Microtomy - The art of sectioning

Once the blocks are cooled they are ready for cutting. The correct name for this process is microtomy. On the right we can see a typical set-up for microtomy. In the foreground of the photo, is an electric cold-plate which is used to keep the blocks cool. It is much easier to section a cold block than a warm one. In the center we can see a warm water bath. On the right is a microtome. A microtome is, essentially, exactly the same as a butcher's meat slicing machine. The difference between these two types of machine is accuracy. A butcher's machine is designed to cut fresh soft meat in thickness of millimeters or centimeters while a microtome is designed to cut hard processed tissue at a thickness of only a few micrometers. Microtome, water-bath and cold plate.  Essentials!

The microtome mechanism slowly moves a block into the path of an extremely sharp steel knife. Modern blades are disposable and last for around 20 to 30 blocks. Previously, and where disposable blades are unavailable, large heavy dangerous knives were/are used and these must be resharpened regularly. The block and blade are moved closer together until sectioning begins and advances are then tiny - in the order of 3 - 5m. This results in a sliver of paraffin containing a slice of the tissue embedded in the block. In this picture you can see a "ribbon" of these sections floating on the water bath. The temperature of the water is around 40C so that the paraffin spreads a little, straightening folds in the tissue. If the water is any hotter, the paraffin melts and the tissue is lost.
Sections floating on the water

You can also see other sections which have been mounted onto glass microscope slides. Notice that all the sections appear white-grey. This is because they are yet to be stained. Before this can happen, the sections must be baked onto the glass slides for around twenty minutes at a temperature of about 60C. cheddar