The Phase Contrast Microscope
- 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 are used.

Growing Cells
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.

Fixing Cells
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.

Sectioning Specimen
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.