Samples destined for viewing in the transmission electron microscope (TEM) must be cut into very thin slices. Allan Mitchell (Microscopy Otago) explains why this is important and describes how the design of the ultramicrotome (the cutting instrument) makes it possible to cut so thinly.
Point of interest: The ultramicrotome at Microscopy Otago cuts slices of samples using a gem-quality diamond (5000 carats). Remarkably, it can cut slices that are several times thinner than the wavelength of white light.
Ultrathin sectioning really means cutting the sections at 100 nanometres or thinner so that the electron beam is able to pass through it. 100 nanometres is 100 millionths of a millimetre – that is the same as four times thinner than white light. Another way of looking at it from a biological context is it’s the same thickness as five ribosomes.
To be able to cut something thin enough to be able to see something in the electron microscope, we use a special instrument called an ultramicrotome. The ultramicrotome has a specimen arm which moves up and down, and it has a knife carrier which holds a very special knife that is able to cut the sections so thin. The knife is made from a gem-quality diamond which has been polished to molecular sharpness, and this will allow our very, very thin slices to be cut from our hard resin block. We have to put a water trough behind the diamond knife for the sections to float out on, otherwise we cannot see them – they are too small – and they are too fragile to be handled.