Xenobiotics, xenoestrogens and reproduction disturbances in fish

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Abstract
Fish maturation and reproduction are complex biological processes that are regulated by endogenous substances (hormones), and synchronized by exogenous factors (photoperiod and temperature), thus ensuring that reproduction occurs at a time of the year optimal for survival of the offspring. The survival of any fish species is ultimately determined by the ability of its members to reproduce successfully in a fluctuating environment and thereby maintain a viable population. Several reports have documented that many compounds introduced into the environment by human activity either deliberately or unintentionally are capable of affecting reproductive processes in fish. Zonagenesis and vitellogenesis (eggshell protein and egg yolk precursor production, respectively) are two estrogen-regulated processes that are integral aspects of fish oogenesis. Several in vivo and in vitro studies have reported that some xenobiotics (xenoestrogens) possess the ability to mimic natural estrogens and therefore initiate precocious or unscheduled zonagenesis and vitellogenesis. Aspects of these effects and other xenobiotically-induced responses will be discussed here, with special reference to their possible consequences for fish populations.