61. Animal Devel I 10 - Animal Development Fertilization...

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Animal Development Intricate acrosomal reaction induces influx of sodium and depolarization of the egg membrane. This is a “fast block” to polyspermy. The cortical reaction is a slow block. Both ensure fertilization occurs by correct, and fully functional, sperm Fertilization The cortical reaction is caused by a spike in [Ca 2+ ] occurring as a wave. [Ca 2+ ] is a common signal in cells. In early evolution it was necessary to keep most Ca 2+ out of cells because it forms insoluble phosphates. So the [Ca 2+ ] is low, making it a good signal. Fertilization In mammals, sperm matures in the female reproductive tract. it must then penetrate the follicle cells and the zona pellucida (zp). There is no fast block, but the slow block is similar to the sea urchin. The zp is often removed in fertility clinics. A human sperm can fertilize a hamster egg with no zp. Fertilization can also occur by injecting sperm into an egg.
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Cleavage in the sea urchin egg There is an animal-vegetal axis, though not visible. Divisions are approximately equal.
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61. Animal Devel I 10 - Animal Development Fertilization...

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