Ultramicrotomy is a method for cutting specimen into extremely thin slices or sections, that can be viewed in a transmission electron microscope (TEM). Sections must be very thin because the 50 to 125kV electrons of the standard electron microscope cannot pass through biological material much thicker than 150nm. For best resolutions, sections should be from 30 to 60nm. This is roughly the equivalent to splitting a 0.1mm thick human hair into 2,000 slices along its "Y" axis, or cutting a single red blood cell into 100 slices. Ultramicrotomy is a demanding technique that requires much practice and patience.

Ultramicrotomy process

There are many equipments involved in the ultramicrotomy process. "Thin" sections, meaning sections from 50 to 100nm thick are able to be viewed in the TEM. Semithin or "thick" sections range from 0.5 to 2µm, and are almost 10 to 20 times thicker than "thin" sections. These thick sections also known as survey sections are viewed in a light microscope to determine whether the right area of the specimen is in a position for thin sectioning. It is a very common practice to view the thick section in the light microscope first before proceeding with ultramicrotomy or thin sectioning.
A small sample about the size of a head of a pin, is taken from the specimen to be investigated. Specimens may be from living matter, like human tissue, animal or plant, or from inorganic material such as rock, metal, magnetic tape, plastic, film, etc. The sample block is first trimmed to create a block face 1 mm by 1 mm in size. "Thick" sections (1 μm) are taken to be looked at on an optical microscope

Optical microscope

The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...

. An area is chosen to be sectioned for the TEM and the block face is re-trimmed to a size no larger than 0.7 mm on a side. Block faces usually have a square, trapezoid, rectangle or triangle shape. Finally, thin sections are cut with a glass or diamond knife using an ultramicrotome and the sections are left floating on water that is held in a boat or trough. The sections are then retrieved from the water surface and mounted on a copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, nickel

Nickel

Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

, gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

or other metal grid. Ideal section thickness for transmission electron microscopy with accelerating voltages between 50kV and 120kV is about 50–100 nm.

Advances

It was in 1952, that Fernandez-Moran introduced cryo ultramicrotomy, which is a similar technique but done at freezing temperatures between -20 and -150°C. Cryo ultramicrotomy can be used to cut ultrathin frozen biological specimens. One of the advantages over the more "traditional" ultramicrotomy process is speed, since it should be possible to freeze and section a specimen in 1 to 2 hours.