The main sources of polysaccharides are endogenous glycosaminoglycans (GAGs), Marine polysaccharides and terrestrial plant polysaccharides, especially polysaccharides from Chinese herbal medicines. GAGs are naturally-derived linear polysaccharides that are expressed in the intracellular compartments, cell surface, and extracellular environments, and they interact with various molecules to regulate many cellular processes associated with health and disease [22]. GAGs are comprised distinct O-linked disaccharide units, which are typically composed of a combination of iduronic acid, glucuronic acid, glucosamine, galactose or galactosamine monosaccharides [23,24]. The widely studied GAGs mainly include chondroitin sulfate (CS), heparan sulfate (HS) and heparin (HP) in animal tissues (Fig. 3 ) [22,25]. GAG chains are in most cases sulfated, except hyaluronan (HA) (Fig. 3), which are biodegradable and non-immunogenic in the body [26,27]. The chemical structures of typical GAGs are shown in Fig. 3.
Fig. 3 The structures of several polysaccharides (GAGs, marine polysaccharides, and terrestrial plant polysaccharides).