Embedding media for cryomicrotomy: an applicative reappraisal

We reassess here the formulation of cryoembedding media in connection with recent developments in commercial cryomicrotomes. Water-based solutions of polyvinyl alcohols were our starting media, and each of 2 different polymers (56-98, MW ∼195000; and 6-98, MW ∼47000) showed a critical concentration for optimum sectioning. At higher or lower polymer concentrations, wrinkles and folds became apparent in tissue areas of sections, or in the sectioned embedding medium areas between tissues, respectively. Addition of polyethylene glycol (MW 380-420) further facilitated and improved sectioning, resulting in frozen tissue blocks that cut well in the 2 to 100 μm range and further, using disposable blades throughout. Applying a wide temperature differential between tissue specimen (-11°C to -13°C) and cutting knife (-33°C to -35°C), serial adjacent sections were reproducibly obtained at a 2-μm setting, singly or in short ribbons. Embedding media of low and high viscosity were obtained, depending on the polyvinyl alcohol polymer used, and could be applied sequentially for tissue infiltration followed by embedding with precise sample orientation. When required, media were made semisolid by addition of carboxymethylcellulose