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Q.With suitable diagrams, describe the process of oogenesis in human female. OR What is menstrual cycle? Briefly describe the various phases of this cycle.
Meghalaya MboseMBOSE Meghalaya Intermediate Board 2021Subjective· 5mImportance★★★★★
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Start your 14-day free trial to unlock the full solution →Oogenesis is the formation of the female gamete in the ovary: oogonia become primary oocytes (arrested in meiosis I until puberty), one of which completes meiosis I each cycle to give a secondary oocyte that is ovulated, completing meiosis II only upon fertilization.
Oogenesis — the process (described, as a diagram cannot be drawn here)
- Fetal stage — formation of oogonia: In the fetal ovary, diploid germ cells called oogonia multiply by mitosis. Most degenerate; the surviving ones enter meiosis and grow into primary oocytes, each of which gets surrounded by a layer of granulosa cells to form a primordial follicle. A female is born with all the primary oocytes she will ever have.
- Meiotic arrest: Each primary oocyte proceeds into prophase I of meiosis I but then arrests at the diplotene/dictyotene stage — this arrest lasts from before birth until puberty (and until that specific follicle is recruited in a menstrual cycle).
- Follicular growth (at puberty, each cycle): From puberty onward, in each menstrual cycle a few primordial follicles are activated; they grow through primary → secondary → Graafian (tertiary) follicle stages, developing a fluid-filled antrum, and are surrounded by the zona pellucida and corona radiata.
- Completion of meiosis I: Just before ovulation, triggered by the mid-cycle LH surge, the primary oocyte in the dominant Graafian follicle completes meiosis I, forming a large, cytoplasm-rich secondary oocyte and a tiny, non-functional first polar body (the unequal division conserves cytoplasm/nutrients for the future embryo).
- Meiosis II begins but arrests: The secondary oocyte immediately begins meiosis II but again arrests, this time at metaphase II.
- Ovulation: The Graafian follicle ruptures and releases this metaphase-II-arrested secondary oocyte (with its surrounding corona radiata) into the fallopian tube.
- Completion of meiosis II: Meiosis II is completed only if a sperm fertilizes the secondary oocyte, producing the mature ovum and a second polar body; the ovum fuses genetic material with the sperm to form the zygote. If fertilization does not occur, the secondary oocyte degenerates. …
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