Question

Explain how the concentration of the the hormones HP and MIH affect oocyte maturation in zebrafish

Answer 1

ANS.

Female zebrafish that have a full-grown ovary were anesthetized using 3-aminobenzoicacid ethyl ester . The ovaries were removed and follicles greater than 0.52 mm in diameter were collected, since previous studies in the zebrafish have shown that only follicles of this size can undergo maturation in vitro in response to hormones.

Approximately 20 follicles were placed into each well of a 24-well culture plate and pre-treated at 26°C in either 1 ml of modified Cortland's medium (MCM) or MCM + chemicals such as Actinomycin D or Cyclohexamide for 2 hours. Pre-treated follicles were then incubated with the control medium, human recombinant TGF-β1 hCG Maturation was scored after 18 hours of incubation. Follicles that underwent germinal vesicle breakdown (GVBD) could be identified by their acquired translucency.

The oocyte maturation of zebrafish is triggered by maturation inducing hormone (MIH, -Dihydroxy-4-pregnen-3-one). In almost all animals, the oocyte maturation is governed by activation of pre-MPF which consists of cyclinB and inactive Cdk1. In the oocyte of Xenopus and mice, the activity of Cdk1 is regulated in two ways, one is the interaction with cyclinB and the other is phosphorylation/dephosphorylation of T14/Y15 residues on the Cdk1 by Wee1 and Cdc25. Unlike Xenopus and mice that have a sufficient amount of pre-MPF, pre-MPF is absent in GV oocyte of most teleost including zebrafish.

Therefore, the activation of MPF during zebrafish oocyte maturation might totally depend on de novo synthesis of cyclinB proteins. It is reported that the translation of maternal mRNA is regulated by combination of several RNA binding proteins such as CPEB, Dazl, Pum1/Pum2, and insulin-like growth factor2 mRNA-binding protein 3 in the zebrafish oocytes. However, the definitive mechanism of these proteins to regulate the translation of stored maternal mRNAs remains to be elucidated. Therefore, the investigation of the maturation process of the zebrafish oocyte will provide new information that can help identify the role of translational control in early vertebrate oocyte maturation.