Microtomy << my KROT uh mee >> originally meant microscopic cutting. But it now means the art of preparing objects for examination with a microscope. Without preparation, few objects can be properly examined with a microscope (see Microscope). A piece of metal, for example, must be highly polished and etched before its structure can be seen with a microscope.
Scientists prepare biological materials either as smears, squashes, wholemounts, or sections. Smears are made by applying a thin layer of blood or other organic fluid to a microscopic slide. Technicians dry and stain the layer so the cells can be seen. Geneticists make squashes by crushing cells in order to see the number and shape of the chromosomes (see Chromosome). Wholemounts are prepared from whole microscopic organisms that are killed in a fixative to keep their shape. They are then stained. Alcohol removes the water, and clove or cedar oil makes the objects transparent. Technicians next mount the objects in a drop of resin on a glass slide, which they cover with a glass coverslip about 1/5,000 inch (. 005 millimeter) thick.
Scientists study plant and animal tissues in sections about 1/2,500 inch (. 01 millimeter) thick. The tissues are hardened and dried out, then soaked in wax and shaped into blocks. The wax supports the tissues so they can be sliced into sections on a microtome (see Microtome). The sections are cemented to slides with egg white and the wax is dissolved. They are then stained and preserved under a coverslip. An ultramicrotome is used to prepare specimens for the electron microscope. It cuts sections about 1/1,300,000 inch (. 02 micrometer) thick. The specimens are so thin that the electron beam of the microscope can penetrate them.
