The hepatocytes of the brown trout (Salmo trutta f. fario): A quantitativestudy using design-based stereology

A stereological study was performed on brown trout hepatocytes aiming to di sclose whether there are basic gender differences when minimal levels of se x hormones exist, and also to establish a platform for both interspecific c omparisons and physiological correlations. We used the so-called "design-ba sed stereology" (with no shape, size or orientation assumptions) and also s ome new related statistics. Two-year-old brown trout were collected in Apri l, and the livers were fixed by perfusion. From liver slicing to microscopi cal field selection, systematic sampling was used. Stereology was applied a t light and electron microscopy. Target parameters were the relative and to tal hepatocyte number, the mean individual hepatocyte volume and surface, a nd also both relative and total volumes, and surfaces, either of organelles or of cell compartments. Observed variability was usually high, but the pr ecision of estimates was proved to be globally adequate facing the true bio logical variation amongst specimens. Females had more hepatocytes per liver (1.79x10(9) vs. 1.12x10(9)). Considering the individual hepatocytes, where as no gender differences were detected in the cell volume, males had higher values of nuclear volume (199 vs. 151 mum(3)) and surface (170 vs. 131 mum (2)), endoplasmic reticulum volume (1300 vs. 824 mum(3)), and microvilli vo lume (82 vs. 54 mum(3)) and surface (1445 vs. 975 mum(2)). However, when de aling with quantities per liver, gender differences were found only in the volumes of dense bodies (56 vs. 97 mm(3)) and of residual cytoplasm (169 vs . 341 mm(3)) - both volumes were higher in females. Functional implications of data are discussed, namely that females seem to have basic structural t raits for coping with the later demands of breeding. Data also support that structural remodelling of hepatocytes occurs after breeding, urging to pur sue seasonal studies (namely on lysosomes). We advanced the hypothesis that genders differ in microvilli surface just to maintain an optimal physiolog ical surface-to-volume ratio. Interspecific similarities and differences we re disclosed. For example, the number of hepatocytes/cm(3) of parenchyma of brown trout was much lower than those reported in rainbow trout, but in bo th trouts females seem to have an higher cell number. In addition, when com paring the size of hepatocytes of brown trout with that from other fish and mammals it was suggested that major interspecific differences exist.