20. Amphibian oocytes remain for years in the diplotene stage of meiotic prophase. Resumption of meiosis is initiated by
    (1) gonodatropic hormone. (2) growth hormone.
    (3) oestrogen.                                       (4) progesterone.

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Introduction

Amphibian oocytes arrest for prolonged periods at the diplotene stage of prophase I during their growth phase. This arrest can last for years until hormonal cues initiate meiosis resumption, allowing the oocyte to mature and become fertilization competent. Among these cues, progesterone is the primary hormone responsible.

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Role of Progesterone in Meiotic Resumption

• Progesterone is secreted by the follicular cells surrounding the amphibian oocyte in response to luteinizing hormone (LH) signals.

• It acts on the oocyte to trigger a cascade of intracellular events leading to activation of maturation promoting factor (MPF), a key protein complex controlling entry into the meiotic M-phase.

• Activation of MPF results in germinal vesicle breakdown (GVBD), chromosomal condensation, and progression beyond the diplotene arrest.

• Progesterone promotes the translation of specific mRNAs such as c-mos, which regulate MPF activation and meiotic progression.

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Supporting Biological Mechanisms

• Studies in Xenopus laevis and other amphibians show that progesterone acts via a non-genomic pathway involving signal transduction and protein synthesis to trigger meiosis.

• Other steroids can mimic progesterone’s action, but progesterone remains the most potent physiological inducer in amphibians.

• Progesterone-induced maturation prepares the oocyte to complete meiosis and ovulate as a mature egg.

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Why Other Hormones Are Less Relevant

• Gonadotropic hormones like LH stimulate follicle cells to produce progesterone but do not directly resume meiosis in the oocyte.

• Growth hormone and estrogen have no direct role in resuming meiosis in amphibian oocytes.

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Conclusion

Progesterone is the key steroid hormone initiating the resumption of meiosis in amphibian oocytes arrested at diplotene. This hormonal trigger transitions oocytes into the maturation phase necessary for fertilization and embryo development.

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This detailed overview highlights progesterone’s essential role in amphibian reproductive biology, providing insights into oocyte maturation and meiotic control mechanisms.