Fish Diseases and Management

7.1.4.Histological techniques

Histology is the study of tissues by microscopic examination of thin, stained tissue sections in order to study their structure and function. Histopathology on the other hand tries to determine changes that occur in the tissues due to pathogens and disease. Histopathology can be used as a diagnostic tool for identifying a variety of pathological conditions associated with many of the disease conditions. Therefore, histopathology plays a central role in disease diagnosis. In analyzing the histopathology of an infection, stained fish tissue sections are prepared and examined by light microscopy for changes resulting from infectious or non-infectious disease. Immunohistochemical methods are also used to detect specific pathogens in tissue sections.

For histological analysis, fish tissues are put in aqueous fixative, which preserves the morphology of tissues and cells, so that they withstand further preparatory steps without change. Tissues should be fixed immediately after death of fish to avoid disintegration of tissues or cells by the action of their own enzymes. After fixation, tissues are gradually dehydrated using a graded series of alcohol. The tissues are then subjected to clearing by a liquid that mixes completely with both alcohol and wax, the embedding agent. The tissues are then embedded in molten paraffin wax and the blocks are cooled to harden the wax blocks are subjected  section for cutting using a microtome. The thin sections (3-5 $$\mu$$m) was made and then spread on warm water bath and mounted onto glass microscope slides. The wax is removed from the sections before staining.

The tissue sections on the microscopic slides are stained using haematoxylin and eosin (H&E) stain, which is able to reveal a wide range of different tissue components. Depending on the specific requirements, various stains are used for differentiating different pathology of infecting agents.