A proposed standard for histopathologic evaluation of medaka used in field and laboratory tests

Since histopathologic changes in tissues of fish are being scrutinized as biomarkers of exposure of fish to environmental contaminants, standards are desirable for the histopathologic evaluation of fish used as test systems in environmental toxicity studies. Medaka (Oryzias latipes) from studies over the past six years have been evaluated for toxic/carcinogenic effects after exposure to a particular chemical or a natural groundwater or surface water. Thirty tissues in each fish were examined in 5 micron, hematoxylin and eosin stained sections prepared from whole fish embedded in Paraplast{reg_sign}. The medaka were step-sectioned in a longitudinal plane such that two left paramedian, one mid sagittal, and two right paramedian sections were cut. A duplicate set of slides from the same areas was made in the event that special stains were required for diagnosis. Systematic evaluation of the same tissues in every fish resulted in a data base of spontaneous and exposure-related histopathologic lesions in all the systems of the medaka.