Histological examination is important to detect and differentiate among different types of tissue changes at the microscopic level. An enlarged organ can be hyperplastic, hypertrophied, or neoplastic. It can also be merely inflamed or infected. Only a histological examination can differentiate among the various growth disturbances and pathological changes at the microscopic level. Histopathology is indispensable when neoplasia induced by transgenes is the subject of study (1-4).

Tissues removed from the animal need to be processed before they can be examined under the microscope. The steps involved are fixation, dehydration, clearing, impregnation, embedding, sectioning, and staining. The aim of fixation is to prevent tissue decay, and maintain tissue and cellular structure for study. Tissue should be treated as soon as possible after removal. The amount of fluid used should be at least ten times the volume of the tissue. The most common fixative used is 10% (v/v) buffered formalin. The aim of dehydration is to remove water from the tissue for the later step of paraffin embedding. Clearing involves replacing alcohol with a wax solvent, since alcohol and wax are not miscible. Toluene is a suitable reagent for this purpose. Impregnation of the tissues supports them when they are sectioned. Tissue is finally embedded with molten paraffin wax and sectioned.

The hematoxylin and eosin (H and E) stain is routinely used for preliminary examinations. This is usually sufficient to give initial information on the nature of the lesion. For further analysis of tissue sections, various types of special staining can be employed ranging from staining for tissue components and microorganisms to immunohis-tochemical staining.

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