Specimen preparation for Microscopy

The biggest problem in any form of microscopy is specimen preparation. In biology this is impossible to achieve without changing the specimen in some way. To minimize this problem many ingenious methods have been devised. Observing living specimens is of course the best way and this can be done with small animals such as protozoa, bacteria and microscopic plants. Living cultures of human and animal cells can also be studied. The specimen is trapped in a small amount of liquid between two glass surfaces and can be observed in more or less natural conditions. But after a time the oxygen becomes depleted and the population changes. But it is possible to study these changes and gain valuable micro-ecological insight. Smears of blood, bacteria and other small specimens can be mounted on a slide, fixed, stained and examined directly without much difficulty. But in general biological specimens require careful preparation.

One of the oldest methods of preparing tissue samples from plants, animals or humans is by making sections on a microtome. A microtome is an instrument that cuts very thin slices of tissue that can be examined in the microscope. To do this the tissue is first fixed by chemical means, embedded in wax or a plastic material and then sliced in the microtome. The wax is removed and the tissue is stained to reveal the various cellular structures, nuclei, cell walls, chromosomes, mitochondria and other parts of the cell. For electron microscopy Ultra-microtomes are used that can cut sections as thin as 40 Nanometers (40 millionths of a millimeter) or even less.

A microtome for cutting wax embedded biological specimens. This instrument is strong enough to cut bone.

 

 

Geologists who wish to examine thin sections of rock first make slices with a diamond saw and then grind and polish them down to fractions of a millimeter in thickness. The mineral sections are also embedded in plastic to support them while they are prepared. Metals may be thinned by electrolysis or by a beam of ions in a vacuum chamber.