Microtomes are cutting instruments for the preparation of preparations used in microscopy. To meet the high demands of such preparations, microtomes allow extremely fine cuts. Normally modern microtomes allow cuts of a thickness of 0.1 to 100 μm. In a comparative way: Human hair has a thickness between 50 and 70 μm.
The history of the microtomes began with the beginning of light microscopes. In order to analyze objects, they had to be thin enough for light to penetrate them. The first microtomes were in the beginning simple blades (usually razor blades) with which cuts were made manually. As the demands on the preparations were increasing, it was necessary for the microtomes to develop.
The first microtomes, as we know them today, were developed in 1770. With these you could fix the test and adjust the thickness of the cut by means of screws. Nowadays, the mechanical microtomes are composed of a block, a sample holder and technical equipment for the control of the advance.
The quality of the preparations depends on the type of advance, the geometry of the blade and the declination (angle between the blade and the cutting direction). In addition, the result can be influenced in the preparation of the sample (for example by freezing). In addition to the mechanical microtomes, laser microtomes are now increasingly used, with which it is possible to prepare samples without contact.
Here is the operation of some microtomes:
Sliding microtomes
These microtomes are composed of a fixed sample holder and a blade that is fixed on a slide. To guarantee a stable cut, normally the sliders of the microtomes usually weigh a lot. During cutting the blade is pressed through the sample. Sliding microtomes allow cuts with a thickness of 1 to 60 μm.
Advantage:
- • Due to its design, it causes few breakdowns.
- • It allows to regulate exactly the pressure of the blade on the fabric.
- • Because of the size of the blade, it is possible to section large size head blocks.
- • By the arrangement of this blade allows cutting blocks included in celloidin.
Disadvantages:
- • Does not allow serial cuts, which slows down the process.
- • Exposure of the blade can cause accidents
- • It is almost impossible to obtain sections with a thickness of less than 8 microns.
Rotating microtomes
These microtomes, also known as Minot microtomes, have a fixed blade and a mobile sample holder. The name of the rotation microtome is given because the sample holder is activated by a handwheel. The movement of rotation of the steering wheel becomes a straight movement. Normally the sample holder of these microtomes moves in the downward direction. The prepared samples accumulate on the blade. The advantage of these microtomes is that the high mass of the flywheel matches the different hardness in the same test, resulting in a uniform cut. The rotation microtomes allow to prepare samples between 1 and 60 μm.
Advantage:
- • By having more weight, it has more precision, it allows obtaining very thin serrated sections.
- • The advance mechanism is more accurate.
Disadvantages:
- • The high price due to the complexity of the advance mechanism, which also makes repairs more difficult and expensive.
- • The impossibility of cutting with it tissues included in celloidin, in gelatin and in propylene glycol.
Freezing microtomes
Freezing microtomes are a subcategory of rotation microtomes. The test is in a freezer container which is cooled, for example, with nitrogen. The low temperature increases the hardness of the test.
Ultramicrotomes
With the ultramicrotomes, samples are prepared for the transmission electron microscopes. Because the preparations must be extremely thin, these microtomes have special blades and a very fine advance, which is often driven by thermal expansion. The use of these microtomes allows a thickness of 10 to 500 nm.
Laser microtomes
Laser microtomes use a special laser for cutting. They stand out for their strong focus and their very short durations of momentum. This allows the tests to be cut very thinly without causing thermal damage to the test material. These microtomes allow to prepare samples with a thickness between 10 and 100 μm.
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