Histology and design-based estimation of hepatocellularity and volumes of hepatocytes in control and ethynylestradiol exposed males of platyfish (Xiphophorus maculatus).

The liver hosts numerous vital functions, such as biotransformation and excretion of xenobiotics. Synthetic oestrogens influence liver structure and function, leading to adaptations or to dysfunctions/injury. They are often stated to induce increases in fish liver weight, but there is controversy regarding how: if by changes in hepatocyte size (hypertrophy) and/or number (hyperplasia). Using platyfish as the experimental model, our primary aim was to assess if/how hepatocytes reacted to a sub-acute oestrogenic exposure. A complementary aim was to generate fundamental structural data for the liver of that model organism. Adult males were injected intramuscularly with 17α-ethinylestradiol (EE2) (25 μg/g), every 72 h for two weeks. Control fish were given solvent only. Body and liver morphometry were registered, and hepatocytes examined through histology and stereology at light microscopy. Immunohistochemistry evaluated hepatocytic vitellogenin (VTG) content. Treated and control fish did not differ as to quantitative parameters. Nevertheless, exposed fish were sensitive to EE2. VTG tagging was positive in their hepatocytes and these tended to be more basophilic, though not fully oestrogenized. We hypothesise that the platyfish liver is not particularly sensitive to the disrupting action of EE2 because of its reproductive mode; with no production peaks of VTG and no huge changes in endogenous sex-steroids. The fish may have had no evolutionary pressure for hepatocytes to be particularly reactive to oestradiol (E2). In the end, this study offers the first unbiased estimation of the liver cellularity in the platyfish, as well of the hepatocytic volume, serving now as a baseline reference.