Polyester Wax

Polyester wax is a low melting temperature (37°C) wax used as an embedding medium for light microscopy. It was developed by Dr. H.F. Steedman as an alternative to paraffin wax (1). In exchange for the extra steps taken during sectioning and slide coating, it allows for heat-sensitive proteins to be preserved for immunostaining methods. In addition, some notable advantages of using polyester wax medium were made by Sidman et al. (2) that the sections fragment less, hard tissues do not split away, and ribbons do not generate static electricity.

Making polyester wax entails mixing poly-ethylene-glycol (PEG) distearate 400 and cetyl alcohol (1-hexadecanol), melted at a temperature of 45°C. The wax is poured into 50ml tubes, allowed to solidify at room temperature, and stored at 4°C until needed to process tissue.  Polyester wax preparations are commercially available, but we find we get better and more consistent results when we make it ourselves.  

We use polyester wax largely due to the vastly improved immunolocalization compared to paraffin, likely due to its low melting temperature. Tissues are only heated to 37°C to infiltrate vs 56-58°C necessary for paraffin infiltration.  With polyester wax sections, we often detect antigens that are undetectable in paraffin sections and we typically can dilute primary antibodies 100-fold compared to what we need for immunolocalization for paraffin sections.  This not only conserves expensive or scarce primary antibodies but also diminishes non-specific labeling.   

The increased sensitivity of antigen detection in polyester wax sections comes at a cost.  Compared to paraffin embedding, polyester wax is expensive, not only due to the cost of the reagents, but also the labor to make the wax and embed manually. Many people find sectioning polyester wax somewhat more difficult than sectioning paraffin, largely because the polyester wax is softer at room temperature than paraffin. Polyester wax sections will not adhere to conventional slides; they require slides triple-coated with chrom-alum gelatin, which are not commercially available but must be made in the lab.  Because of the gelatin coating, polyester wax sections cannot be used in procedures that require protease digestion for antigen retrieval or in situ hybridization.

Nonetheless, we find using polyester wax so much better for immunolocalization that it is worth the expense and effort to use it for virtually all our immunohistochemistry work.

References
  1. Steedman, H.F. (1957). Polyester Wax: A New Ribboning Embedding Medium for Histology. Nature 179: 1345.
  2. Sidman RL, Mottla PA and Feder N (1961). Improved polyester wax embedding for histology. Stain Techn 36: 279-284