Let's talk about sex.Why should something that is as integral a part of life as breathing be so difficult to discuss openly? We think about sex from the time our infant hands first discover that our genitals are as fun to play with as our toes (and are easier to reach!), yet it's a subject that is shrouded in mystery to most children and even many adults. Read on, and open your eyes to the miracles of the human body at its most vulnerable and its most powerful.

Approximately once a month, a woman's body gears up for reproduction. Her ovary releases an egg. While her body waits to see if that egg is destined to become an embryo, her uterus prepares so that it will be ready to incubate the potential life from conception to birth. The egg can be fertilized only during the brief window of time in which it is viable. If a sperm wriggles through the egg's sugary shell during this time, an embryo is formed. That embryo, given a bit of luck and nine months of living the high life in its mother's womb, will be born into the world as a son ...or daughter.

One of the many seemingly magical things about human reproduction is that only one sperm can ever penetrate an egg. Many will attach to it, striving not to have their genetic material wasted after their exhausting journey through the female reproductive tract. There they swim valiantly and frantically in a hostile environment that only the strongest sperm can survive. But only one will eventually get through.

As soon as it does, the egg changes, sealing off its borders in a process known as the zonal reaction, leaving the remaining sperm to their doom.

This is the first sign of how much the woman controls in human reproduction, but there is one aspect that is biologically forever out of reach: her body has no say in the sex of her future child. All eggs contain a single x chromosome, common to both men and women. It's the sperm that contain the complementary X or Y that will determine whether the child is a boy (XY), or a girl (XX). The man's genes determine a child's sex.

Sex, however, is not just for reproduction—not in humans. That may be its primary biological mandate, but it serves other purposes as well; otherwise it wouldn't be so much fun. It helps strengthen relationships, encourages a man to stick around to care for his child, and affects not only health but also well-being. It makes us fascinating to others, and we continue to explore its fascination to us.

Unisex isn't just a term forclothing; it's also an accurate description of human beings during their earliest development in the womb. For more than a month, all embryos, whether they are destined to be male or female, grow in a way that looks exactly the same. They take after their mother. It's only natural, really, since she provides the vast majority of the support structure in which they grow. The cytoplasm of the egg, a complex bath of essential nutrients; mitochondria, the biological engines that provide the energy of life; and the womb, the incubator in which the embryo starts to thrive—all are given by Mom. At this stage of development, the father has provided his DNA, a smidgen of cytoplasm, a structure called the centrosome—which is necessary for the fertilized egg to undergo cell division—and, at the site of sperm penetration, the point of reference around which the left and right sides of the body will eventually form.

Adam may have been created first in the Bible, but it's Eve who is the original child of every womb. All embryos, male (XY) or female (XX), start off along the developmental pathway to womanhood, and for the first six weeks of growth they look exactly the same. In order for a boy to be formed, the Y chromosome must not only send out the correct chemical signals to the brain and body, but those signals must be properly received.

The definition of biological sex is a difficult one. You might think that a look at the 23rd chromosome pair should be enough to tell us whether a child is a boy or a girl, but sometimes it's not that simple. When we look at a newborn baby, we don't take blood and examine the shapes of the chromosomes to figure out if it's male or female; we look at the child and know. Or at least we think we do. Most of the time, what you see is what you get, but the process of sexual development is complex and can occasionally lead to subtle or fascinating surprises. If some sex-determining genes did not function correctly, or if the growing fetus is exposed to an external source of hormones, what you see isn't always what you get. A child who, at birth, appears to be a boy may actually have the genes of a girl. The girl you expect to see grow into a woman may turn out, at puberty, to actually be a man. Life is full of surprises, and one of them is how little difference there is between the sexes. After all, even by the most generous estimate, the number of genes unique to men represents only around one percent of the human genetic code ... that's far less than the relative size of an extra rib.

The spark that starts male development along its unique pathway comes from the sex-determining region Y (SRY) gene on the Y chromosome. This gene's products, unique to male fetuses, kick off the development of the testicles at around the 6th week of development. If SRY is not present, or not working, by its second month the fetus will start to grow a uterus and continue along the road to womanhood.

By the 8th week of development, the fetal gonads—the organs that will develop into the ovaries or testicles—can be clearly determined. The development of the gonads is critical to the correct formation of the male and female sex organs. Before this point, the internal genitalia consisted of two undifferentiated sets of ducts: the Mullerian ducts and the Wolffian ducts. In healthy fetuses, the Mullerian ducts will grow into the uterus, fallopian tubes, and inner vagina—unless anti-mullerian hormone (AMH) is produced by the testicles to tell them to degenerate. The name AMH clearly describes its purpose: it tells the Mullerian ducts to wither. Without it, since the Mullerian ducts need no hormonal stimulation to grow, XY fetuses would end up both male and female on the inside. The Wolffian ducts, on the other hand, will degenerate unless they're stimulated by testosterone to grow into the epididymis, vas deferens, and seminal vesicles.

The testicles start producing AMH and testosterone after they differentiate in week six. If, in a male fetus, the testicles don't form correctly, they may not produce AMH or testosterone. Either mistake can have serious effects. For example, a fetus that produces and responds to AMH, but not testosterone, may have no obvious internal genital development. A fetus that produces or is exposed to testosterone, but doesn't make or respond to AMH, may actually have characteristics of both male and female genitalia. Fortunately, these abnormalities are rare.

Finally, by the 28th week, the testicles have descended to their final location in the scrotum. The inguinal canal, which remains after birth in both men and women, will have been used almost like an elevator—the gubernacula, a cord-like ligament inside it, pulling the testicles down. The ovaries, in contrast, stay where they are in the abdomen—tethered by the body in their well-protected location.

When we look at a child, it's not the differences on the inside that we see, but those on the outside. The irony is that although the internal genitalia develop from two separate duct systems, the external genitalia of men and women are actually derived from the same tissues! The genital tubercle (which will become the penis or the clitoris), urogenital groove and sinus, and labioscrotal folds appear in all fetuses by the end of the 7th week of development. What they develop into depends on whether or not they're stimulated by the fetal androgens (testosterone and dihydrotestosterone, or DHT). If they're not stimulated by these androgens, they develop into the female sex organs the clitoris, urethra, vagina, and labia. If they are stimulated by androgens, they merge into the two male sex structures—the penis and scrotum. The external genitalia are, therefore, thought of as homologous structures ... which is just a way of saying that they're really very similar. In a male, what would be the clitoris in a female lengthens into the penis, the urethra becomes enclosed in its shaft, and the labia fold to enclose the testicles as the scrotum.

Male fetuses become visibly different from female fetuses in the second and third month of pregnancy. It's not the commonly known hormone testosterone, but instead its highly active derivative DHT, that is primarily responsible for this differentiation. Male fetuses that lack 5-alpha reductase, the enzyme that transforms testosterone into DHT, may be incompletely masculinized and have indeterminate external genitalia that are neither completely masculine nor completely feminine. Similar indeterminate genitalia can be seen in female fetuses that are exposed to androgens while in the womb. Such androgen exposure can be caused by environmental exposures, certain drugs, and maternal or fetal health problems. At puberty, the external genitalia develop further, and in individuals with an abnormal hormone mix the appearance of the genitals may change completely. But in general, adult sexual anatomy is established by the end of the third trimester of pregnancy.

Where Did These Come From?

Shopping for the first bra is a pivotal moment at the start of womanhood, but breasts aren't the only things that grow during puberty. In girls, the growth spurt begins before puberty, and actually tapers out at around the time of the first period, which is known as menarche, from the Greek words for "moon” (or monthly) and “beginning."

At around the time that breast growth begins, the brain increases its production of the hormones that will regulate a woman's reproductive life. The increase in follicle stimulating hormone (FSH), luteinizing hormone (LH), and gonadotropin-releasing hormone (GnRH) stimulates the development of the ovaries. Estrogen produced by the ovaries causes most of the changes in the external genitalia and also plays an important role in bone growth. Estrogen is also created in fatty tissue, however, an extremely slender young woman may have delayed puberty. This is most commonly seen in athletes like dancers and gymnasts, who sometimes maintain a childlike appearance well into their late teens or 20s. A minimum amount of body fat is desirable and even necessary, because too little body fat or too-severe dieting may delay and even prevent menarche. For most young women, it takes almost two years of change for her body to prepare for her first period.

Menarche is not the only "first” for women that involves blood. Blood is also associated with another important moment in a young woman's life: the time of her first sexual intercourse. In fact, not all women bleed at this time, and those who do experience varying amounts of discomfort. In some cultures, the "blood on the sheets” is a traditional way to confirm a young woman's virginity the morning after she is wed. Bleeding during first intercourse is the result of the disruption of the hymen—a piece of membranous tissue that blocks the vagina to varying degrees. The hymen almost always has an opening wide enough to allow for unimpeded menstrual flow or for the insertion of a tampon. But the size of the hymen can range from a thin ring that stretches easily and doesn't impede penetration at all, to a thick covering that actually must be opened by a medical professional to allow intercourse to occur. This condition is called having an imperforate hymen, and it occurs in approximately one in every thousand women.

One of the first signs of puberty in girls is breast growth. At first, small lumps may appear under the nipples, which then expand as puberty continues and hormone levels rise. While a girl's breasts are still growing, it is not uncommon for the two breasts to be of different size or shape, simply because they are growing at different rates. It's important to remember that breast size has nothing to do with femininity.

During puberty, a girl's body reshapes itself in many ways. The hips widen, which will ease the birth of future children. Girls begin to take on a womanly shape as their waist narrows and fat is deposited in the abdomen, waist, and buttocks. Adolescence can be a difficult time when biology wants teenage girls to fatten up, just when they're most concerned about staying slender.

As estrogen levels rise, hair begins to grow in new locations—the underarms, the legs, and the pubic region. This hair tends to be thicker and curlier than the hair on the scalp. Girls also start to sweat more, and their scent may change, since glands throughout their body and skin are growing as well.

During adolescence, the genitals begin to assume their adult appearance. Fat is deposited throughout the body, including the genital area, giving the pubic mound its soft and rounded appearance. The outer labia—the vaginal lips—fatten as well, while the inner labia remain as folds of connective tissue and skin.

Go With the Flow

The curse. The gift. The visitor. These are only a few of the words and phrases that people use to describe the shedding of the uterine lining that signals a young woman's body is capable of bearing children. In other words, they're euphemisms for menstruation—and they reflect the wide range of attitudes toward this pivotal monthly event. A woman's first menstrual period may be exciting or exhausting, painful or profound, depending on how and where she was raised. In some cultures it's a source of pride and celebration, while in others it's barely discussed. To various degrees, many women are segregated from their society when they're bleeding: they may be prohibited from prayer, required to abstain from sex, or physically separated from their families during the time of menses. One of the most interesting rituals surrounding menstruation is undertaken by Orthodox Jewish women who follow the rules of Taharat Hamishpachah, or family purity. These women partake in a ritual bath, the mikvah, after the end of each menstrual period before they can resume sexual relations with their husbands. In recent years, many less religious women have also turned to this activity as a way of regularly reaffirming both their sexuality and their faith.

The uterus is a self-cleaning organ: every month it cycles out the residue of reproduction to start fresh again. During the early phase of the menstrual cycle, rising estrogen levels cause the lining of the uterus to thicken. This rise in estrogen is stimulated by the FSH, which is made in the anterior pituitary gland. The follicle is the receptacle in the ovary in which the egg develops, and the FSH stimulates the egg's growth while it is in the follicle. At the middle of the cycle, there is a surge of another pituitary hormone, the LH. Now ovulation, release of the egg, occurs. At this point, the cells that had been nurturing the egg start producing the sex hormone progesterone, which causes the uterine lining to further thicken and prepare for pregnancy. If pregnancy does not occur, estrogen and progesterone levels fall and the uterus sheds its thickened lining to prepare for another reproductive cycle. This shedding, menstruation, usually lasts three to seven days and removes both blood and tissue from the uterus.

Sometimes menstruation can be a very unpleasant process. The hormones involved, and the process of expelling blood and tissue from the uterus, can cause emotional distress, cramping, stomach pains, diarrhea, breast tenderness, and numerous other symptoms. Around 15 percent of women have pain so severe during their periods that it's actually disabling and affects their ability to go to school or work. A smaller percentage have mood swings so serious that they consider suicide. Fortunately, most women's periods cause only a moderate amount of discomfort.

Just like the uterus, the vagina is also self-cleaning. Vaginal secretions clean out bacteria and debris, and the vaginal interior does not need to be washed. Douching is, in fact, a major source of vaginal and reproductive health problems, not a cure for them. Instead of cleaning the vagina, douching is far more likely to wash bacteria and other organisms up into the uterus where they can change from being a minor irritation to a source of serious complication.

Menstruation is not only an important part of a woman's life, it's also the basis for a major industry. Most woman menstruate every month that they aren't pregnant, and they do so for more than 20 years. Over this time women use a huge number of menstrual collection devices. Although the most common products work by absorbing blood either externally (menstrual pads) or internally (tampons), a growing number of women are experimenting with different types of menstrual collection devices. Reusable menstrual cups or pads, for example, are appealing to women who dislike the waste associated with disposable products. Menstrual cups are also good for women who enjoy having sex during their periods, because unlike tampons or pads they can be used to block the flow of menstrual blood during penetration.

Size Doesn't Matter

In men, unlike in women, the onset of puberty is not visible to the outside world. Instead, the first sign that adulthood is imminent is the growth of the testicles, which increase in size as their hormone production begins to increase. These hormones, in turn, cause the growth of pubic hair, the thinning of the scrotum, and the enlargement of the penis. The locker room question, though, of how large a boy's penis is will rarely be answered before he turns 16, and some boys maintain penis growth for several more years. As puberty continues, boys' muscle structure increases, their voice "breaks," and they finally start to grow—after several uncomfortable years of being shorter than most of the girls in their grade. Although most men are taller than most women, it's estrogen that is in charge of bone growth. Boys don't have enough estrogen in their system to start their growth spurt until girls are already reaching their final height.

Puberty in males can be an audible experience. Although vocal changes occur in girls as well, these changes are much more noticeable in boys. In boys, when the larynx increases in size, it also experiences a growth of cartilage into what is commonly called the Adam's apple. As the voice deepens and the vocal cords lengthen, these changes can cause some loss of vocal agility. This "cracking" has been the source of humor in many teen movies and television shows. Fortunately, it resolves over time.

Although it has profound effects on fetal sexual development, during puberty the effects of DHT are only slightly stronger than those of testosterone. The large volumes of testosterone that are suddenly present more than make up for the strength of DHT, which is why some genital abnormalities may actually resolve at this time. Genetically male children lacking in DHT may appear to be female until puberty, at which point the increase in testosterone causes the testicles to descend and the apparent clitoris to enlarge into a penis.

It's not just a boy's body that changes during puberty; so does his brain. Although men have the same regulatory hormones as women (GnRH, LH, and FSH), their brains express them in a completely different way. Instead of varying monthly to create a menstrual cycle, the hormones in men's brains are constantly active. Maybe men and women really do have different ways of thinking about sex…

Nocturnal Emissions

Just as a girl's first period is a defining moment of womanhood, a boy's first nocturnal emission, or wet dream, is a defining moment of manhood. It happens when hormones have reached new, higher levels in a young man's system. The increase in sperm production and sexual tension can cause frequent involuntary erections, both at night and during the day. The three-ring binder might have been invented as a discreet way for young men to hide an unfortunate erection occurring during the school day! Masturbation is a natural response to this increased level of sexual arousal. For teenage boys, it is an instinctive and effective way to reduce the number of spontaneous erections and the embarrassment they sometimes cause.

So Many Changes

The voice deepens during puberty because testosterone causes the larynx, or voice box, to increase in size. The vocal cords also lengthen at this time. Some boys may experience their voice "cracking" in this period, jumping back and forth between different types of sounds, as their body tries to adjust to the changing shape of their vocal organs. In the not-so-distant past, boy sopranos with beautiful voices were castrated to keep their voices intact.

Shaving for the first time can be a very exciting experience for boys. During puberty, facial hair begins to grow on the face. It also begins to grow on the underarms and legs, in the pubic area, and on the chest. Not all men develop chest hair, and for some men it begins to grow much later in life.

Chest hair, like acne, is highly affected by a man's levels of dihydrotestosterone, which can change over time.

Body shape changes dramatically as boys grow into manhood. Height increases, the shoulders broaden, and muscle growth is vastly increased. At this time, there may be some swelling of a boy's nipples, but this is normal and does not mean that he is growing breasts.

Penis and testicular growth can be a stressful component of puberty. Boys can feel compelled to compare their penis size to that of their friends, but it is important to remember that size has nothing to do with masculinity. And, although in most cases the penis has reached its final size by age 16, in some men it can continue to grow for several more years.

Sperm and Testosterone: The Next Generation

Testosterone Production

Just as the pituitary hormones FSH and LH are important in establishing the menstrual cycle, they also have important effects on male hormone production. LH stimulates the growth of the leydig cells, which are the primary site of testosterone production. FSH is far more involved in the stimulation of sertoli cells, which are involved in the formation of sperm, or spermatogenesis.

Testosterone is mostly produced in the leydig cells. but low levels are also synthesized in the adrenal cortex. Testosterone starts out as cholesterol and then goes through four steps before reaching its final form. Leydig cells are found in the body of the testicles, along with blood vessels and macrophages, not in the seminiferous tubules where sperm are produced. Leydig cells are vital to spermatogenesis since without their testosterone production it would not occur. That the level of sperm production is directly proportional to the number of leydig cells further shows their importance.

Spermatogenesis

The production of sperm takes almost three months from the first meiotic cell division until the fully grown sperm are ready for ejaculation. Spermatogenesis is near its peak in the late teens and early twenties. Unlike women, who are born with a fixed number of gametes, or reproductive cells, that slowly are used or die off over time, men are constantly making new sperm. This means that men may remain fertile throughout their lifespan, although as they age the level of sperm production may drop. This decline can be due to decreased testosterone levels or to loss of elasticity, or sclerosis, in the seminiferous tubules. Although the prostate makes important contributions to the volume of seminal fluid, it is possible for a man without one to remain fertile.

Five Stages of Puberty

The progress of puberty can be assessed using the Tanner Stages of Puberty Scale, developed by the British physician Jim Tanner. During stage one, boys grow slowly, around two inches per year; they have no coarse pubic hair and no penile growth. During stage two, growth remains similar, but pubic hair begins to appear and the penis begins to increase in both length and width. At this time, fat levels also begin to decrease and the scrotum thins and reddens. During stage three, growth increases to three inches per year, pubic hair is found all over the pubis, and the voice may break. It is also during this stage that muscle mass begins to increase and there may be some swelling behind the nipples. During stage four, growth is at its fastest rate of around four inches per year, underarm hair appears, vocal changes continue, and acne begins to be a problem for some young men. Finally, in stage five, growth of both the body and the penis ceases, facial hair appears along with a mature male physique, and pubic hair spreads to the thighs. Although there are no set durations for any of the stages, they can be used to monitor a teenager's progression to adulthood and may help to flag any abnormalities in growth or hormonal development while they are still easy to treat.

Contrary to popular belief, masturbation is not a behavior that begins at puberty. It has been observed in children and infants, and in general it is a healthy behavior that causes no physical or psychological problems. In other words, it's not going to make you go crazy or blind. And, although men are more likely to masturbate than women and tend to masturbate more frequently, it also isn't going to make hair grow on your palms ... or anywhere else. Since masturbation carries no risk of pregnancy or sexually transmitted disease, some people, including former Surgeon General Joycelyn Elders, consider it to be the safest form of sexual expression. It may also have health benefits: masturbation is known to reduce the severity of menstrual cramps, and one study has suggested that frequent ejaculation during a man's 20s and 30s may actually decrease his risk of developing prostate cancer.

Sexual differentiation isn't a process that only happens in the body-it also happens in the brain. Exposure to hormones, like testosterone and DHT, affects the way the brain develops during pregnancy as well as a person's behavior after birth. Although the genes that control their development are largely the same, male and female brains are functionally quite different. For example, women are better at distinguishing colors, while men are better at tracking motion. There are also many subtle developmental differences that affect both sex and sexuality. A great deal of research suggests that hormone exposure before birth shapes sexual identity throughout life.

The three main sex hormones are considered to be estrogen, testosterone, and DHT. But despite their designation as sex hormones, their roles are not confined to the genital tract. They're not even made just in the gonads, but in the skin, adrenals, and other organs as well. One of the main sites of estrogen production is actually the fatty tissue. This is why weight can have such a profound effect on the timing of puberty. The effects of sex hormones aren't limited to reproduction; they work on many of the body's systems. Estrogen, for instance, affects bone growth, and testosterone and DHT influence the formation of hair and muscle.

One of the most interesting things about sex hormones is that, contrary to popular belief, all three hormones are present in both sexes. Estrogen is not just a female hormone, any more than testosterone just belongs to males. Yes, women have more estrogen than men, but some research suggests that estrogen may be essential for maintenance of the male libido, or sex drive. Men have greater levels of testosterone, but without enough of it sexual desire and function in women is greatly reduced. Some scientists think that it's actually the asymmetrical levels of hormones that create healthy sexual responses in women and men.