Human Reproduction, Sex, and Sexuality

By Enger, E.D., Ross, F.C., Bailey, D.B.

Edited by Paul Ducham

SEXUALITY FROM VARIOUS POINTS OF VIEW

Probably nothing interests people more than sex and sexuality. Sex is the nature of the biological differences between males and females. By sexuality, we mean all the factors that contribute to one’s female or male nature. A person’s sexuality includes the structure and function of the sex organs, sexual behavior, and the ways in which culture influences sexual behavior. Males and females have different behavior patterns for a variety of reasons. Some behavioral differences are learned (e.g., patterns of dress, the use of facial makeup), whereas others appear to be less dependent on culture (e.g., degree of aggressiveness, the frequency of sexual thoughts).
          There are several ways of looking at human sexuality. The behavioral sciences tend to focus on the behaviors associated with being male and female and what is considered appropriate or inappropriate sexual behavior. Psychologists consider sexual behavior to be a strong drive, appetite, or urge (figure 27.1). They describe the sex drive as a basic impulse to satisfy a biological, social, or psychological need. Other social scientists, such as sociologists and cultural anthropologists, are interested in sexual behavior as it occurs in various cultures. When a variety of cultures are examined, it becomes very difficult to classify various kinds of sexual behavior as normal or abnormal; What is considered abnormal in one culture may be normal in another. For example, public nudity is considered abnormal in many cultures, but not in others.
         Biologists have studied the sexual behavior of nonhuman animals for centuries. They have long considered the function of sexuality in light of its value to the population or species. Sexual reproduction results in new combinations of genes, which are important in the process of natural selection. Many biologists are attempting to look at human sexual behavior from an evolutionary perspective and speculate on why certain sexual behaviors are common. The behaviors of courtship, mating, the raising of the young, and the division of labor between the sexes are complex in all social animals, including humans, as demonstrated in the elaborate social behaviors surrounding picking a mate and forming a family. It is difficult to draw the line between the biological development of sexuality and the social customs related to the sexual aspects of human life. However, the biological mechanism that determines whether an individual will develop into a female or a male has been well documented.

Figure 27.1

SEXUAL ATTRACTION, SEX, AND SEXUAL RESPONSE

Heterosexual sex takes place between members of the opposite sex. The primary biological goal of sexual intercourse (coitus, mating) is the union of sperm and egg to form offspring. However, this interaction can also increase the health and bring about pleasure in willing partners.
         Sexual attraction involves many factors, including sight and smell. How one person appears to another is usually what catches the other’s attention. If we find a person pleasing in appearance, we say, they are “attractive”, that is, we want to be closer. As with many other organisms, humans release chemicals that act as attractants. These chemicals are called pheromones. The existence of pheromones has been well documented even though we are not conscious of their actions. The cosmetic and fashion industries are founded on these fundamentals of sexual attraction (figure 27.3). After initial attraction, the couple will usually talk. Some of this conversation will better acquaint the two, present the idea that there is an attraction, and may suggest that they are interested in sexual intercourse. It is not unusual for gift giving and dining to take place as a part of this mating ritual. The male may give gifts to the female or the female to the male. Depending on the culture and the couple, such interactions may be brief or may take years. Marriage may or may not take place before sexual interaction occurs.
        Foreplay is the term used to describe sexual stimulation that precedes sexual intercourse. Hugging, kissing, and fondling (petting) arouse sexual excitement and desire. This leads to increased hormone production and blood flow in both male and female genitals. In males, tissues in the penis become engorged with blood, causing the penis to stiffen or become erect. An erection allows the penis to be more easily placed inside the vagina during vaginal intercourse, increasing the chance that sperm will find their way to the egg. Prior to the penis being inserted into the vagina, lubricating fluids are released from male and female reproductive tracts. Throughout arousal, the heart rate increases, breathing quickens, and blood pressure increases. Once the penis is inside the vagina, pelvic movements result in stimulation of both the male and female and can result in ejaculation. Ejaculation is the release of sperm and other fluids (semen) from the penis. It occurs with a pulsating of smooth muscle. As in females, this release is generally accompanied by a sensation called orgasm. Orgasm is the pleasurable climax of sexual activity. Muscles throughout the body (especially those in the vagina, uterus, anus, and pelvic floor) begin to spasm. During female orgasm, all the muscles that were tightened during sexual arousal relax, resulting in this enjoyable sensation. The sensations vary from one woman to another and some women may not recognize that they have experienced orgasm. Following male orgasm, blood leaves the penis and the erection ends. Two other forms of sexual intercourse practiced are anal (penis in anus) and oral (penis in mouth or oral stimulation of vagina, clitoris).
         Maintaining an active, enjoyable sex life in a long-term relationship requires the same amount of effort that was extended at the beginning of the relationship. Things that can interfere include work (time away), family (kids, relatives), physical exhaustion (sports), other people (affairs), illness (for example, diabetes), emotions (for example, depression) and a person's overall health (for example, being overweight). The use of alcohol, street drugs, or certain prescription medications can lead to a reduction of sex drive and may include erectile dysfunction (ED). Since sight plays an initial and important role in sexual attraction and self-image, changes in a person's anatomy can also dampen sexual activity, for example, changes such as a mastectomy due to cancer or loss of penis due to accident. In addition, acquiring a sexually transmitted disease can place limits on sexual activity in socially responsible individuals.

figure 27.3

MOVING ACROSS THE SPECTRUM

In humans, incidences of partial development of the genitalia (sex organs) of both sexes in one individual may be more frequent than most people realize. These people are referred to as pseudo-hermaphrodites, because they do not have complete sets of male and female organs. Sometimes, this abnormal development occurs because the hormone levels are out of balance at critical times in the development of the embryo. This hormonal imbalance may be related to an abnormal number of sex-determining chromosomes, or it may be the result of abnormal functioning of the endocrine glands.
           When children with abnormal combinations of sex organs are born they are usually assessed by a physician in consultation with the parents to determine which sexual structures should be retained or surgically reconstructed. The physician might also decide that hormone therapy might be a more successful treatment. These decisions are not easily made, because they involve children who have not fully developed their sexual nature. An increasingly vocal group advocates that children who are diagnosed with this condition not be surgically “corrected” as infants, recommending that, if the parents can cope with the unusual genitalia, they allow the child to grow older without surgery. They believe that people should choose for themselves once they are more mature. However, few long-term studies have examined the stability of reconstructive surgery in children or the extent of the social adjustment required.
           More and more frequently, we are becoming aware of individuals whose physical gender does not match their psychological gender. Gender is the sexual identity based on a person’s anatomy. A male with normal external male genitals may “feel” like a female. The same situation may occur with structurally female individuals. Because some of these individuals may dress as a member of the opposite sex, they are sometimes called cross-dressers or transvestites. In private, such people may dress as the other sex, behaving in social situations appropriately for their sex. Others completely change their public and private behavior to reflect their inner desire to function as the other sex. A male may dress as a female, work in a traditionally female occupation, and make social contacts as a female. Tremendous psychological and emotional pressures develop from this condition. Frequently, many of these individuals choose to undergo gender reassignment surgery, a sexchange operation. This surgery and the follow-up hormonal treatment can cost tens of thousands of dollars and take several years.
         Individuals with an occasional or a consistent feeling of being another gender than that indicated by their genitalia experience gender dysphoria. People who feel gender dysphoria (also known as trans-gendered or transsexuals) most intensely may be candidates for gender reassignment surgery, including hormonal treatments, counseling, and even training on how to act as a member of the other gender. Their goal is to interact with society as a member of the other gender without being detected as having been the other gender at one time, called “passing” as a man or woman.
         There have been some studies on brain structures and trans-gendered individuals. A small region of the hypothalamus is proportionally larger in normal males than in normal females. In one study, it was found that, in trans-gendered persons, the proportions of this region of the brain are switched: Males in a female body have a larger region than expected for a normal female; females in a male body have a larger region than expected for a normal male. This study was originally done in rats, in which malelike behavior was induced in females who were exposed to testosterone during their fetal development. Brain structure correlations were first established in the rat. Follow-ups on trans-gendered humans have shown similar results. Scientists don’t know what those brain cells actually do. They may be only involved in activating certain hormones.
         Similar studies were done to correlate brain structure with homosexual orientation. However, these studies were laden with controversy. Scientists have difficulty defining homosexuality in practical terms, so control groups were hard to establish. Many of the homosexual brain donors in this research were HIV + and had taken antiviral medications. The controls did not consider HIV status or time spent on the antiviral drugs. This is a significant issue, because HIV can infect and destroy certain brain cells, causing AIDS-associated dementia. AZT and its ability to affect neural cell function were not known at the time of the studies.
         Homosexuality is a complex behavioral pattern. Some females who are attracted to females (“lesbians”) are genderconfused whereas others are not. Certain studies suggest that genetic regions on chromosomes 7, 8, and 10 may influence homosexuality. Some regions on chromosome 7 and 8 have been linked with male sexual orientation, regardless of whether the male receives the chromosomes from his mother or father. The regions on chromosome 10 appear to be linked with male sexual orientation only if they were inherited from the mother.

CHROMOSOMAL ABNORMALITIES AND SEX

Evidence that the Y chromosome and its SRY gene control male development comes from many kinds of studies, including research on individuals who have an abnormal number of chromosomes. An abnormal meiotic division that results in sex cells with too many or too few chromosomes is a form of nondisjunction. If nondisjunction affects the X and Y chromosomes, a gamete might be produced that has only 22 chromosomes and lacks a sex-determining chromosome. On the other hand, it might have 24, with 2 sexdetermining chromosomes. If a cell with too few or too many sex chromosomes is fertilized, sexual development is usually affected. If a normal egg cell is fertilized by a sperm cell with no sex chromosome, the offspring will have only 1 X chromosome. These people, always women, are designated as XO. They develop a collection of characteristics known as Turner’s syndrome (figure 27.5)
          About 1 in 2,000 girls born has Turner’s syndrome. A female with this condition is short for her age and fails to mature sexually, resulting in sterility. In addition, she may have a thickened neck (termed webbing), hearing impairment, and some abnormalities in her cardiovascular system. When the condition is diagnosed, some of the physical conditions can be modified with treatment. Treatment involves the use of growth-stimulating hormone to increase her growth rate and female sex hormones to stimulate sexual development, although sterility is not corrected.
        An individual who has XXY chromosomes is basically male (figure 27.6). This genetic condition is termed Klinefelter’s syndrome. It is one of the most common examples of abnormal chromosome number in humans. This condition is present in about 1 in 500 to 1,000 men. Most of these men lead healthy, normal lives and it is impossible to tell them apart from normal males. However, those with Klinefelter’s syndrome may be sterile and show breast enlargement, incomplete masculine body form, lack of facial hair, and some minor learning problems. These traits vary greatly in degree, and many men are diagnosed only after they undergo testing to determine why they are infertile. Treatments include breastreduction surgery and testosterone therapy.

figure 27.5

FETAL DEVELOPMENT AND SEX

The development of embryonic gonads begins very early during fetal growth. First, a group of cells begins to differentiate into primitive gonads at about week 5 (figure 27.7). By week 5 to 7, if a Y chromosome is present, the gene product (testes determining factor) from the chromosome begins the differ entiation of these embryonic gonads into testes. They will develop into ovaries beginning about week 12 if 2 X chromosomes are present (and the Y chromosome is absent).
         As soon as the gonad has differentiated into an embryonic testis at about week 8, it begins to produce testosterone. The presence of testosterone results in the differentiation of male sexual anatomy, and the absence of testosterone results in the differentiation into female sexual anatomy in the developing embryo.

figure 27.7

THE MATURATION OF FEMALES

Girls typically begin to produce sex hormones from several glands, marking the onset of puberty. The hypothalamus, pituitary gland, ovaries, and adrenal glands begin to produce sex hormones at 9 to 12 years of age (figure 27.8).
        The hypothalamus is an endocrine gland that controls the functioning of many glands throughout the body, including the pituitary gland. At puberty, the hypothalamus begins to release gonadotropin-releasing hormone (GnRH), which stimulates the pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Increased levels of FSH stimulate the development of follicles, saclike structures that produce eggs in the ovaries. The increased luteinizing hormone stimulates the ovary to produce larger quantities of estrogens. The increasing supply of estrogens is responsible for the many changes in sexual development. These changes include breast growth, changes in the walls of the uterus and vagina, changes in the pelvic bone structure, and increased blood supply to the clitoris, a small, elongated erectile structure located at the head of the labia; it is equivalent to the penis.
          Estrogens also stimulates the female adrenal gland to produce androgens, male sex hormones. The androgens are responsible for the production of pubic hair, and they seem to have an influence on the female sex drive. The features that are not primarily involved in sexual reproduction but are characteristic of a sex are called secondary sexual characteristics. In women, the distribution of body hair, the patterns of fat deposits, and a higher voice are secondary sexual characteristics.
           A major development during this time is the establishment of the menstrual cycle, the periodic growth and shedding of the lining of the uterus, the endometrium. These changes are under the control of a number of hormones produced by the pituitary gland and ovaries. The ovaries are stimulated to release their hormones by the pituitary gland, which is in turn influenced by the ovarian hormones, both follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH causes the maturation and development of the ovaries, and LH is important in causing ovulation, the periodic release of sex cells from the surface of the ovary (figure 27.9), and converting the ruptured follicle into a structure known as the corpus luteum. In addition to their role in bringing about sexual development LH and FSH are also involved in regulating the menstrual cycle.
           The corpus luteum produces the hormone progesterone, which is important in maintaining the lining of the uterus. Changes in progesterone levels result in a periodic buildup and shedding of the lining of the uterus known as the menstrual cycle (table 27.1). Initially, as girls go through puberty, menstruation and ovulation may be irregular; however, in most women hormone production eventually becomes regulated, so that ovulation and menstruation take place on a monthly basis, although normal cycles vary from 21 to 45 days.
        As girls progress through puberty, curiosity about their changing body form and new feelings lead to self-investigation. Studies have shown that sexual activity, such as stimulation of the clitoris, vagina, or anus, which causes a pleasurable sensation, orgasm, is performed by a large percentage of young women. This stimulation is termed masturbation, and it is a normal part of sexual development.

Figure 27.8

Figure 27.9

Table 27.1

THE MATURATION OF MALES

Males typically reach puberty about 2 years later (ages 11–14) than females, but puberty in males also begins with a change in hormone levels. At puberty, the hypothalamus releases increased amounts of gonadotropin-releasing hormone (GnRH), resulting in increased levels of folliclestimulating hormone (FSH) and luteinizing hormone (LH). These are the same changes that occur in female development. Luteinizing hormone is often called interstitial cellstimulating hormone (ICSH) in males. LH stimulates the testes to produce testosterone, the primary sex hormone in males. The testosterone produced by the embryonic testes causes the differentiation of internal and external genital anatomy in the male embryo. At puberty, the increase in testosterone is responsible for the development of male secondary sexual characteristics and is important in the maturation and production of sperm.
         The major changes during puberty include growth of the testes and scrotum, pubic hair development, and increased penis size. Secondary sex characteristics also begin to become apparent; facial hair, underarm hair, and chest hair are some of the most obvious. The male voice changes as the larynx (voice box) begins to change shape. Body contours also change, and a growth spurt increases height. In addition, the proportion of the body that is muscle increases and the proportion of body fat decreases. At this time, a boy’s body begins to take on the characteristic adult male shape, with broader shoulders and heavier muscles.
        In addition to these external changes, increased testosterone causes the production of semen, also known as seminal fluid, a mixture of sperm and secretions from three accessory glands—the seminal vesicles, prostate, and bulbourethral glands. They produce secretions that nourish and activate the sperm. They also lubricate the tract and act as the vehicle to help propel the sperm.
          Seminal vesicles secrete an alkaline fluid, containing fructose, and hormones. Its alkaline nature helps neutralize the acidic environment in the female reproductive tract, improving the sperm’s chances of reaching the egg. The fructose provides energy for the sperm. Seminal vesicle secretions make up about 60% of the seminal fluid. The prostate gland produces a thin, milky fluid, which makes up about 25% of semen. It contains sperm-activating enzymes. Bulbourethral gland secretions make up the remaining 15%; they, too, are alkaline.
          FSH stimulates the production of sperm cells. The release of sperm cells and seminal fluid, ejaculation, begins during puberty. This release is generally accompanied by a pleasurable sensation, orgasm. During sleep, males frequently have erections, sometimes resulting in ejaculation of seminal fluid. This is termed a wet dream. It is normal and related to the amount of seminal fluid produced. Wet dreams occur less often in men who engage in frequent sexual intercourse or masturbation. Masturbation is a common and normal activity as a boy goes through puberty. Studies of adult sexual behavior have shown that nearly all men masturbate at some time during their lives.

SPERMATOGENESIS

The process of producing gametes is called gametogenesis (gamete formation), and it includes meiosis (figure 27.10). Spermatogenesis is gametogenesis that takes place in the testes of males, producing sperm. The two, olive-shaped testes are composed of many small, sperm-producing tubes, called seminiferous tubules, and collecting ducts that store sperm. These are held together by a thin covering membrane. The seminiferous tubules join together and eventually become the epididymis, a long, narrow, twisted tube in which sperm cells are stored and mature before ejaculation (figure 27.11).
           Leading from the epididymis is the vas deferens, or sperm duct; this empties into the urethra, which conducts the sperm out of the body through the penis (figure 27.12). Before puberty, the seminiferous tubules are packed solid with diploid cells. These cells, which are found just inside the tubule wall, undergo mitosis, thus providing a continuous supply of cells. Beginning about age 11, some of these cells specialize and begin the process of meiosis, whereas others continue to divide by mitosis. Spermatogenesis needs to occur below body temperature, which is why the testicles are in a sack, the scrotum, outside the body. Once spermatogenesis begins, the seminiferous tubules become hollow and can transport the mature sperm.
           Spermatogenesis consists of several steps. It begins when some of the cells in the walls of the seminiferous tubules differentiate and enlarge. These diploid cells undergo the first meiotic division, which produces two haploid cells. These cells go through the second meiotic division, resulting in four haploid cells called spermatids. Spermatids then lose much of their cytoplasm, develop long tails, and mature into sperm. Sperm have only a small amount of food reserves. Therefore, once they are released and become active swimmers, they live no more than 72 hours. However, if the sperm are placed in a protective solution, the temperature can be lowered drastically to 196°C. Under these conditions, the sperm become deactivated, freeze, and can survive for years outside the testes. This has led to the development of sperm banks. The artificial insemination (placing stored sperm into the reproductive tract of a female) of cattle, horses, and other domesticated animals with sperm from sperm banks is a common breeding practice. This technique is also used in humans who experience infertility.
          Spermatogenesis takes place continuously throughout a male’s reproductive life, although the number of sperm produced decreases as a man ages. Sperm counts can be taken and used to determine the probability of successful fertilization. A healthy male releases about 150 million sperm per milliliter with each ejaculation. A man must be able to release at least 100 million sperm per milliliter to be fertile. Many men with sperm counts under 50 million per milliliter are infertile; those with sperm counts below 20 million per milliliter are clinically infertile. These vast numbers of sperm are necessary because so many die during their journey. Large numbers are needed in order for the few survirors to reach the egg. In addition, each sperm contains enzymes in its cap that are able to digest through the mucus and protein found in the female reproductive tract. Millions of sperm contribute in this way to the process of fertilization, but only one fertilizes the egg.

figure 27.10

figure 27.11

figure 27.12

 

OOGENESIS

Oogenesis is the production of egg cells. This process starts before a girl is born, during prenatal development of the ovaries. This occurs when diploid cells in each ovary cease dividing by mitosis and enlarge in preparation to divide by meiosis. These are the potential egg cells that will become the eggs. All of these cells form in the embryonic ovaries before a female is born. At birth, they number approximately 2 million, but that number has been reduced by cell death to between 300,000 and 400,000 cells by puberty. These cells stop their development in an early stage of meiosis and remain just under the surface of the ovary inside a follicle.
         At puberty and on a regular monthly basis thereafter, the sex hormones stimulate one of these cells to continue meiosis. However, in telophase I, the two cells that form receive unequal portions of cytoplasm, a kind of lopsided division (figure 27.13). The smaller of the two cells is called a polar body, and the larger haploid cell is commonly referred to as an egg or ovum, although technically it is not. Just prior to ovulation, the follicle of the soon-to-be-released egg grows and moves near the surface of the ovary. When this maturation is complete, ovulation occurs, when the follicle ruptures and the egg is released. It is swept into the oviduct (Fallopian tube) by ciliated cells and travels toward the uterus (figure 27.14). Because of the action of luteinizing hormone, the follicle from which the egg was ovulated develops into a glandlike structure, the corpus luteum. This gland produces hormones (progesterone and estrogen), which prevent the release of other eggs. If the egg is fertilized, it completes meiosis with the sperm DNA inside and the haploid egg and sperm nuclei unite to form the zygote. If the egg is not fertilized, it passes through the vagina to the outside of the body during menstruation.
            One distinguishing characteristic is the relative age of male and female sex cells. In males, sperm production continues throughout the life span. Sperm do not remain in the tubes of the male reproductive system for very long; They are either released shortly after they form or they die and are harmlessly absorbed. In females, meiosis begins before birth, but the oogenesis process is not completed, and an egg cell is not released for many years. An egg released when a woman is 37 years old began meiosis 37 years earlier. During that time, the cell was exposed to many influences, a number of which may have damaged the DNA or interfered with the meiotic process. The increased risk for abnormal births in older mothers may be related to the age of their eggs. Such alterations are less likely to occur in males, because new gametes are being produced continuously.
         Hormones control the cycle of changes in breast tissue, the ovaries, and the uterus. In particular, estrogen and progesterone stimulate milk production by the breasts and cause the uterine lining to become thicker and filled with blood vessels prior to ovulation. This ensures that, if fertilization occurs, the resultant embryo will be able to attach itself to the uterine wall and receive nourishment. If the cell is not fertilized, the lining of the endometrium is shed. This is known as menstruation, menstrual flow, the menses, or a period. Once the endometruim has been shed, it begins to build up again (figure 27.15)  
       The activities of the ovulatory cycle and the menstrual cycle are coordinated. During the first part of the menstrual cycle, increased amounts of FSH cause the follicle to increase in size. Simultaneously, the follicle secretes increased amounts of estrogen, which cause the uterine lining to thicken. When ovulation occurs, the remains of the follicle are converted into a corpus luteum by LH. The corpus luteum begins to secrete progesterone and the nature of the uterine lining changes as a result of the development of many additional blood vessels. This is organized so that, if an embryo arrives in the uterus shortly after ovulation, the uterine lining is prepared to accept it. If pregnancy does not occur, the corpus luteum degenerates, resulting in a reduction in the amount of progesterone needed to maintain the uterine lining, and it is shed. At the same time that hormones are regulating ovulation and the menstrual cycle, changes are taking place in the breasts. The same hormones that prepare the uterus to receive the embryo also prepare the breasts to produce milk. These changes in the breasts, however, are relatively minor unless pregnancy occurs.

Figure 27.13

figure 27.14

figure 27.15

THE HORMONAL CONTROL OF FERTILITY

An understanding of how various hormones influence the menstrual cycle, ovulation, milk production, and sexual behavior has led to the medical use of certain hormones. Some women are unable to have children because they do not release eggs from their ovaries, or they release them at the wrong time. Physicians can regulate the release of eggs using certain hormones, commonly called fertility drugs. These hormones can be used to stimulate the release of eggs for capture and use in what is called in vitro fertilization, also called testtube or IV fertilization. The hormones can also be used to stimulate the release of eggs in women with irregular cycles to increase the probability of natural conception; that is, in vivo fertilization (in-life fertilization) (How Science Works 27.1).
         The use of these techniques often results in multiple embryos being implanted in the uterus. This is likely to occur because the drugs may cause too many eggs to be released at one time. In the case of in vitro fertilization, because there is a high rate of failure and the process is expensive, typically several early-stage embryos are inserted into the uterus to increase the likelihood that one will implant. If several are successful, multiple embryos implant. The implantation of multiple embryos makes it difficult for one embryo to develop properly and be carried through the entire nine-month pregnancy. When scientists understand the action of hormones better, they may be able to control the effects of fertility drugs and eliminate the problem of multiple implantations.
          Another medical use of hormones is the control of conception with birth-control pills—oral contraceptives (table 27.2). Birth-control pills have the opposite effect of fertility drugs. They raise the levels of estrogen and progesterone, which slows the production of FSH and LH, preventing the release of eggs from the ovaries. They can also help relieve premenstrual syndrome (PMS), which causes irritability, emotional instability, depression, headache, and other aches and pains.
        The hormonal control of fertility is not as easy to achieve in men, because there is no comparable cycle of gamete release. However a new, reversible male conception control method for males has been developed. It relies on using an implant containing testosterone, and injections of a progestin, a hormone used in female contraceptive pills. The combination of the two hormones temporarily turns off the normal brain signals that stimulate sperm production. The extra injection of testosterone are given to maintain sex drive.

HOW SCIENCE WORKS 27.1

Assisted Reproductive Technology

The Centers for Disease Control and Prevention defines assisted reproductive technology (ART) as “all fertility treatments in which both eggs and sperm are manipulated. In general, ART involves surgically removing eggs from a woman’s ovaries, combining them with sperm in the laboratory, and returning them to the woman’s body or donating them to another woman. It does NOT include procedures in which only sperm are manipulated (i.e., artificial insemination or intrauterine insemination) or procedures in which a woman takes drugs only to stimulate egg production, without the intention of having eggs retrieved.” There are three types of ART: in vitro fertilization, (IVF) gamete intra-fallopian transfer, (GIFT), and zygote intra-fallopian transfer ZIFT.
        In vitro fertilization is a method that uses hormones to stimulate egg production, removing the egg or eggs from the ovary, and fertilizing it with donated sperm. The fertilized egg is incubated to stimulate cell division in a laboratory dish and then placed in the uterus.
       GIFT relies on the same hormonal treatment as IVF to stimulate ovulation. With ultrasound, the physician observes the transfer of unfertilized eggs and sperm into the woman’s oviduct through small incisions in her abdomen. Once fertilized, the zygote moves down through the oviduct into the uterus and implants. GIFT is only an option for women with open fallopian tubes.
        In the ZIFT procedure, a woman’s mature eggs are collected and fertilized in the laboratory. Then, a scope is inserted into the vagina to find a place for the implantation. Another instrument is used to guide the fertilized egg into the identified position.

Table 27.2

FERTILIZATION, PREGNANCY, AND BIRTH

In most women, an egg is released from the ovary about 14 days before the next menstrual period. The menstrual cycle is usually said to begin on the first day of menstruation. Therefore, if a woman has a regular 28-day cycle, the cell is released approximately on day 14 (review figure 27.15). If a woman normally has a regular 21-day menstrual cycle, ovulation occurs about day 7 in the cycle. If a woman has a regular 40-day cycle, ovulation occurs about day 26. Some women, however, have very irregular menstrual cycles, and it is difficult to determine just when the egg will be released to become available for fertilization. Once the cell is released, it is swept into the oviduct and moved toward the uterus. If sperm are present, they swarm around the egg as it passes down the oviduct, but only one sperm penetrates the outer layer to fertilize it and cause it to complete meiosis. The other sperm contribute enzymes, which digest away the protein and mucous barrier between the egg and the successful sperm (How Science Works 27.2).
         During the second meiotic division, the second polar body is pinched off and the larger of the two cells, the true ovum, is formed. Because chromosomes from the sperm are already inside, they simply intermingle with those of the ovum, forming a diploid zygote. As the zygote continues to travel down the oviduct, it begins to divide by mitosis into smaller and smaller cells, without an increase in the mass of cells (figure 27.16). This division process is called cleavage.
        Eventually, a solid ball of cells is produced, known as the morula stage of embryological development. The morula continues down the oviduct and continues to divide by mitosis. The result is called a blastocyst. The blastocyst becomes embedded or implanted in the uterine lining when it reaches the uterus.
         The blastocyst stage of the embryo eventually develops a tube, which becomes the gut. The formation of the primitive digestive tract is just one of a series of changes that result in an embryo that is recognizable as a miniature human being.
        Most of the time during its development, the embryo is enclosed in a water-filled membrane, the amnion, which protects it from blows and keeps it moist. Two other membranes, the chorion and allantois, fuse with the lining of the uterus to form the placenta (figure 27.17). A fourth sac, the yolk sac, is well developed in reptiles, fish, amphibians, and birds. The yolk sac in these animals contains a large amount of food used by the developing embryo. Although a yolk sac is present in mammals, it is small and does not contain yolk. The embryo’s nutritional needs are met through the placenta. The placenta also produces the hormone chorionic gonadotropin, which stimulates the corpus luteum to continue producing progesterone and thus prevents menstruation and ovulation during pregnancy.
            As the embryo’s cells divide and grow, some of them become differentiated into nerve cells, bone cells, blood cells, or other specialized cells. In order to divide, grow, and differentiate, cells must receive nourishment. This is provided by the mother through the placenta, in which both fetal and maternal blood vessels are abundant, allowing for the exchange of substances between the mother and embryo. The materials diffusing across the placenta include oxygen, carbon dioxide, nutrients, and a variety of waste products. The materials entering the embryo travel through blood vessels in the umbilical cord. The diet and behavior of the mother are extremely important. Any molecules consumed by the mother can affect the embryo. Cocaine; alcohol; heroin; the chemicals in cigarette smoke; and medications, such as Phenytoin (or Dilantin) and Accutane (acne treatment), can all cross the placenta and affect the development of the embryo. Infections by such microbes as rubella virus (German measles), Varicella (chicken pox), and the protozoan Toxoplasma can also result in fetal abnormalities or even death. The growth of the embryo results in the development of major parts of the body by the tenth week of pregnancy. After this time, the embryo continues to increase in size, and the structure of the body is refined.

HOW SCIENCE WORKS 27.2

History of Pregnancy Testing

The answer to the question “Am I pregnant?” once demanded a combination of guesswork, “old wives’ tales” and time. An ancient Egyptian papyrus described a test in which a woman who might be pregnant urinated on wheat and barley seeds over the course of several days: “If the barley grows, it means a male child. If the wheat grows, it means a female child. If both do not grow, she will not bear at all.”
         In the Middle Ages, so-called piss prophets said they could tell if a woman was pregnant by the color of her urine. Urine was described as a “clear pale lemon color leaning toward offwhite, having a cloud on its surface” if she was pregnant. Some mixed wine with urine and watched for changes. In fact, alcohol reacts with certain proteins in urine, so they may have had a little help in guessing.
         In 1927, a test was developed that required the injection of a women’s urine into an immature female rat or mouse. If the woman was not pregnant, the animal showed no reaction. If she was pregnant, the animal went into heat (displayed behaviors associated with a desire to mate) despite its immaturity. This test implied that, during pregnancy, there was an increased production of certain hormones, and that they were excreted in the urine. In the 1930s, a similar test involved injecting urine into rabbits, frogs, toads, and rats. If pregnancy hormones were in the urine, they induced ovulation in the animals.
          However, in 1976, advertisements proclaimed “a private little revolution,” the first home pregnancy tests. The FDA granted approval to four tests: Early Pregnancy Test, Predictor, ACU-TEST, and Answer. All the tests identified changes in hormone levels in the urine of pregnant women.
       The next generation of home pregnancy tests arrived in 2003, when the FDA approved Clearblue Easy’s digital pregnancy test. Instead of showing a thin blue line, the indicator screen now displayed the words “pregnant” or “not pregnant.”

figure 27.16

Figure 27.17

TWINS

In the United States, women giving birth have a 1 in 40 chance of delivering twins and a 1 in 650 chance of triplets or other multiple births. Twins happen in two ways. In the case of identical twins (approximately one-third of all twins), the embryo splits during cleavage into two separate groups of cells. Each group develops into an independent embryo. Because they come from the same single fertilized ovum, they have the same genes and are the same sex.
            Fraternal twins result from the fertilization of two separate eggs by two different sperm. Therefore, they resemble each other no more than do regular brothers and sisters. They do not contain the same genes and are not the same sex.
           Should separation be incomplete, the twins would be born attached to one another, a condition referred to as conjoined twins. Conjoined twins occur once in every 70,000 to 100,000 live births.

BIRTH

The process of giving birth is also known as parturition, or birthing. At the end of about 9 months, hormonal changes in the mother’s body stimulate contractions of the uterine muscles during a period prior to birth called labor. These contractions are stimulated by the hormone oxytocin, which is released from the posterior pituitary. The contractions normally move the baby headfirst and face down through the vagina, or birth canal. One of the first effects of these contractions may be the bursting of the amnion (“bag of water”) surrounding the baby. Following this, the uterine contractions become stronger, and shortly thereafter the baby is born. In some cases, the baby becomes turned in the uterus before labor. If this occurs, the feet or buttocks appear first. Such a birth is called a breech birth. This can be a dangerous situation, because the baby’s source of oxygen may be cut off as the placenta begins to separate from the mother’s body before the baby’s head emerges (figure 27.18). If for any reason the baby does not begin to breathe on its own, it will not receive enough oxygen to prevent the death of nerve cells; thus, brain damage or death can result.
       Occasionally, a baby cannot be born vaginally because of the position of the baby in the uterus. Other factors include the location of the placenta on the uterine wall, the size of the birth canal. A procedure to resolve this problem is the surgical removal of the baby through the mother’s abdomen. This procedure is known as a cesarean, or C-section. The procedure was apparently named after Roman Emperor Julius Caesar, who was said to have been the first child to be delivered by this method. While C-sections are known to have been performed before Caesar, the name stuck.  Following the birth of the baby, the placenta, also called the afterbirth, is expelled. Once born, the baby begins to function on its own. The umbilical cord collapses and the baby’s lungs, kidneys, and digestive system must now support all bodily needs. Birth is quite a shock, but the baby’s loud protests fill the lungs with air and stimulate breathing.
     Over the next few weeks, the mother’s body returns to normal, with one major exception. The breasts, which underwent changes during pregnancy, are ready to produce milk to feed the baby. Following birth, prolactin, a hormone from the pituitary gland, stimulates the production of milk, and oxytocin stimulates its release. If the baby is breast-fed, the stimulus of the baby’s sucking prolongs the time during which milk is produced. This response involves both the nervous and endocrine systems. The sucking stimulates nerves in the nipples and breasts, resulting in the release of prolactin and oxytocin from the pituitary.
        Recent studies have found that breast-fed babies are 20% less likely to die between the ages of 1 and 12 months than are those who are not breast-fed. In addition, the longer babies are breast-fed, the lower is their risk for early death. The American Academy of Pediatrics has recommended exclusive (that is, only human breast milk; no formula, water, or solids) breast-feeding for the first 6 months of life. Breast-feeding should continue throughout the second half of the baby’s first year with the introduction of solid foods. In some cultures, breast-feeding continues for 2 to 3 years, and the continued production of milk often delays the reestablishment of the normal cycles of ovulation and menstruation. Many people believe that a woman cannot become pregnant while she is nursing a baby, a method of birth control called Lactation Amenorrhea Method (LAM). For a woman breast-feeding, the probability of becoming pregnant during the first three months is practically zero. However, because there is so much variation among women, relying on this as a natural conception-control method after the first three months is not a good choice. Many women have been surprised to find themselves pregnant again a few months after delivery.

FIG 27.18

CHEMICAL METHODS

Contraceptive jellies and foams make the vaginal environment more acidic, which diminishes the sperm’s chances of survival. Spermicidal (sperm-killing) foam or jelly is placed in the vagina before intercourse. When the sperm make contact with the acidic environment, they stop swimming and soon die. Aerosol foams are an effective method of conception control, but interfering with the hormonal regulation of ovulation is more effective.

HORMONAL CONTROL METHODS

The first successful method of hormonal control was “the pill,” or “birth control pill.” However, the primary way the birth-control pill works is by preventing ovulation and secondarily by interfering with implantation. The quantity and balance of hormones (estrogen and progesterone) in the pill fools the ovaries into functioning as if the woman were already pregnant. Therefore, ovulation does not occur so conception is highly unlikely. The emergency contraceptive pill (ECP), or “morning-after pill,” uses a high dose of the same hormones found in oral contraceptives, which prevents the woman from becoming pregnant in the first place. In fact, “the pill” in higher dosages can be used as an ECP. ECPs prevent conception in five ways: (1) by delaying or inhibiting ovulation; (2) by inhibiting the transport of the egg or sperm; (3) by preventing fertilization; (4) by inhibiting the implantation of a fertilized egg; or (5) by stimulating an autoimmune response, which kills the sperm. This method is effective if used within 72 hours after unprotected intercourse. One more form of oral contraceptive, the progesteron-only pill (POP or mini pill), is to be taken within the same three hour time period each day, and taken continuously without a break.
               Another method of conception control that may again become available in the United States also involves hormones. The hormones are contained within small rods, or capsules, known as implants, which are placed under a woman’s skin. These rods, when properly implanted, slowly release hormones and prevent the maturation and release of eggs from the follicle. The major advantage of the implant is its convenience. Once the implant has been inserted, the woman can forget about contraceptive protection for several years. If she wants to become pregnant, the implants are removed and her normal menstrual and ovulation cycles return over a period of weeks.
            The hormone-infused contraceptive patch, an adhesive square that can be attached to the abdomen, buttocks, upper arm, or upper torso. The patch works by slowly releasing a combination of estrogen and progestin through the skin, preventing ovulation. The hormones also cause the cervical mucus to thicken, creating a barrier to prevent sperm from entering the uterus. When used correctly, it is about 99% effective. It does not protect against reproductive tract infections, STDs.
           The vaginal ring also uses hormones. The ring is inserted into the vagina, where it releases a continuous low dose of estrogen and progestin for 21 days. At the end of the 21 days, the ring is removed for 7 days to allow the menstrual period to occur. A new ring is then inserted monthly.
         Contraceptive hormones can also be used to stop menstrual periods preventing the symptoms of premenstrual syndrome and improving a woman's sex life. The birth control pill, Seasonale, limits periods to four a year. The first continuous use birth control pill, Lybrel, may soon be available in the United States.

THE TIMING METHOD

Killing sperm and preventing ovulation are not the only methods of preventing conception. Any method that prevents sperm from reaching the egg prevents conception. One method is to avoid intercourse during the times of the month when an egg may be present. This is known as the rhythm method of conception control. Although at first glance it appears to be the simplest and least expensive, determining when an egg is likely to be present can be very difficult. A woman with a regular 28-day menstrual cycle typically ovulates about 14 days before the onset of the next menstrual flow. To avoid pregnancy, couples need to abstain from intercourse a few days before and after this date. However, if a woman has an irregular menstrual cycle, there may be only a few days each month for intercourse without the possibility of pregnancy. In addition to calculating safe days based on the length of the menstrual cycle, a woman can better estimate the time of ovulation by keeping a record of changes in her body temperature and vaginal pH. Both changes are tied to the menstrual cycle and can therefore help a woman predict ovulation. In particular, at about the time of ovulation, a woman has a slight rise in body temperature—less than 1°C. Thus, she should use an extremely sensitive thermometer. A digital-readout thermometer on the market spells out the word yes or no.

BARRIER METHODS

Other methods of conception control that prevent sperm from reaching the egg include the diaphragm, vaginal cap, sponge, and condom. A diaphragm is a specially fitted membranous shield that is inserted into the vagina before intercourse and positioned so that it covers the cervix, which contains the opening of the uterus. Because of anatomical differences among females, diaphragms must be fitted by a physician. The diaphragm’s effectiveness is increased if spermicidal foam or jelly is also used. A vaginal cap functions in a similar way. A contraceptive sponge, as the name indicates, is a small amount of absorbent material soaked in a spermicide. The sponge is placed within the vagina, where it chemically and physically prevents sperm cells from reaching the egg.
           The male condom is probably the most popular contraceptive device. It is a thin sheath placed over the erect penis before intercourse. In addition to preventing sperm from reaching the egg, this physical barrier also helps prevent the transmission of the microbes that cause sexually transmitted diseases (STDs), such as syphilis, gonorrhea, and AIDS, from being passed from one person to another during sexual intercourse. The most desirable condoms are made of a thin layer of latex that does not reduce the sensitivity of the penis. Latex condoms have also been determined to be the most effective in preventing transmission of HIV. The condom is most effective if it is prelubricated with a spermicidal material, such as nonoxynol-9. This lubricant also has the advantage of providing some protection against the spread of HIV.
     Condoms for women are also available. One, called the Femidom, is a polyurethane sheath, which, once inserted, lines the contours of the woman’s vagina. It has an inner ring, which sits over the cervix, and an outer ring, which lies flat against the labia. Research shows that this device protects against STDs and is as effective a contraceptive as the condom used by men. The intrauterine device (IUD) is not a physical barrier that prevents the gametes from uniting; in fact, the way in which this mechanical device works is not completely known. This synthetic apparatus might interfere with the implantation of the embryo. The IUD must be fitted and inserted into the uterus by a physician, who can also remove it if pregnancy is desired. While not 100% effective, IUDs are used successfully in many countries. The IUD can also be used for “emergency contraception”—in cases of unprotected sex, forced sex, or the failure of a conception-control method (e.g., a broken condom). The IUD must be inserted within 7 days of unprotected sex.

SURGICAL METHODS

Two contraceptive methods that require surgery are tubal ligation and vasectomy (figure 27.19). Tubal ligation is the cutting and tying off of the oviducts; it can be done on an outpatient basis, in most cases. An alternative to sterilization by tubal ligation involves the insertion of small, flexible devices, called Micro-inserts, into each oviduct (Fallopian tube). Once inserted, tissue grows into the inserts, blocking the tubes. Ovulation continues as usual, but the sperm and egg cannot unite.
        Vasectomy is not the same as castration. Castration is the surgical removal of testes. A vasectomy can be performed in a physician’s office. A small opening is made above the scrotum, and the spermatic cord (vas deferens) is cut and tied. This prevents sperm from moving through the ducts to the outside. Because most of the semen is produced by the seminal vesicles, prostate gland, and bulbourethral glands, a vasectomy does not interfere with normal ejaculation. The sperm that are still being produced die and are reabsorbed in the testes.
        Neither tubal ligation nor vasectomy interferes with normal sex drives. However, these medical procedures are generally not reversible and should not be considered by those who may want to have children at a future date. These procedures are not 100% effective since, in rare instances, the tubes might reattach, allowing gametes to pass from one severed side of the duct to the other.

FIG 27.19

TERMINATION OF PREGNANCY—ABORTION

Another medical procedure often associated with birth control is abortion, which has been used throughout history. Abortion is the death and removal of a developing embryo through various medical procedures. Abortion is not a method of conception control; rather, it prevents the normal development of the embryo. Abortion is a highly charged subject: Some people believe that abortion should be prohibited by law in all cases; others think that abortion should be allowed in certain situations, such as in pregnancies that endanger the mother’s life or in pregnancies that are the result of rape or incest. Still others think that abortion should be available to any woman under any circumstances. Regardless of the moral and ethical issues that surround abortion, it is still a common method of terminating unwanted pregnancies. The three most common techniques performed early in pregnancy are scraping the inside of the uterus with instruments, (called dilation and curettage, or D and C); injecting a saline solution into the uterine cavity; and using a suction device to remove the embryo from the uterus. In the future, abortion may be accomplished by a medication prescribed by a physician. One drug, RU-486, is currently used in about 15% of the elective abortions in France, and it has received approval for use in the United States. The medication is administered orally under the direction of a physician, and several days later, a hormone is administered. This usually results in the onset of contractions, which expel the fetus. A follow-up examination of the woman is made after several weeks to ensure that there are no serious side effects. Late-term abortions are done after the 20th week of gestation. Late-term abortion is not a medical term, and the exact point when a pregnancy becomes late term has not been defined. Nor is there exact data for the number performed. Late-term abortions performed for fetal abnormality are involved procedures called intact dilation and extraction (IDX or D&X), also known as partial-birth abortion.

CHANGES IN SEXUAL FUNCTION WITH AGE

Although there is a great deal of variation, at about age 50, a woman’s hormonal balance begins to change because of changes in the ovaries’ production of hormones. At this time, the menstrual cycle becomes less regular and ovulation is often unpredictable. Over several years, the changes in hormone levels cause many women to experience mood swings and physical symptoms, including cramps and hot flashes. Menopause is the period when a woman’s body becomes nonreproductive, because reproductive hormones stop being produced. This causes the ovaries to stop producing eggs, and menstruation ends. Occasionally, the symptoms associated with menopause become so severe that they interfere with normal life and the enjoyment of sexual activity. A physician might recommend hormone replacement therapy (HRT) to augment the natural production of the hormones estrogen and progestin or progestin or other therapies may be used to help manage symptoms. Normally, the sexual enjoyment of a healthy woman continues during menopause and for many years thereafter. Human males do not experience a relatively abrupt change in their reproductive or sexual lives. However, as men age, their production of sperm declines and they may experience a variety of problems related to their sexuality. The word impotence is used to describe problems that interfere with sexual intercourse and reproduction. These may include a lack of sexual desire, problems with ejaculation or orgasm, and erectile dysfunction (ED). Erectile dysfunction is the recurring inability to get or keep an erection firm enough for sexual intercourse. Most incidences of ED at any age are physical not psychological. In older men, this is usually the result of injury, disease, or the side effects of medication. Damage to nerves, arteries, smooth muscles, and other tissues associated with the penis is the most common cause of ED. Diseases linked with ED include diabetes, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease, and neurologic disease. Blood pressure drugs, antihistamines, antidepressants, tranquilizers, appetite suppressants, and certain ulcer drugs have been associated with ED. Other possible causes are smoking, which reduces blood flow in veins and arteries, and lowered amounts of testosterone. ED is frequently treated with psychotherapy, behavior modification, oral or locally injected drugs, vacuum devices, and surgically implanted devices. ED is not an inevitable part of aging. Rather, sexual desires tend to wane slowly as men age. They produce fewer sperm cells and less seminal fluid. Nevertheless, healthy individuals can experience a satisfying sex life during aging. Recent evidence also indicates that men also experience hormonal and emotional changes similar to those seen as women go through menopause.
      Human sexual behavior is quite variable. The same is true of older persons. The range of responses to sexual partners continues, but generally in a diminished form. People who were very active sexually when young continue to be active, but less so as they reach middle age. Those who were less active tend to decrease their sexual activity also. It is reasonable to state that one’s sexuality continues from before birth until death.