The initial adhesion of sperm to the oocyte is mediated in part by the interaction of integrins on the oocyte and their ligands, disintegrins, on sperm. After adhesion, the plasma membranes of the sperm and egg fuse (Fig. 2.5). Because the plasma membrane covering the acrosomal head cap disappears during the acrosome reaction, actual fusion is accomplished between the oocyte membrane and the membrane that covers the posterior region of the sperm head (Fig. 2.5). In the human, both the head and tail of the spermatozoon enter the cytoplasm of the oocyte, but the plasma membrane is left behind on the oocyte surface. As soon as the spermatozoon has entered the oocyte, the egg responds in three ways:
1. Cortical and zona reactions. As a result of the release of cortical oocyte granules, which contain lysosomal enzymes, (a) the oocyte membrane becomes impenetrable to other spermatozoa, and (b) the zona pellu-cida alters its structure and composition to prevent sperm binding and penetration. These reactions prevent polyspermy (penetration of more than one spermatozoon into the oocyte).
2. Resumption of the second meiotic division. The oocyte finishes its second meiotic division immediately after entry of the spermatozoon. One of the daughter cells, which receives hardly any cytoplasm, is known as the second polar body; the other daughter cell is the definitive oocyte. Its chromosomes (22 + X) arrange themselves in a vesicular nucleus known as the female pronucleus (Figs. 2.6 and 2.7).
3. Metabolic activation of the egg. The activating factor is probably carried by the spermatozoon. Postfusion activation may be considered to encompass the initial cellular and molecular events associated with early embryogenesis.
The spermatozoon, meanwhile, moves forward until it lies close to the female pronucleus. Its nucleus becomes swollen and forms the male pronucleus (Fig. 2.6); the tail detaches and degenerates. Morphologically, the male and female pronuclei are indistinguishable, and eventually, they come into close contact and lose their nuclear envelopes (Fig. 2.7A). During growth of male and female pronuclei (both haploid), each pronucleus must replicate its DNA. If it does not, each cell of the two-cell zygote has only half of the normal amount of DNA. Immediately after DNA synthesis, chromosomes organize on
the spindle in preparation for a normal mitotic division. The 23 maternal and 23 paternal (double) chromosomes split longitudinally at the centromere, and sister chromatids move to opposite poles, providing each cell of the zygote with the normal diploid number of chromosomes and DNA (Fig. 2.6, D and E). As sister chromatids move to opposite poles, a deep furrow appears on the surface of the cell, gradually dividing the cytoplasm into two parts (Figs. 2.6F and 2.7B).
The main results of fertilization are as follows:
• Restoration of the diploid number of chromosomes, half from the father and half from the mother. Hence, the zygote contains a new combination of chromosomes different from both parents.
• Determination of the sex of the new individual. An X-carrying sperm produces a female (XX) embryo, and a Y-carrying sperm produces a male (XY) embryo. Hence, the chromosomal sex of the embryo is determined at fertilization.
• Initiation of cleavage. Without fertilization, the oocyte usually degenerates 24 hours after ovulation.
Barrier techniques of contraception include the male condom, made of latex and often containing chemical spermicides, which fits over the penis; and the female condom, made of polyurethane, which lines the vagina. Other barriers placed in the vagina include the diaphragm, the cervical cap, and the contraceptive sponge.
The contraceptive pill is a combination of estrogen and the progesterone analogue progestin, which together inhibit ovulation but permit menstruation.
Both hormones act at the level of FSH and LH, preventing their release from the pituitary. The pills are taken for 21 days and then stopped to allow menstruation, after which the cycle is repeated.
Depo-Provera is a progestin compound that can be implanted subder-mally or injected intramuscularly to prevent ovulation for up to 5 years or 23 months, respectively.
A male "pill" has been developed and tested in clinical trials. It contains a synthetic androgen that prevents both LH and FSH secretion and either stops sperm production (70-90% of men) or reduces it to a level of infertility.
The intrauterine device (IUD) is placed in the uterine cavity. Its mechanism for preventing pregnancy is not clear but may entail direct effects on sperm and oocytes or inhibition of preimplantation stages of development.
The drug RU-486 (mifepristone) causes abortion if it is administered within 8 weeks of the previous menses. It initiates menstruation, possibly through its action as an antiprogesterone agent.
Vasectomy and tubal ligation are effective means of contraception, and both procedures are reversible, although not in every case.
Infertility is a problem for 15% to 30% of couples. Male infertility may be a result of insufficient numbers of sperm and/or poor motility. Normally, the ejaculate has a volume of 3 to 4 ml, with approximately 100 million sperm per ml. Males with 20 million sperm per ml or 50 million sperm per total ejaculate are usually fertile. Infertility in a woman may be due to a number of causes, including occluded oviducts (most commonly caused by pelvic inflammatory disease), hostile cervical mucus, immunity to spermatozoa, absence of ovulation, and others.
In vitro fertilization (IVF) of human ova and embryo transfer is a frequent practice conducted by laboratories throughout the world. Follicle growth in the ovary is stimulated by administration of gonadotropins. Oocytes are recovered by laparoscopy from ovarian follicles with an aspirator just before ovulation when the oocyte is in the late stages of the first meiotic division. The egg is placed in a simple culture medium and sperm are added immediately. Fertilized eggs are monitored to the eight-cell stage and then placed in the uterus to develop to term. Fortunately, because preimplantation-stage embryos are resistant to teratogenic insult, the risk of producing malformed offspring by in vitro procedures is low.
A disadvantage of IVF is its low success rate; only 20% of fertilized ova implant and develop to term. Therefore, to increase chances of a successful pregnancy, four or five ova are collected, fertilized, and placed in the uterus. This approach sometimes leads to multiple births.
Another technique, gamete intrafallopian transfer (GIFT), introduces oocytes and sperm into the ampulla of the fallopian (uterine) tube, where fertilization takes place. Development then proceeds in a normal fashion. In a similar approach, zygote intrafallopian transfer (ZIFT), fertilized oocytes are placed in the ampullary region. Both of these methods require patent uterine tubes.
Severe male infertility, in which the ejaculate contains very few live sperm (oligozoospermia) or even no live sperm (azoospermia), can be overcome using intracytoplasmic sperm injection (ICSI). With this technique, a single sperm, which may be obtained from any point in the male reproductive tract, is injected into the cytoplasm of the egg to cause fertilization. This approach offers couples an alternative to using donor sperm for IVF. The technique carries an increased risk for fetuses to have Y chromosome deletions but no other chromosomal abnormalities.
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