Proteoglycan histochemistry--a valuable tool for connective tissue biochemists.

The histochemistry of connective tissue proteoglycans (PGs) poses two major requirements, 1. for the specific demonstration of a given PG and, 2. at the ultrastructural level, for the examination of the shape of the PG, and its interactions with other molecules and tissue elements. Techniques for the localisation and identification of PGs are discussed, according to the principles behind their application. The strengths and weaknesses of antibody stains are compared with those of mini-molecular reagents, based on resolution, sensitivity, stoichiometry and tissue penetration. The concept of specificity is analysed in this context. The polyanionic characteristics of PGs are exploited to allow very sensitive detection and localisation by cationic probes, both macro- and mini-molecular. Complex formation by heavy metal cations, basic dyestuffs and polycations with PGs is a simple ion-exchange process (basophilia), which can be manipulated to give considerable specificity, e.g. using the critical electrolyte concentration (CEC) concept. The fundamental physical chemical unity of the phenomena of basophilia, CEC and biochemical fractionations of tissue polyanions by anion-exchange is discussed. This unity allows direct quantitative and qualitative comparisons to be made between histochemical and biochemical results at all levels from the tissue sample to the single molecule of PG in situ. The great value of the partnership between microscopy and analytical biochemistry is emphasised. The expanded solution domain of PGs collapses during formation and processing of the 'stained' complex, obscuring molecular detail and possibly resulting in translocation of the PG. Approaches aimed at restoring the initial situation, and their application to electron microscopy of PGs in tissues are outlined. Complexes of PG with ambient polycations may form, either as artefacts during processing or as an integral part of in vivo ultrastructure. Uptake of stain may be partly or totally blocked, in consequence. Ways of investigating, avoiding, or making use of this phenomenon are described. Application of integrated approaches to the study of PG--collagen and PG--elastin interactions in tissues are discussed. Specific interactions and stoichiometric relationship, particularly of dermatan sulphate proteoglycans with type I collagen have been observed, in skin, tendon, and other non-calcifying tissue, but not in bone.