Question

Why do hormones MIH and Hp contributes to a higher maturation rate in zebrafishes ?

Answer 1

Growth and maturation are the two major phases for ovarian development in fish. During oocyte growth, follicle stimulating hormone (FSH) stimulates the overy to produce of estradiol-17β which in turn stimulates the production of vitellogenin by the liver. This Vitellogenin is then further processed by the developing oocyte and cleaved to form yolk protein, which serves as a nutritional reserve for the developing embryo. This step of oocyte maturation in teleosts is triggered by the release of leutinizing hormone (LH) from the pituitary which in turn is known to activate several signalling cascades resulting in the production of 17α-hydroxyprogesterone (17α-HP) hormone. In the granulosa cells, under the action of 20β-hydroxysteroid dehydrogenase (20β-HSD), 17α-HP is converted to 17α, 20β-dihydroxyprogesterone (DHP), the maturation inducing hormone (MIH) in cyprinids, such as zebrafish and goldfish. MIH activates the cytoplasmic maturation promoting factor (MPF), which is made up of two subunits: cyclin B and cdc2 . MIH stimulates the de novo synthesis of cyclin B. Cyclin B protein binds to cdc2 to form MPF. The newly formed MPF is activated by phosphorylation of cdc2 on threonine 161. The active MPF, then, stimulates all the changes associated with oocyte maturation, such as germinal vesicle break down (GVBD), spindle formation, chromosome condensation and allows the transition from G2/M phase of meiosis.