged skin (Juhlin, 1997).  Experiments have demonstrated that 77% of naturally occurring hyaluronan in the skin is lost by age 70 in humans (Weist, 2008).