The rotary microtome is the most common instrument found in a histology laboratory. Although most microtomes are manual, some are automatic or semi-automatic, where the advancement of the block and speed of cutting are controlled by a foot pedal or a digital keypad at one's fingertips. Automatic and semi-automatic microtomes greatly improve ergonomics by reducing repetitive stress on joints. Microtomes have become more precise and easier to use since the first versions. Although a good microtome can last decades, most laboratories are equipped with modern microtomes with current design innovations.
Microtomes are very heavy, weighing 40 to 60 pounds. This is to reduce vibration during microtomy, in which stability is critical during sectioning to prevent undulations (washboarding) in the paraffin sections. Daily cleaning from paraffin debris and yearly preventive maintenance will keep a microtome cutting optimally for many years.
The main components of a rotatory microtome are described below. Although some microtomes have more bells and whistles, the standard microtome remains relatively simple to operate.
- Microtome base plate or stage: A platform which has rails that secure the knife holder base.
- Knife holder base: A part that anchors the knife holder to the microtome stage. The knife holder base can be moved toward or away from the block, but MUST be stationary and locked during microtomy.
- Knife holder: This part is comprised of several components including the blade clamp that holds the blade, the knife tilt for adjusting the knife angle, and the face plate that guides that ribbons away from the blade and towards the operator.
- Cassette clamp or block holder: Holds the paraffin block in place. Typically, the block moves up and down with each revolution while the blade is stationary. The block holder may have knobs that allow the user to manipulate the block face in various directions to bring the tissue in alignment with the blade.
- Coarse handwheel: Moves the block holder either toward the knife or away from the knife.
- Advancement handwheel: Turns in one direction and advances the block toward the knife at the specified microns. Most handwheels are equipped with a safety lock to prevent the wheel from releasing and having the block holder come down towards the blade while a block is inserted or removed. The safety lock should be used anytime the microtomist is not actively sectioning paraffin blocks.
- Micron adjustment: Micron settings for section thickness can range from 1 to 60 microns on most microtomes.
After becoming familiar with the major components of a microtome, we can examine the knife angles in detail.