Fertilization

  • Overview

    Upon ejaculation, about 200 – 600 million sperm are deposited near the cervical os of the uterus and posterior fornix of the vagina.
    They must endure a period of
    capacitation before they are capable of fertilization.

    • Only capacitated sperm can go through the corona radiata to contact the zona pellucida and initiate the acrosome reaction.
    • Upon contact with the plasmalemma of the secondary oocyte, the sperm cell initiates cortical reaction, which causes the secondary oocyte membrane to become impermeable to any other spermatozoa.
    • After the sperm cell nucleus enters the secondary oocyte, the secondary oocyte completes meiosis II to become an ovum and second play body.
    • Fertilization generally occurs in the distal 1/3 of the uterine tube (oviduct), the portion called the ampulla.

  • Capacitation

    • Capacitation is a period of conditioning of the spermatozoa that occurs in the female reproductive tract. It lasts approximately 7 hours; much of it taking place in the uterine tube and involving interactions between the sperm and the mucosal cells of the uterine tube.
    • It involves removal of seminal plasma proteins and a glycoprotein coat over the acrosome region of the sperm cells.
    • Only capacitated sperm can penetrate the corona radiate and undergo the acrosome reaction.

  • Acrosome Reaction
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    Acrosome reaction occurs after the sperm cell attaches to the zona pellucida and is induced by zonal proteins. It culminates in the release of trypsin like enzymes from the acrosome and penetration of the zona pellucida by the sperm cell.

  • Phases of Fertilization

    The following events occur during fertilization:

    • Passage of the sperm through the corona radiata. May be due to hyaluronidase released from the acrosome
    • Penetration of the zona pellucida. Due again to enzymes in the acrosome - acrosome reaction
    • Once a single sperm penetrates the zona pellucida and contacts the surface of the oocyte, cortical and zonal reactions occur.
      • cortical reaction involves release of cortical oocyte granules (lysosomes) making the oocyte membrane impermeable to other sperm cells.
      • zona reaction alters the structure of the zona pellucida so that no other sperm can bind to it or penetrate it.
    • Fusion of the plasma membranes of the oocyte and sperm
    • Completion of the second meiotic division to form the mature ovum and the second polar body
    • The nucleus of the ovum becomes the female pronucleus
    • Within the cytoplasm of the ovum the sperm nucleus enlarges to become the male pronucleus
    • Fertilization is complete with the fusion of the female and male pronuclei to re-establish the diploid number of chromosomes.
    The new individual formed after the fusion of the male and female pronuclei, is called a zygote. It has the diploid number of chromosomes for the species and it begins a process of cell division called cleavage.

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