Sectioning polyester wax block is unique due to the extra step taken to prepare the slide. For the tissue to adhere to the slides during immunochemistry experiments, the slides are coated with chrome alum gelatin. The extra coating allows for the sections to bind to the slides more securely and prevents them from falling off during heated incubations such as the antigen retrieval steps. This is necessary because polyester wax sections do not produce static electric ribbons that paraffin sections do. While most people spend extra on the convenience of electrostatically charged slides for their paraffin sections, we find it more economical to utilize inexpensive slides and coat them in gelatin to get the same effect.
Chrome alum gelatin solution is made by dissolving 3.85g of porcine gelatin in 750ml dH2O heated to about 40-50°C. Once the gelatin is dissolved, add 0.375g of chromium potassium sulfate and then filter through a Whatman No. 1 filter paper while it is still warm. The cleaned and dried slide racks are dipped once for 2 minutes and placed in the 4°C fridge for 30 minutes, then dipped briefly again with 30 minutes in the fridge, followed by another brief dip and stored in the fridge overnight. A total of three coatings is used to maximize the tissue adherence effect. The slides are usable after removing them from the fridge and allowing them to settle for at least a day.
Before sectioning, we must note that polyester wax is a soft medium compared to paraffin wax. Therefore, it is slightly more difficult to section. However, just like paraffin wax blocks, cooling the blocks on a cold plate with a wet icy surface helps even more when cutting the specimen. The cold wax provides support to the tissue and allows for thinner sections to be cut. The moisture penetrates the block face, swelling the tissue and preventing dehydrated tissues from crumbling too much during cutting.
While there are many technical aspects to consider when cutting, such as maximizing blade life or trimming and facing the blocks, the most important things to consider are the thickness of the sections and the consistency of getting the same thickness. The following are fundamental steps to help you produce quality and consistent sections:
- Make sure wax blocks are chilled and re-chilled every time a new ribbon is cut.
- Designate a portion of the blade for facing or course trimming only.
- Be consistent with the timing and the speed of the cutting stroke.
- Do not stop and restart cutting while collecting a ribbon of sections as it will produce different thickness.
- The first couple of sections are usually thicker so only take sections in the middle of the ribbon.
- Collect sections in the ribbon of the same shade of transparency as this indicates multiple sections with the same thickness.
Due to the difference in melting temperature, the polyester wax section is floated out at a different temperature compared to paraffin section. The temperature that works the best for us is about 10°C below the melting point of the wax as it will give us plenty of time to see the tissue expand to its original flat dimension. In the case of polyester wax sections, room temperature water works just fine. Instead of using a warm water bath to float the paraffin sections, droplets of water are poured on top of the coated slide and polyester wax sections are then floated on top of the slides directly (shiny side down). Once flattened, the water is removed using a Kim-wipe at an angle. The slides are dried at an angle and then incubated overnight at the melting point of the wax before using for staining or immunohistochemistry.