Advances in reproductive endocrinology of fish

In teleosts two gonadotropins (GtHs) are produced: GtH I which stimulates steroidogenesis and incorporation of vitellogenin into the oocytes and GtH II, which stimulates steroidogenesis in the last stage of maturation and ovulation. Synthesis and release of GtHs are under control of gonadotropin releasing hormones, growth hormone, gonadotropin releasing inhibitory factor (GRIF)--dopamine, neuropeptide Y (NPY) gamma-aminobutyric acid (GABA) and melatonin. It was also found that calcium ions play a role of an intracellular mediator in GtH release. In ovaries, GtH stimulates production of 17 alpha-hydroxyprogesterone in the thecal cells. This steroid is transferred to granulosa cells where it is converted to 17 alpha 20 beta dihydroxy-4-pregnen-3-one (17 alpha 20 beta DHP). This steroid acts on the oocyte surface causing an appearance cytoplasmatic maturation promoting factor (MPF) which initiates the nuclear membrane breakdown and a subsequent cell division in both mitosis and meiosis. Teleosts are the only animals in which it is possible to change sex and to have population of fish of one sex. This possibility is widely applied in fisheries practice.