The Phase Contrast
- Preparation of Specimen
Perhaps the greatest benefit of light microscopy
is that there is no need to prepare the specimen before examination.
However, the problem is how to visualize the specimen, as most
specimens are colorless and transparent. For studying unstained
material, including living cells and tissues, light microscopes
with special optical systems, so called phase contrast microscopes
To prepare cells for examination in a phase contrast microscope,
the cells can be grown in a monolayer directly on a cover
slip bathed in cell culture medium. The cell culture medium
provides all the nutritional requirements for the cells to
divide and prosper. When the cells have reached the desired
population, the cover slip is removed from the cell culture
medium and mounted upside down on a microscope slide. The
specimen can then be viewed in the microscope.
To preserve the morphology of the cells or tissues, the specimen has to be
fixed. The best fixatives maintain the specimen in as near a living state
as possible. Drying or freezing the specimen are two methods but the most
commonly used fixatives are chemicals.
Thick specimens, such as tissues, have to be sectioned before they are mounted
on a microscope slide. Previously, the specimen was cut using a razor blade,
but the result was crude so better methods were developed. Today, a device called
a microtome, that automatically cuts thin slices of the specimen, is used. If
the specimen is frozen, a cryostat is used instead to keep the specimen in a
frozen state while slicing it.