A glycosaminoglycan that is part of the ground substance of connective tissue. Hyaluronic acid is a major component of articular cartilage and skin, helping in tissue repair and the lubrication of joints. It also plays a role in cell movement and proliferation and may contribute to the growth of certain tumours. Consisting of around 25 000 disaccharide residues, it is made by a complex of enzymes on the outer cell surface, and absorbs water molecules to form a hydrated gel. Hyaluronan is commonly used as the basis for fillers in cosmetic surgery. Hyaluronidases break down hyaluronic acid.