Ovarian follicle growth, maturation, and ovulation in teleost fish

Knowledge of ovarian follicle growth, maturation, and ovulation in teleosts is important to many aspects of basic and biomedical research, fisheries management, aquaculture, and environmental science. Follicle growth includes previtellogenic and vitellogenic stages. Although little progress has been made in our understanding of previtellogenic development, information on the regulation and mechanisms of vitellogenic growth has increased substantially in recent years. This new information pertains to the multiplicity of vitellogenin (Vg) genes, the oocyte Vg receptor, Vg processing enzymes (cathepsins) and their specific yolk products, and synthesis of various vitelline envelope precursor proteins in liver and ovary. Oocyte acquisition of lipids from sources other than Vg dominates follicle growth in many teleosts, especially perciformes, yet our understanding of the basic processes involved remains rudimentary. Considerable advances also have been made in our understanding of the endocrine regulation and mechanisms of ovarian follicle maturation. We have learned that ovarian follicle maturation involves a number of events including luteinizing hormone (LH)-dependent acquisition of oocyte maturational competence, LH induction of maturation-inducing hormone (MIH) synthesis, and MIH-dependent meiotic resumption (nuclear maturation) and cytoplasmic maturation. While much has been learned about mechanisms of maturational competence, MIH synthesis and meiotic resumption, our knowledge of cytoplasmic maturation is limited to descriptions of MIH-dependent yolk protein hydrolysis and associated oocyte hydration in a few species. It has become apparent that ovulation requires genomic activation regulated by nuclear MIH receptors, but the transduction pathways for the ovulatory MIH signal appear to be complex and are poorly understood.