During each menstrual cycle

During each menstrual cycle - zona pellucida and corona...

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During each menstrual cycle, one primary oocyte, enclosed in its follicle, resumes  meiosis I to produce two daughter cells (each haploid with 23 chromosomes). One  daughter cell, the secondary oocyte, contains most of the cytoplasm, ensuring that  adequate amounts of stored food, as well as mitochondria, ribosomes, and other  cytoplasmic organelles, will be available for the developing embryo. The other daughter  cell, a first polar body, is much smaller and contains little cytoplasm and few if any  organelles. The secondary oocyte then begins meiosis II (equatorial division) but again  stops at prophase (this time prophase II). The first polar body may also begin meiosis II,  but it will eventually degenerate. Ovulation occurs when a secondary oocyte and its first polar body, surrounded by the 
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Unformatted text preview: zona pellucida and corona radiata, rupture from their mature follicle and are expelled from the surface of the ovary. The oocyte is then swept up into the uterine (fallopian) tube and advances toward the uterus. If a sperm cell penetrates the corona radiata and zona pellucida and enters the secondary oocytes, meiosis II resumes in the secondary oocytes, producing an ovum and a second polar body. If a first polar body is present, it too, may resume meiosis II, producing daughter polar bodies. Fertilization occurs when the nuclei of the sperm cell and ovum unite, forming a zygote (fertilized egg). Any polar bodies present ultimately degenerate...
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