The hepatocytes of the brown trout (Salmo trutta f. fario): a quantitative study using design-based stereology

A stereological study was performed on brown trout hepatocytes aiming to disclose whether there are basic gender differences when minimal levels of sex hormones exist, and also to establish a platform for both interspecific comparisons and physiological correlations. We used the so-called "design-based stereology" (with no shape, size or orientation assumptions) and also some new related statistics. Two-year-old brown trout were collected in April, and the livers were fixed by perfusion. From liver slicing to microscopical field selection, systematic sampling was used. Stereology was applied at light and electron microscopy. Target parameters were the relative and total hepatocyte number, the mean individual hepatocyte volume and surface, and also both relative and total volumes, and surfaces, either of organelles or of cell compartments. Observed variability was usually high, but the precision of estimates was proved to be globally adequate facing the true biological variation amongst specimens. Females had more hepatocytes per liver (1.79x10(9) vs. 1.12x10(9)). Considering the individual hepatocytes, whereas no gender differences were detected in the cell volume, males had higher values of nuclear volume (199 vs. 151 microm3) and surface (170 vs. 131 microm2), endoplasmic reticulum volume (1,300 vs. 824 microm3), and microvilli volume (82 vs. 54 microm3) and surface (1,445 vs. 975 microm2). However, when dealing with quantities per liver, gender differences were found only in the volumes of dense bodies (56 vs. 97 mm3) and of residual cytoplasm (169 vs. 341 mm3)--both volumes were higher in females. Functional implications of data are discussed, namely that females seem to have basic structural traits for coping with the later demands of breeding. Data also support that structural remodelling of hepatocytes occurs after breeding, urging to pursue seasonal studies (namely on lysosomes). We advanced the hypothesis that genders differ in microvilli surface just to maintain an optimal physiological surface-to-volume ratio. Interspecific similarities and differences were disclosed. For example, the number of hepatocytes/cm3 of parenchyma of brown trout was much lower than those reported in rainbow trout, but in both trouts females seem to have an higher cell number. In addition, when comparing the size of hepatocytes of brown trout with that from other fish and mammals it was suggested that major interspecific differences exist.