54 KERATIN AND KERATINIZATION 



The work of Meyer (1945, 1951 and 1957) in particular has led to the 

 recognition of the importance of a special class of polymers forming the 

 colloidal ground-substance of the intercellular spaces, the mucopoly- 

 saccharides. These are proteins linked to polysaccharides containing hexos- 

 amine or glucuronic acid. They are distinguished partly by the amount 

 and type of polysaccharide and by the presence or absence of sulphuric 

 acid, e.g. chondroitin sulphuric acid, contains sulphuric acid and hyaluronic 

 acid does not. Meyer (1957) distinguishes four or five mucopolysaccharides : 

 hyaluronic acid, chondroitin sulphates A B and C differing in their 

 optical rotary power [a] D (see p. 194). Heparin may also be classed 

 among them. The amount and types of mucopolysaccharides vary from 

 site to site which suggests some relation with the overlying tissue. Their 

 importance in tissue maintenance is sufficiently indicated by the dramatic 

 effects produced by cortisone, by the adrenocorticotrophic hormone 

 (ACTH) and by hyaluronidase. The two histochemical tests used to 

 distinguish mesenchymal elements: the periodic acid-Schiff test (PAS) 

 and metachromatic staining are often believed to stain mucopolysaccharides. 



Metachromasy is the phenomenon of a change in colour of a dye on 

 becoming associated with a structure, an effect believed to be partly due 

 to the state of aggregation of the combined dye. The commonly used 

 dye, toluidine blue, is blue in dilute solutions and stains (basement 

 membranes) metachromatically to give a purple colour. The PAS test 

 is an application of the well-known test for aldehydes using a bleached 

 solution of basic fuchsin (Pearse, 1953). It may be used histochemically in 

 cases where a chemica ltreatment can cause the release of aldehydes from 

 polysaccharides or sugar-containing complexes. The Feulgen test (p. 80) 

 is based on the fact that mild acid hydrolysis liberates an aldehyde from 

 DNA which restores the colour of the dye. Other polysaccharide com- 

 plexes yield aldehydes after oxidization by periodic acid and can then be 

 stained by the Schiff reagent. Reticulin and basal membranes are 

 strongly PAS-positive. Formerly the basal membrane was on these 

 grounds said to contain a fine network of reticulin fibrils; probably it 

 contains an amorphous mucopolysaccharide as suggested by its appearance. 

 Leblond and his associates have cast doubt on the interpretation of these 

 tests by showing that extracts of pure components do not always give the 

 expected reactions. 



The PAS-positive and metachromatically-staining substances are 

 invariably present in sites of rapid growth (p. 221) and seem therefore 

 associated with the process, but their role remains obscure. They are 

 strongly hydrophilic and could help to retain water and to form a viscous 

 gelatinous scaffolding in advance of more permanent formations. It also 

 seems likely that in a more condensed form they can function as com- 

 ponents of intercellular adhesives and the related basement membranes. 



