PVP Mounting Medium
polyvinylpyrrolidone is usually referred to as PVP. It is a water soluble polymer which can form thick, syrupy solutions and dries into a clear film suitable as a mounting medium.
Mix the PVP and water to a paste and heat in a paraffin oven overnight or longer, stirring occasionally, until a smooth syrup is obtained. Add the glycerol and mix well. Leave in the oven until all bubbles have been eliminated. The solution is stable at room temperature for a long time (years). If it becomes too thick, add a small volume of water and mix well, then leave in an oven overnight. Preserve with a small crystal of thymol.
There are two structural formulas to the left. The top one is N-vinylpyrrolidone. PVP is a polymer of this compound, linked as illustrated by the bottom one.
Although this solution has a refractive index a little lower than others, it will increase as the mountant dries. After a few days it can approach that of resinous media. It is useful when greater clarity is needed than is given by the more commonly used aqueous media, for enzymes or IHC for instance. It is suitable for mounting sections stained for lipids, but is not suitable for sections stained to demonstrate metachromasia, whether of mucin or amyloid, as the dye is leached fairly rapidly.
When coverslipping, ensure that the coverslip is initially placed correctly as the medium exposed at the edge of the coverslip hardens rapidly and the coverslip may not be moved after a few minutes. After several months the film of mounting medium is prone to crazing in unprotected mounts. Important slides should be ringed and photographed for records before then.
Kiernan. J.A., (1999)
Histological and histochemical methods: Theory and practice, 3rd ed. pp.54.
Butterworth Heinemann, Oxford, UK.