The brown trout (Salmo trutta) is a commercially and ecologically significant salmonid fish, yet its hepatic cellular and functional dynamics throughout the reproductive cycle remain poorly characterised, particularly in males. This study investigated seasonal and sex-specific liver plasticity across four reproductive stages: spawning capable (December), regressing (March), regenerating (July), and developing (November). We quantified mRNA and protein abundance of key oestrogen-responsive targets—vitellogenin (VtgA) and zona pellucida (ZP) proteins—alongside cell turnover markers, caspase 3 (Casp3) and proliferating cell nuclear antigen (PCNA). These molecular endpoints were integrated with stereological analyses to estimate hepatocyte, nuclear, and cytoplasmic volumes. Results revealed stage-dependent mobilisation and transient hepatic retention of reproductive proteins; females exhibited stronger vitellogenic signatures and more pronounced seasonal shifts than males. Although VtgA and ZP mRNA levels peaked during the developing and spawning-capable stages, males maintained low but consistent levels throughout the cycle, indicating constitutive hepatic oestrogen sensitivity. Regarding cell turnover, PCNA protein data indicated heightened proliferative activity during the spawning-capable and regressing stages. In contrast, while Casp3 mRNA levels remained stable across all stages, protein detection suggested a post-transcriptional increase in apoptotic signalling during the developing phase, consistent with controlled tissue remodelling rather than extensive cell loss. Stereological data confirmed enlarged hepatocyte and nuclear volumes during periods of high secretory and proliferative demand. Overall, these findings demonstrate significant stage-dependent and sex-specific plasticity in brown trout liver, providing a robust reference framework for ecological monitoring, endocrine disruption assessments, and studies of teleost reproductive physiology.

Introduction: The liver is a central metabolic regulator responsible not only for energy storage and allocation but also for the production of essential egg proteins. In seasonal breeders such as brown trout, physiological demands increase during spawning, requiring strong metabolic and structural adaptations. Understanding the cellular mechanisms underlying these fluctuations is essential for clarifying how liver function is regulated during reproduction. Objective: The aim of this study was to investigate cellular mechanisms underlying liver remodelling during the reproductive cycle of male and female brown trout by examining proliferation and apoptosis markers at both molecular and protein levels during four distinct reproductive stages: spawning capable, regressing, regenerating, and developing. Methodology: Adult male and female brown trout were sampled at four reproductive stages. Liver gene expression of proliferating cell nuclear antigen (PCNA) and caspase-3 was analysed using qRT-PCR. At the protein level, liver tissue sections were immunolabelled with antibodies against PCNA and caspase-3. Caspase-3 staining intensity was quantified using ImageJ, while PCNA expression was assessed by counting positively stained nuclei. Results: PCNA gene expression was upregulated during the regressing stage, while PCNA-positive nuclei immunolabelling increased during the regressing and spawning-capable stages. Caspase-3 transcript levels showed no significant differences among stages, whereas immunostaining increased during regenerating and developing stages. Conclusions: Overall, the liver undergoes dynamic cellular remodelling throughout the reproductive cycle, balancing proliferation and apoptosis to meet reproductive demands. The results suggest that proliferation is more evident during regenerating stages, while apoptotic activity may be regulated post-transcriptionally. PCNA also indicates that proliferative activity may persist during spawning capable stages, reflecting ongoing mitotic readiness despite reduced overall proliferation.