Alcianblue/PAS or PAS/alcianblue? Remarks on a classical technic used in carbohydrate histochemistry

The combined alcian blue (AB)/PAS technic is widely used for the detection and characterization of mucosubstances in tissue sections. Mostly the sequence AB/PAS is used, occasionally also the reserved sequence PAS/AB. The present study shows clearly that the sequence of the combined technic, i.e. AB/PAS or PAS/AB is substantially influencing the results. So it could be demonstrated that by using the combination PAS/AB originally PAS-positive and AB-negative reacting mucosubstances become AB-positive. This could be caused by periodic acid oxidation followed by addition of hydrogen sulfite to aldehyde group thus providing secondary basophilic resp. AB positive material.