A microtome is a tool that can be used to cut a slice of tissue that is so thin that it can be used for light or electron microscopy. The slice is called a coupe and must be about 10-15 µm thick for plant tissue and 1-7 µm thick for animal tissue (1 µm = 1 micrometer = 1 thousandth of a millimeter).
As already described on the Histology page, the tissue is usually molded in paraffin (for ultra-thin cuts, a synthetic resin is used). This embedded tissue is mounted on a stamp, block of wood or cassette and clamped to the microtome. A particularly sharp knife is then used to cut slices of tissue. There are different types of microtomes:
- hand microtome
- rotational microtome
- sledge microtome
- cryostat (freezing microtome)
A hand microtome is a simple microtome that can be used for some preparations. Especially plant cuttings (stems, buds and leaves) can be cut reasonably well with it. On this site you can see some specimens that are cut this way. The big disadvantage of this type of microtome is that very thin and regular cutting is almost impossible, so the specimen soon turns out to be unusable. In a coupe of plant material this is less annoying and therefore useful for plant material. An additional advantage of this microtome is the purchase price. It is just under 100 euros and can be ordered at e.g. Euromex.
Rotational Microtome, (Text from fellow microscopist Yvan Lindekens)
Rotational microtomes are probably the most widely used devices in zoological microtechnology. The rotational microtome was invented at the end of the 19th century by the American Pfeifer (1883). Three years later Charles Minot came up with an analogue but better machine. Since then the machine is also called "Minot-microtome". The working principle of the rotational microtome is essentially simple. Its practical realisation is by no means that. The circular motion of turning the microtome's hand wheel is transformed into a straight-line motion, which moves the object holder up and down with the object to be cut. At the same time, each movement cycle moves the object holder a preset distance towards the knife. In other systems, the knife moves towards the object to be cut. The microtome knife is located perpendicular to the upward and downward movement of the object holder. With each downward movement, a slice of the object is cut off in this way. Rotational microtomes are almost impossible to beat when it comes to the speed at which an object can be cut. Another important advantage is that by heating up the paraffin when the paraffin block touches the knife, the sections stick together. As a result, whole series of coupes can be fitted under the same cover glass, making it easy to study them in their interrelationship. Other methods of cutting can do the same, but are considerably more complicated.
The paraffin samples on this site have been cut with this type of microtome. It is a fairly inexpensive device (is occasionally offered for sale on e-bay for
≈ € 500,-) and gives fantastic cutting results for the amateur.
The plastic preparations on this website are cut with an electrically driven microtome with retraction which is a bit more expensive to buy. This microtome is the LKB Historange 2218.
By clicking on one of the images below a video clip will be shown.
Current professional versions of the rotational microtome look like the images below. On the left a manually operated microtome, on the right an electrically driven microtome. However, they all work according to the same principle: tissue moves along a stationary knife, tissue shifts a set thickness and another cut etcetera follows.
There are two types of knives available for this type of microtome: the fixed knife and the disposable knives.
Fixed knives come in many types with many cutting angles and the like. look at Downloads for instructions. Sharpening knives is an art to be learned. Sharpening knives is easy but of course quite expensive. The cheapest solution is to use disposable knives. This requires a special knife holder. The author has very good experiences with Leica Microsystems knives (see picture).
In the case of a sledge microtome, it is not the object that moves, as in the case of a rotational microtome, but the blade. This type of microtome is very suitable for cutting woody plant parts.
Cryostat (freezing microtome),
On a freezing microtome, frozen tissue is quickly cut with a glass knife. A major advantage of this type is that the tissue requires little preparation. It does not need the long dehydration times as with paraffin techniques. The freezing technique is used in some surgical procedures because the results of microscopic examination are available to the surgeon very quickly.
This type of microtome is used for making ultra-thin cuts suitable for light and electron microscopy. Tissue here is not poured into relatively soft paraffin but soaked with a much harder synthetic resin. Coupes are cut with a glass or diamond knife.