312 GUNNAR BLIX 



wherever it occurs, is a high-molecular, poly-anionic, highly hydrated, 

 essentially unbranched polysaccharide, the molecules of which seem to be 

 present in living tissues as randomly kinked coils. As might be expected, 

 aqueous solutions of this substance are highly viscous. The viscous 

 character of the joint fluids is due to hyaluronic acid, which occurs dis- 

 solved in the fluid together with some serum proteins. In this environment 

 the properties of hyaluronic acid make it suitable to serve as a lubricant, 

 protecting the cartilaginous surfaces in the joints against mechanical 

 damage. This may be regarded as the main function of the hyaluronic 

 acid of the joint fluid. 



In the skin, subcutaneous, and other connective tissues the hyaluronic 

 acid has quite other functions. The characteristic mechanical properties 

 of these tissues, their rigidity, degree of compressibility, resistance to 

 injected fluids and so on are no doubt due to the particular arrangement 

 in which a sol or gel of hyaluronic acid (and of some other mucopoly- 

 saccharides) is included into the fine three-dimensional network of 

 collagen fibres. Variation in pore-size, in fibre-width, fibre-length and 

 orientation and in concentration, polymerization and ionization of the 

 polysaccharides may of course influence and modify the mechanical and 

 other properties of the tissues, but these fields are largely unexplored. 



Karl Meyer and his collaborators have made the interesting observation 

 that the ratio between hyaluronate and chondroitin sulphate and the 

 ratio between different chondroitin sulphates vary with age. In pig's skin 

 the ratio between chondroitin sulphate B and hyaluronate was found to 

 be 1 : 5 in the new-born but a good i : i in the adult. In new-born infants 

 the rib cartilage contains a high concentration of chondroitin-4-sulphate. 

 This concentration decreases with increasing age, whereas the concentra- 

 tion of keratosulphate increases from the first year to adulthood and then 

 remains constant. Furthermore chondroitin-4-sulphate is practically all 

 replaced by the 6-sulphate. The physiological significance of these changes 

 is unknown, but since they concern chemical substances with different 

 properties they must undoubtedly influence the mechanical and other 

 properties of the tissues. The mechanism by which these chemical changes 

 are initiated may be something essential in the process of ageing. 



There is evidently a long way to go before arriving at a clear under- 

 standing of the precise physiological significance of these and other 

 structural polysaccharides. The work still to be done must comprise 

 investigations of their primary and conformation structure, as well as 

 studies on their interactions with fibre and globular proteins. It should 

 also include a close inquiry into the physicochemical properties of the 

 pure substances and of the complexes formed between them and proteins. 

 It should of course involve direct studies on living tissues with optical 

 and other methods. Investigation on artificial models may also be valuable. 



