Lh

Ovulation

Regressive corpus luteum

Ovulation

Luteal phase

Corpus albicans

Follicular phase

Luteal phase

Corpus albicans

Proliferative phase

Secretory phase

Menstruation

Menstruation

Figure 22.33

Proliferative phase

Secretory phase

Menstruation

Menstruation

Figure 22.33

Major events in the female ovarian and menstrual cycles. Follicular development parallels FSH concentration. Ovulation is preceded and triggered by the LH surge.

characteristics may change. The breasts, vagina, uterus, and uterine tubes may shrink, and the pubic and axillary hair may thin. The epithelial linings associated with urinary and reproductive organs may thin. There may be increased loss of bone matrix (osteoporosis) and thinning of the skin. Because the pituitary secretions of FSH and LH are no longer inhibited, these hormones may be released continuously for some time.

About 50% of women reach menopause by age fifty, and 85% reach it by age fifty-two. Of these, perhaps 20% have no unusual symptoms—they simply stop menstruating. However, about 50% of women experience unpleasant vasomotor symptoms during menopause, including sensations of heat in the face, neck, and upper body called "hot flashes." Such a sensation may last for thirty seconds to five minutes and may be accompanied by chills and sweating. Women may also experience headache, backache, and fatigue during menopause. These vasomotor symptoms may result from changes in the rhythmic secretion of GnRH by the hypothalamus in response to declining concentrations of sex hormones. To reduce the unpleasant side effects of menopause, women are often treated with estrogen replacement therapy (ERT), which consists of estrogens delivered through a transdermal (skin) patch or oral estrogens.

At age fifty-three, Mary Shearing gave birth to twins, Amy Leigh and Kelly Ann. In the last stages of menopause, Mary already had three grown children from a previous marriage, and two grandchildren. She and her thirty-two-year-old husband, Don, decided to try for pregnancy. Don's sperm was used to fertilize donated eggs in a laboratory dish, and some of the fertilized ova were implanted in Mary's uterus. The Shearings' success showed that it is not the condition of the uterine lining in an older woman that makes conceiving, carrying, and delivering a healthy baby difficult, but the age of the egg. Women in their sixties have since given birth, using donated oocytes. However, most women who have the treatment that Shearing underwent are under forty-five years of age and have lost ovaries to disease. Often the donor is a sister. College newspapers sometimes run ads offering young women with high grades large sums of money to donate eggs.

Trace the events of the female menstrual cycle.

What effect does progesterone have on the endometrium?

What causes menstrual flow?

What are some changes that may occur at menopause?

Pregnancy

Pregnancy (preg'nan-se) is the presence of a developing offspring in the uterus. It results from the union of the genetic packages of an egg cell and a sperm cell—an event called fertilization.

Nausea and vomiting in pregnancy — more commonly known as morning sickness—may be a protective mechanism to shield a fetus from foods that might contain toxins or pathogens. The condition affects two in three pregnancies and coincides with the time in gestation when a woman's immune system is at its weakest. An analysis of more than 80,000 pregnant women found that they tend to have aversions to foods that spoil easily, such as eggs and meats, as well as coffee and alcohol. Yet many pregnant women eat more fruits and vegetables than they otherwise do. In addition, in societies where the diet is mostly grains with little if any meat, incidence of morning sickness is much lower than in groups with more eclectic, and possibly dangerous, diets. Rates of morning sickness are highest in Japan, where raw fish is a dietary staple, and European countries, where undercooked meat is often eaten. Coincidence? Maybe. But more likely, evolution has selected for morning sickness where it correlates to, and possibly contributes to better birth outcomes.

Transport of Sex Cells

Ordinarily, before fertilization can occur, a secondary oocyte must be ovulated and enter a uterine tube. During sexual intercourse, the male deposits semen containing sperm cells in the vagina near the cervix. To reach the secondary oocyte, the sperm cells must then move upward through the uterus and uterine tube. Lashing of sperm tails and muscular contractions within the walls of the uterus and uterine tube, stimulated by prostaglandins in the semen, aid the sperm cells' journey. Also, under the influence of high concentrations of estrogens during the first part of the menstrual cycle, the uterus and cervix contain a thin, watery secretion that promotes sperm transport and survival. Conversely, during the latter part of the cycle, when the progesterone concentration is high, the female reproductive tract secretes a viscous fluid that hampers sperm transport and survival (fig. 22.34).

Sperm movement is inefficient. Even though as many as 200 million to 600 million sperm cells may be deposited in the vagina by a single ejaculation, only a few hundred ever reach a secondary oocyte. The journey to the upper portions of the uterine tube takes less than an hour following sexual intercourse. Although many sperm cells may reach a secondary oocyte, usually only one sperm cell actually fertilizes it (fig. 22.35). About one in a million births produces a severely deformed child who has inherited three sets of chromosomes. This may be the result of two sperm cells entering a single egg cell.

A secondary oocyte may survive for only twelve to twenty-four hours following ovulation, whereas sperm cells may live up to seventy-two hours within the female reproductive tract. Consequently, sexual intercourse probably must occur earlier than seventy-two hours before ovulation or within twenty-four hours following ovulation if fertilization is to take place. Clinical Application 22.3 describes assisted routes to conception.

Fertilization and Implantation

When a sperm reaches a secondary oocyte, it invades the follicular cells that adhere to the oocyte's surface (corona radiata) and binds to the zona pellucida that surrounds the oocyte's cell membrane. The acrosome of a sperm cell attached to the zona pellucida releases an enzyme that helps the motile sperm penetrate the zona pellucida (fig. 22.36).

In "zona blasting," an experimental procedure to aid certain infertile men, an egg cell cultured in a laboratory dish is chemically stripped of its zona pellucida. The more vulnerable egg now presents less of a barrier to a sperm, and is more easily fertilized.

The cell membranes of the sperm cell and the secondary oocyte fuse and sperm movement ceases. The sperm cell sheds its tail. At the same time, the oocyte

Figure

The paths of the egg and sperm cells through the female reproductive tract.

Figure

The paths of the egg and sperm cells through the female reproductive tract.

Figure

Scanning electron micrograph of sperm cells on the surface of an egg cell (1,200x).

Figure

Scanning electron micrograph of sperm cells on the surface of an egg cell (1,200x).

cell membrane becomes unresponsive to other sperm cells. The union of the oocyte and sperm cell membranes also triggers some lysosomelike granules (cortical granules) just beneath the oocyte cell membrane to release enzymes that harden the zona pellucida. This reduces the chance that other sperm cells will penetrate, and it forms a protective layer around the newly formed fertilized egg cell.

The sperm nucleus enters the oocyte's cytoplasm and swells. The secondary oocyte then divides unequally to form a large cell and a tiny second polar body, which is later expelled. Therefore, female meiosis completes only after the sperm enters the egg. Next, the nuclei of the male and female cells unite. Their nuclear membranes disassemble, and their chromosomes mingle, completing the process of fertilization, diagrammed in figure 22.36.

Because the sperm cell and the egg cell each provide twenty-three chromosomes, the product of fertilization is a cell with forty-six chromosomes—the usual number in a human cell. This zygote is the first cell of the future offspring.

About thirty hours after forming, the zygote undergoes mitosis, giving rise to two cells. These cells, in turn, divide into four cells, which divide into eight cells, and so forth. Meanwhile, the developing mass of cells moves through the uterine tube to the uterine cavity, aided by the beating of cilia of the tubular epithelium and by weak peristaltic contractions of smooth muscles in the tubular wall. Secretions from the epithelial lining may provide the developing organism with nutrients. The trip to the uterus takes about three days. The ball of cells remains free within the uterine cavity for about three days. By the end of the first week of development, the ball of cells superficially implants in the endometrium.

Assisted Reproductive Technologies

Conception requires the meeting and merging of sperm and egg, which naturally occurs in the woman's reproductive tract. Abnormal gametes or blockages that impede this meeting of cells can result in infertility (inability to conceive). Assisted reproductive technologies can help couples conceive. The procedures usually involve a laboratory technique and sometimes participation of a third individual. These techniques are often costly and may take several attempts, and some have very low success rates. Most assisted reproductive technologies were developed in nonhuman animals. For example, the first artificial inseminations were performed in dogs in 1782, and the first successful in vitro fertilization was accomplished in 1959, in a rabbit. Here is a look at some procedures.

donated sperm-

Artificial Insemination

In artificial Insemination a doctor places donated sperm in a woman's reproductive tract. A woman might seek artificial Insemination if her partner is infertile or carries a gene for an inherited illness or if she desires to be a single parent. More than 250,000 babies have been born worldwide as a result of this procedure. The first human artificial inseminations by donor were done in the 1890s. For many years, physicians donated sperm, and this became a way for male medical students to earn a few extra dollars. By 1953, sperm could be frozen and stored for later use. Today, sperm banks freeze and store donated sperm and then provide it to physicians who perform artificial insemination.

A woman or couple choosing artificial insemination can select sperm from a catalog that lists the personal characteristics of the donors, including blood type; hair, skin, and eye color; build; and even educational level and interests. Of course, not all of these traits are inherited. Although artificial insemination has helped many people to become parents, it and other assisted reproductive technologies have led to occasional dilemmas (table 22C).

In Vitro Fertilization

In in vitro fertilization (IVF), which means "fertilization in glass," sperm meets egg outside the woman's body. The fertilized ovum divides two or three times and is then introduced into the egg donor's (or another woman's) uterus. If all goes well, a pregnancy begins.

A woman might undergo IVF if her ovaries and uterus work but her uterine tubes are blocked. To begin, she takes a hormone that hastens maturity of several oocytes. Using a laparoscope to view the ovaries and uterine tubes, a physician removes a few of the largest eggs and transfers them to a dish, then adds chemicals similar to those in the female reproductive tract, and sperm.

If a sperm cannot penetrate the egg in vitro, it may be sucked up into a tiny syringe and injected using a

1. A physician in California used his own sperm to artificially inseminate

15 patients and told them that he had used sperm from anonymous donors.

1. A physician in California used his own sperm to artificially inseminate

15 patients and told them that he had used sperm from anonymous donors.

2. A plane crash killed the wealthy parents of two early embryos stored at -320° F (-195° C) in a hospital in Melbourne, Australia. Adult children of the couple were asked to share their estate with two eight-celled balls.

3. Several couples in Chicago planning to marry discovered that they were half-siblings. Their mothers had been artificially inseminated with sperm from the same donor.

4. Two Rhode Island couples sued a fertility clinic for misplacing embryos.

5. Several couples in California sued a fertility clinic for implanting their eggs or embryos in other women without consent from the donors. One woman is requesting partial custody of the resulting children if her eggs were taken and full custody if her embryos were used, even though the children are of school age and she has never met them.

6. A man sued his ex-wife for possession of their frozen embryos as part of the divorce settlement.

tiny needle into the female cell (fig. 22C). This variant of IVF, called intracytoplasmic sperm injection (ICSI), is very successful, resulting in a 68% fertilization rate. It can help men with very low sperm counts, high numbers of abnormal sperm, or injuries or illnesses that prevent them from ejaculating. Minor surgery is used to remove testicular tissue, from which viable sperm are isolated and injected into eggs. A day or so later, a physician transfers some of the resulting balls of 8 or 16 cells to the woman's uterus. The birth rate following IVF is about 17%, compared with 31% for natural conceptions (fig. 22D).

lire 22C

Figure

Intracytoplasmic sperm Injection (ICSI) enables some infertile men and men with spinal cord injuries and other illnesses to become fathers. A single sperm cell is injected into the cytoplasm of an egg.

gamete

Intrafallopian Transfer

One reason that IVF rarely works is the artificial fertilization environment. A procedure called GIFT, which stands for gamete intrafallopian transfer, circumvents this problem by moving fertilization to the woman's body. A woman takes a superovulation drug for a week and then has several of her largest eggs removed. A man donates a sperm sample, and a physician separates the most active cells. The collected eggs and sperm are deposited together in the woman's uterine tube, at a site past any obstruction so that implantation can occur. GIFT is 26% successful.

In zygote intrafallopian transfer (ZIFT), a physician places an in vitro fertilized ovum in a woman's uterine tube. This is unlike IVF because the site of introduction is the uterine tube and unlike GIFT because fertilization occurs in the laboratory. Allowing the fertilized ovum to make its own way to the uterus seems to increase the chance that it will implant. ZIFT is 23% successful. ■

Figure 22D

IVF worked too well for Michele and Ray L'Esperance. Five fertilized ova implanted in Michele's uterus are now Erica, Alexandria, Veronica, Danielle, and Raymond.

Figure

Steps in fertilization: (1) Sperm cell reaches corona radiata surrounding the egg cell. (2) Acrosome of sperm cell releases protein-digesting enzyme. (3 and 4) Sperm cell penetrates zona pellucida surrounding egg cell. (5) Sperm's cell membrane fuses with egg's cell membrane.

What factors aid the movements of the egg and sperm cells through the female reproductive tract?

Where in the female reproductive system does fertilization normally take place?

List the events of fertilization.

Occasionally, the developing mass of cells implants in tissues outside the uterus, such as those of a uterine tube, an ovary, the cervix, or an organ in the abdominal cavity. The result is called an ectopic pregnancy. If a fertilized egg implants within the uterine tube, it is called a tubal pregnancy. A tubal pregnancy is dangerous to a pregnant woman and the developing offspring because the tube usually ruptures as the embryo enlarges and is accompanied by severe pain and heavy vaginal bleeding. Treatment is prompt surgical removal of the embryo and repair or removal of the damaged uterine tube.

Hormonal Changes During Pregnancy

During a typical menstrual cycle, the corpus luteum degenerates about two weeks after ovulation. Consequently, concentrations of estrogens and progesterone decline rapidly, the uterine lining is no longer maintained, and the endometrium sloughs off as menstrual flow. If this occurs following implantation, the embryo is lost (spontaneously aborted).

A hormone called hCG (human chorionic gonadotropin) normally helps prevent spontaneous abortion. A layer of cells, called a trophoblast, that secretes hCG and later helps form the placenta, surround the developing embryo (see chapter 23, pp. 947 and 949). This hormone has properties similar to those of LH, and it maintains the corpus luteum, which continues secreting estrogens and progesterone. Thus, the uterine wall continues to grow and develop. At the same time, release of FSH and LH from the anterior pituitary gland is inhibited, so normal menstrual cycles cease (fig. 22.37).

Trophoblast cells secrete hCG
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