Introduction
The production and maturation of gametes in vertebrates 1s tightly controlled by several hormones. Oocyte maturation refers to the process in which fully-grown oocytes become fertilizable. In fish, fully-grown oocytes are arrested in meiosis I and resumption of meiosis occurs during final oocyte maturation (Nalahama et al., 1995; Peyton and Thomas, 2011).
During oocyte maturation, the oocyte nucleus (germinal vesicle, GV) migrates from the center to the animal pole and subsequently, the nuclear envelope breaks down. This process is referred to as germinal vesicle break down (GVBD). The GV in immature fish oocytes is difficult to observe because the cytoplasm of the immature oocyte is opaque (Figure 1A). However, during final oocyte maturation, oocytes become translucent (Figure 1B). To determine if GVBD has occurred, follicles can be chemically fixed and observed under a dissection microscope equipped with a transmitted light. The GVBD can also be scored easily by observing if oocytes have become translucent.
It has been shown that three major factors are important in the induction of oocyte maturation including the gonadotropin luteinizing hormone (LH). When conditions are favourable for spawning, a surge in LH secretion by the pituitary, acts on the thecal cells of follicles to stimulate the production of 17a-hydroxyprogesterone (HP). HP is then converted into 17a,20ß -dihydroxyprogesterone, known as maturation-inducing hormone MIH) in the granulosa cells. Subsequently, MIH acts on the oocyte surface to activate maturation promoting factor, which in turn, induces final oocyte maturation Nagahama et al., 1995; Tokumoto et al.,
2011; see Lessman, 2009 for a review).
