Fertilization

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Fertilization normally occurs in the uterine tubes. Each sperm contains a large, enzyme-filled vesicle above its nucleus, known as an acrosome, that is central to this task (fig. 20.40). The interaction of sperm with particular molecules in the zona pellucida triggers an acrosomal reaction. This involves the progressive fusion of the acrosomal membrane with the plasma membrane of the sperm, creating pores through which the acrosomal enzymes can be released by exocytosis. These enzymes, including a protein-digesting enzyme and hyaluronidase (which digests hyaluronic acid, a constituent of the extracellular matrix), allow the sperm to digest a path through the zona pellu-cida to the oocyte.

Before activation

Binding to zona pellucida stimulates fusion and exocytosis

After acrosomal reaction

Exocytose Sicule Acrosomique

Figure 20.40 The acrosome reaction. Prior to activation, the acrosome is a large, enzyme-containing vesicle over the sperm nucleus. After the sperm binds to particular proteins in the zona pellucida surrounding the egg, the acrosomal membrane fuses with the plasma membrane in many locations, creating openings through which the acrosomal contents can be released by exocytosis. When the process is complete, the inner acrosomal membrane has become continuous with the plasma membrane.

Figure 20.40 The acrosome reaction. Prior to activation, the acrosome is a large, enzyme-containing vesicle over the sperm nucleus. After the sperm binds to particular proteins in the zona pellucida surrounding the egg, the acrosomal membrane fuses with the plasma membrane in many locations, creating openings through which the acrosomal contents can be released by exocytosis. When the process is complete, the inner acrosomal membrane has become continuous with the plasma membrane.

Primary oocyte

Secondary oocyte

First polar body

First meiotic division

Human Zygote With Polar Body

Secondary oocyte |-Degenerate-1

at metaPhase II First polar ^ Second polar

First polar Spindle body body body

Ovulation apparatus apparatus

Fertilization

Human Fertilization

Chromosomes

Nuclear membrane disappearing

Sperm cell nucleus

■ Figure 20.41 Changes in the oocyte following fertilization. A secondary oocyte, arrested at metaphase II of meiosis, is released at ovulation. If this cell is fertilized, it will complete its second meiotic division and produce a second polar body. The chromosomes of the two gametes are joined in the zygote.

Reproduction

As the first sperm tunnels its way through the zona pellu-cida and fuses with the plasma membrane of the oocyte, a number of changes occur that prevent other sperm from fertilizing the same oocyte. Polyspermy (the fertilization of an oocyte by many sperm) is thereby prevented; only one sperm can fertilize an oocyte. As fertilization occurs, the secondary oocyte is stimulated to complete its second meiotic division (fig. 20.41). Like the first meiotic division, the second produces one cell that contains all of the cytoplasm—the mature ovum—and one polar body. The second polar body, like the first, ultimately fragments and disintegrates.

At fertilization, the sperm cell enters the cytoplasm of the much larger egg cell. Within 12 hours, the nuclear membrane in the ovum disappears, and the haploid number of chromosomes (twenty-three) in the ovum is joined by the haploid number of chromosomes from the sperm cell. A fertilized egg, or zygote, containing the diploid number of chromosomes (forty-six) is thus formed (fig. 20.41).

It should be noted that the sperm cell contributes more than the paternal set of chromosomes to the zygote. Recent evidence demonstrates that the centrosome of the human zygote is derived from the sperm cell and not from the oocyte. As described in chapter 3, the centrosome is needed for the organization of microtubules into a spindle apparatus, so that duplicated chromosomes can be separated during mitosis. Without a cen-trosome to form the spindle apparatus, cell division (and hence embryonic development) cannot proceed.

A secondary oocyte that has been ovulated but not fertilized does not complete its second meiotic division, but instead disintegrates 12 to 24 hours after ovulation. Fertilization therefore cannot occur if intercourse takes place later than 1 day following ovulation. Sperm, by contrast, can survive up to 3 days in the female reproductive tract. Fertilization therefore can occur if intercourse takes place within a 3-day period prior to the day of ovulation.

■ Figure 20.42 In vitro fertilization. A needle (the shadow on the right) is used to inject a single spermatozoon into a human oocyte in vitro.

The process of in vitro fertilization is sometimes used to produce pregnancies in women with absent or damaged uterine tubes or in women who are infertile for a variety of other reasons. A secondary oocyte may be collected by aspiration following ovulation (as estimated by waiting 36 to 38 hours after the LH surge). Alternatively, a woman may be treated with powerful FSH-like hormones that cause the development of multiple follicles, and preovulatory oocytes may be collected by aspiration guided by ultrasound and laparoscopy. The donor's sperm are treated so as to duplicate normal capacitation. The oocytes may be placed in a petri dish for 2 to 3 days, along with sperm collected from the donor, or newer techniques may be used to promote fertilization. These newer techniques include ICSI (for intracytoplasmic sperm injection), which involves the microinjection of sperm through the zona pellucida directly into the ovum (fig. 20.42). A number of embryos may be produced at the same time, and the surplus frozen in liquid nitrogen for later use. The embryos are usually transferred, three or more at a time, to the woman's uterus at their four-cell stage, 48 to 72 hours after fertilization. In some cases, the embryos may be transferred to the end of the uterine tube. The likelihood of a successful implantation is low (around 35%), and the procedure is expensive. The long-term safety of fertility drugs has also been questioned.

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A Beginner's Guide to Healthy Pregnancy. If you suspect, or know, that you are pregnant, we ho pe you have already visited your doctor. Presuming that you have confirmed your suspicions and that this is your first child, or that you wish to take better care of yourself d uring pregnancy than you did during your other pregnancies; you have come to the right place.

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Responses

  • Susanne
    What does the second set of enzymes released by the acrosome appear to do?
    8 years ago
  • MYLA
    Why does the secondary oocyte cannot proceed without fertilization?
    8 years ago
  • faruz
    How is fertilization of an oocyte by more than one sperm prevented?
    8 years ago
  • geraldina palerma
    How are multiple fertilizations of the same ovum prevented?
    7 years ago
  • Sagramor
    Why does a oocyte require multiple sperm for successful fertilization?
    6 years ago
  • alfrida
    Can sperm enter the second polar body?
    6 years ago
  • Arthur
    When acrosomal reaction in secondary oocyte?
    2 years ago

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