Glycosaminoglycans (GAGs)
Glycosaminoglycans are long unbranched sugar chains of repeating disaccharides that hold water in the dermal ground substance and, except for hyaluronan, are sulfated and attached to core proteins.
A glycosaminoglycan is an unbranched polysaccharide built from a repeating disaccharide, typically an amino sugar paired with a uronic acid or a galactose. Sulfate and carboxylate groups make the chains strongly anionic, so they attract counterions and, osmotically, large amounts of water. All except hyaluronan are synthesised attached to a core protein, forming proteoglycans such as decorin, biglycan, and versican.
In human dermis the principal sulfated species is dermatan sulfate, with chondroitin sulfate, heparan sulfate, and keratan sulfate also present. Decorin binds along collagen fibrils and limits their diameter, so glycosaminoglycan biology is structural as well as hydrating. Heparan sulfate additionally sequesters growth factors, including fibroblast growth factors and vascular endothelial growth factor, holding them in the matrix until released. Collectively they are a small fraction of dermal dry weight while accounting for most of its bound water.
For assessing a claim, glycosaminoglycan content is a faster-moving variable than collagen and is measured differently, usually as sulfated glycosaminoglycan by dye binding or as hexosamine content. That speed is exactly why cosmetic in vitro studies report glycosaminoglycan increases so often: it is an easier readout to shift in culture than mature crosslinked collagen, and it does not carry the same structural meaning.
Three errors recur. Treating glycosaminoglycan and hyaluronic acid as synonyms, when hyaluronan is the unusual member of the family rather than the representative one. Presenting a fold increase in culture as skin plumping in vivo. And offering topical chondroitin sulfate, a chain of tens of kilodaltons, as replenishment of dermal ground substance, when nothing that size crosses intact stratum corneum.