In this chapter, you learned about the male and female reproductive systems. Specifically, you learned that:

• The reproductive system is the human organ system responsible for the production and fertilization of gametes and, in females, the carrying of a fetus.
• Both male and female reproductive systems have organs called gonads (testes in males, ovaries in females) that produce gametes (sperm or ova) and sex hormones (such as testosterone in males and estrogen in females). Sex hormones are endocrine hormones that control prenatal development of sex organs, sexual maturation at puberty, and reproduction after puberty.
• The reproductive system is the only organ system that is significantly different between males and females. A Y-chromosome gene called SRY is responsible for undifferentiated embryonic tissues developing into a male reproductive system. Without a Y chromosome, the undifferentiated embryonic tissues develop into a female reproductive system.
• Male and female reproductive systems are different at birth, but immature and nonfunctioning. Maturation of the reproductive system occurs during puberty when hormones from the hypothalamus and pituitary gland stimulate the gonads to produce sex hormones again. The sex hormones, in turn, cause the physical changes experienced during puberty.
• Male reproductive system organs include the testes, epididymis, penis, vas deferens, prostate gland, and seminal vesicles.

• • The two testes are sperm- and testosterone-producing male gonads. They are contained within the scrotum, a pouch that hangs down behind the penis. The testes are filled with hundreds of tiny, tightly coiled seminiferous tubules, where sperm are produced. The tubules contain sperm in different stages of development, as well as Sertoli cells, which secrete substances needed for sperm production. Between the tubules are Leydig cells, which secrete testosterone.
  • The two epididymides are contained within the scrotum. Each epididymis is a tightly coiled tubule where sperm mature and are stored until they leave the body during an ejaculation.
  • The two vas deferens are long, thin tubes that run from the scrotum up into the pelvic cavity. During ejaculation, each vas deferens carries sperm from one of the epididymides to one of the pair of ejaculatory ducts.
  • The two seminal vesicles are glands within the pelvis that secrete fluid through ducts into the junction of each vas deferens and ejaculatory duct. This alkaline fluid makes up about 70% of semen, the sperm-containing fluid that leaves the penis during ejaculation. Semen contains substances and nutrients that sperm need to survive and “swim” in the female reproductive tract.
  • The prostate gland is located just below the seminal vesicles and surrounds the urethra and its junction with the ejaculatory ducts. The prostate secretes an alkaline fluid that makes up close to 30% of semen. Prostate fluid contains a high concentration of zinc, which sperm need to be healthy and motile.
  • The ejaculatory ducts form where the vas deferens joins with the ducts of the seminal vesicles in the prostate gland. They connect the vas deferens with the urethra. The ejaculatory ducts carry sperm from the vas deferens, and secretions from the seminal vesicles and prostate gland that together form semen.
  • The paired bulbourethral glands are located just below the prostate gland. They secrete a tiny amount of fluid into semen. The secretions help lubricate the urethra and neutralize any acidic urine it may contain.
  • The penis is the external male organ that has the reproductive function of intromission, which is delivering sperm to the female reproductive tract. The penis also serves as the organ that excretes urine. The urethra passes through the penis and carries urine or semen out of the body. Internally, the penis consists largely of columns of spongy tissue that can fill with blood and make the penis stiff and erect. This is necessary for sexual intercourse so intromission can occur.
• Parts of a mature sperm include the head, acrosome, midpiece, and flagellum. The process of producing sperm is called spermatogenesis. This normally starts during puberty and continues uninterrupted until death.

• • Spermatogenesis occurs in the seminiferous tubules in the testes, and requires high concentrations of testosterone. Sertoli cells in the testes play many roles in spermatogenesis, including concentrating testosterone under the influence of follicle stimulating hormone (FSH) from the pituitary gland.
  • Spermatogenesis begins with a diploid stem cell called a spermatogonium, which undergoes mitosis to produce a primary spermatocyte. The primary spermatocyte undergoes meiosis I to produce haploid secondary spermatocytes, and these cells in turn, undergo meiosis II to produce spermatids. After the spermatids grow a tail and undergo other changes, they become sperm.
  • Before sperm are able to “swim,” they must mature in the epididymis. The mature sperm are then stored in the epididymis until ejaculation occurs.
• Ejaculation is the process in which semen is propelled by peristalsis in the vas deferens and ejaculatory ducts from the urethra in the penis.
• Leydig cells in the testes secrete testosterone under the control of luteinizing hormone (LH) from the pituitary gland. Testosterone is needed for male sexual development at puberty and to maintain normal spermatogenesis after puberty. It also plays a role in prostate function and the ability of the penis to become erect.
• Disorders of the male reproductive system include erectile dysfunction (ED), epididymitis, prostate cancer, and testicular cancer.

• • ED is a disorder characterized by the regular and repeated inability of a sexually mature male to obtain and maintain an erection. ED is a common disorder that occurs when normal blood flow to the penis is disturbed or there are problems with the nervous control of penile engorgement or arousal.

• • • Possible physiological causes of ED include aging, illness, drug use, tobacco smoking, and obesity, among others. Possible psychological causes of ED include stress, performance anxiety, and mental disorders.
    • Treatments for ED may include lifestyle changes, such as stopping smoking and adopting a healthier diet and regular exercise. However, the first-line treatment is prescription drugs such as Viagra® or Cialis® that increase blood flow to the penis. Vacuum pumps or penile implants may be used to treat ED if other types of treatment fail.
  • Epididymitis is inflammation of the epididymis. It is a common disorder, especially in young men. It may be acute or chronic and is often caused by a bacterial infection. Treatments may include antibiotics, anti-inflammatory drugs, and painkillers. Treatment is important to prevent the possible spread of infection, permanent damage to the epididymis or testes, and even infertility.
  • Prostate cancer is the most common type of cancer in men and the second leading cause of cancer death in men. If there are symptoms, they typically involve urination, such as frequent or painful urination. Risk factors for prostate cancer include older age, family history, a high-meat diet, and sedentary lifestyle, among others.

• • • Prostate cancer may be detected by a physical exam or a high level of prostate-specific antigen (PSA) in the blood, but a biopsy is required for a definitive diagnosis. Prostate cancer is typically diagnosed relatively late in life, and is usually slow growing, so no treatment may be necessary. In younger patients or those with faster-growing tumors, treatment is likely to include surgery to remove the prostate, followed by chemotherapy and/or radiation therapy.
  • Testicular cancer, or cancer of the testes, is the most common cancer in males between the ages of 20 and 39 years. It is more common in males of European than African ancestry. A lump or swelling in one testis, fluid in the scrotum, and testicular pain or tenderness are possible signs and symptoms of testicular cancer.

• • • Testicular cancer can be diagnosed by a physical exam and diagnostic tests, such as ultrasound or blood tests. Testicular cancer has one of the highest cure rates of all cancers. It is typically treated with surgery to remove the affected testis, and this may be followed by radiation therapy, and/or chemotherapy. Normal male reproductive functions are still possible after one testis is removed, if the remaining testis is healthy.
• The female reproductive system is made up of internal and external organs that function to produce haploid female gametes called ova, secrete female sex hormones (such as estrogen), and carry and give birth to a fetus.
• Female reproductive system organs include the ovaries, oviducts, uterus, vagina, clitoris, and labia.

• • The vagina is an elastic, muscular canal that can accommodate the penis. It is where sperm are usually ejaculated during sexual intercourse. The vagina is also the birth canal, and it channels the flow of menstrual blood from the uterus. A healthy vagina has a balance of symbiotic bacteria and an acidic pH.
  • The uterus is a muscular organ above the vagina where a fetus develops. Its muscular walls contract to push out the fetus during childbirth. The cervix is the neck of the uterus that extends down into the vagina. It contains a canal connecting the vagina and uterus for sperm or an infant to pass through. The innermost layer of the uterus, the endometrium, thickens each month in preparation for an embryo but is shed in the following menstrual period if fertilization does not occur.
  • The oviducts extend from the uterus to the ovaries. Waving fimbriae at the ovary ends of the oviducts guide ovulated ova into the tubes where fertilization may occur as the ova travel to the uterus. Cilia and peristalsis help eggs move through the tubes. Tubular fluid helps nourish sperm as they swim up the tubes toward eggs.
  • The ovaries are gonads that produce eggs and secrete sex hormones including estrogen. Nests of cells called follicles in the ovarian cortex are the functional units of ovaries. Each follicle surrounds an immature ovum. At birth, a baby girl’s ovaries contain at least a million eggs, and they will not produce any more during her lifetime. One egg matures and is typically ovulated each month during a woman’s reproductive years.
  • The vulva is a general term for external female reproductive organs. The vulva includes the clitoris, two pairs of labia, and openings for the urethra and vagina. Secretions from Bartholin’s glands near the vaginal opening lubricate the vulva.
  • The breasts are technically not reproductive organs, but their mammary glands produce milk to feed an infant after birth. Milk drains through ducts and sacs and out through the nipple when a baby sucks.
• Oogenesis is the process of producing eggs in the ovaries of a female fetus. Oogenesis begins when a diploid oogonium divides by mitosis to produce a diploid primary oocyte. The primary oocyte begins meiosis I and then remains at this stage in an immature ovarian follicle until after birth.
• After puberty, one follicle a month matures and its primary oocyte completes meiosis I to produce a secondary oocyte, which begins meiosis II. During ovulation, the mature follicle bursts open and the secondary oocyte leaves the ovary and enters a oviducts.
• While a follicle is maturing in an ovary each month, the endometrium in the uterus is building up to prepare for an embryo. Around the time of ovulation, cervical mucus becomes thinner and more alkaline to help sperm reach the secondary oocyte.
• If the secondary oocyte is fertilized by a sperm, it quickly completes meiosis II and forms a diploid zygote, which will continue through the oviducts. The zygote will go through multiple cell divisions before reaching and implanting in the uterus. If the secondary oocyte is not fertilized, it will not complete meiosis II, and will soon disintegrate.
• Pregnancy is the carrying of one or more offspring from fertilization until birth. The maternal organism must provide all the nutrients and other substances needed by the developing offspring, and also remove its wastes. She should also avoid exposures that could potentially damage the offspring, especially early in the pregnancy when organ systems are developing.

• • The average duration of pregnancy is 40 weeks (from the first day of the last menstrual period) and is divided into three trimesters of about three months each. Each trimester is associated with certain events and conditions that a pregnant woman may expect, such as morning sickness during the first trimester, feeling fetal movements for the first time during the second trimester, and rapid weight gain in both fetus and mother during the third trimester.
  • Labour, which is the general term for the birth process, usually begins around the time the amniotic sac breaks and its fluid leaks out. Labour occurs in three stages: dilation of the cervix, birth of the baby, and delivery of the placenta (afterbirth).
• The physiological function of female breasts is lactation, or the production of breast milk to feed an infant. Sucking on the breast by the infant stimulates the release of the hypothalamic hormone oxytocin from the posterior pituitary, which causes the flow of milk. The release of milk stimulates the baby to continue sucking, which in turn keeps the milk flowing. This is one of the few examples of positive feedback in the human organism.
• The ovaries produce female sex hormones, including estrogen and progesterone. Estrogen is responsible for sexual maturation and secondary sex characteristics at puberty. It is also needed to help regulate the menstrual cycle and ovulation after puberty until menopause. Progesterone prepares the uterus for pregnancy each month during the menstrual cycle, and helps maintain the pregnancy if fertilization occurs.
• The menstrual cycle refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus for pregnancy. It involves changes in both the ovaries and uterus and is controlled by pituitary hormones (FSH and LH) and ovarian hormones (estrogen and progesterone).

• • The female reproductive period is delineated by menarche, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.
  • The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle. It includes the follicular phase, when a follicle and its ovum mature due to rising levels of FSH; ovulation, when the ovum is released from the ovary due to a rise in estrogen and a surge in LH; and the luteal phase, when the follicle is transformed into a structure called a corpus luteum that secretes progesterone. In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.
  • The events of the menstrual cycle that take place in the uterus make up the uterine cycle. It includes menstruation, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase, during which the endometrium builds up again until ovulation occurs; and the secretory phase, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo.
• Disorders of the female reproductive system include cervical cancer, vaginitis, and endometriosis.

• • Cervical cancer occurs when cells of the cervix grow abnormally and develop the ability to invade nearby tissues, or spread to other parts of the body. Worldwide, cervical cancer is the second-most common type of cancer in females and the fourth-most common cause of cancer death in females. Early on, cervical cancer often has no symptoms; later, symptoms such as abnormal vaginal bleeding and pain are likely.

• • • Most cases of cervical cancer occur because of infection with human papillomavirus (HPV), so the HPV vaccine is expected to greatly reduce the incidence of the disease. Other risk factors include smoking and a weakened immune system. A Pap smear can diagnose cervical cancer at an early stage. Where Pap smears are done routinely, cervical cancer death rates have fallen dramatically. Treatment of cervical cancer generally includes surgery, which may be followed by radiation therapy or chemotherapy.
  • Vaginitis is inflammation of the vagina. A discharge is likely, and there may be itching and pain. About 90% of cases of vaginitis are caused by infection with microorganisms, typically by the yeast Candida albicans. A minority of cases are caused by irritants or allergens in products such as soaps, spermicides, or douches.

• • • Diagnosis of vaginitis may be based on characteristics of the discharge, which can be examined microscopically or cultured. Treatment of vaginitis depends on the cause, and is usually an oral or topical anti-fungal or antibiotic medication.
  • Endometriosis is a disease in which endometrial tissue grows outside the uterus. This tissue may bleed during the menstrual period and cause inflammation, pain, and scarring. The main symptom of endometriosis is pelvic pain, which may be severe. Endometriosis may also lead to infertility.

• • • Endometriosis is thought to have multiple causes, including genetic mutations. Retrograde menstruation may be the immediate cause of endometrial tissue escaping the uterus and entering the pelvic cavity. Endometriosis is usually treated with surgery to remove the abnormal tissue and medication for pain. If surgery is more conservative than hysterectomy, endometriosis may recur.
• Infertility is the inability of a sexually mature adult to reproduce by natural means. It is defined scientifically and medically as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse.
• About 40% of infertility in couples is due to female infertility, and another 30% is due to male infertility. In the remaining cases, a couple’s infertility is due to problems in both partners or to unknown causes.
• Male infertility occurs when there are no or too few healthy, motile sperm. This may be caused by problems with spermatogenesis or by blockage of the male reproductive tract that prevents sperm from being ejaculated. Risk factors for male infertility include heavy alcohol use, smoking, certain medications, and advancing age, to name just a few.
• Female infertility occurs due to failure to produce viable ova by the ovaries or structural problems in the oviducts or uterus. Polycystic ovary syndrome is the most common cause of failure to produce viable eggs. Endometriosis and uterine fibroids are possible causes of structural problems in the oviducts and uterus. Risk factors for female infertility include smoking, stress, poor diet, and older age, among others.
• Diagnosing the cause(s) of a couple’s infertility generally requires testing both the man and the woman for potential problems. For men, semen is likely to be examined for adequate numbers of healthy, motile sperm. For women, signs of ovulation are monitored, for example, with an ovulation test kit or ultrasound of the ovaries. For both partners, the reproductive tract may be medically imaged to look for blockages or other abnormalities.

• • Treatments for infertility depend on the cause. For example, if a medical problem is interfering with sperm production, medication may resolve the underlying problem so sperm production is restored. Blockages in either the male or the female reproductive tract can often be treated surgically. If there are problems with ovulation, hormonal treatments may stimulate ovulation.
  • Some cases of infertility are treated with assisted reproductive technology (ART). This is a collection of medical procedures in which eggs and sperm are taken from the couple and manipulated in a lab to increase the chances of fertilization occurring and an embryo forming. Other approaches for certain causes of infertility include the use of a surrogate mother, gestational carrier, or sperm donation.
• Infertility can negatively impact a couple socially and psychologically, and it may be a major cause of marital friction or even divorce. Infertility treatments may raise ethical issues relating to the costs of the procedures and the status of embryos that are created in vitro but not used for pregnancy. Infertility is an under-appreciated problem in developing countries where birth rates are high and children have high economic as well as social value. In these countries, poor health care is likely to lead to more problems with infertility and fewer options for treatment.
• More than half of all fertile couples worldwide use contraception (birth control), which is any method or device used to prevent pregnancy. Different methods of contraception vary in their effectiveness, typically expressed as the failure rate, or the percentage of women who become pregnant using a given method during the first year of use. For most methods, the failure rate with typical use is much higher than the failure rate with perfect use.
• Types of birth control methods include barrier methods, hormonal methods, intrauterine devices, behavioural methods, and sterilization. Except for sterilization, all of the methods are reversible.

• • Barrier methods are devices that block sperm from entering the uterus. They include condoms and diaphragms. Of all birth control methods, only condoms can also prevent the spread of sexually transmitted infections.
  • Hormonal methods involve the administration of hormones to prevent ovulation. Hormones can be administered in various ways, such as in an injection, through a skin patch, or, most commonly, in birth control pills. There are two types of birth control pills: those that contain estrogen and progesterone, and those that contain only progesterone. Both types are equally effective, but they have different potential side effects.
  • An intrauterine device (IUD) is a small T-shaped plastic structure containing copper or a hormone that is inserted into the uterus by a physician and left in place for months or even years. It is highly effective even with typical use, but it does have some risks, such as increased menstrual bleeding and, rarely, perforation of the uterus.
  • Behavioural methods involve regulating the timing or method of intercourse to prevent introduction of sperm into the female reproductive tract, either altogether or when an egg may be present. In fertility awareness methods, unprotected intercourse is avoided during the most fertile days of the cycle as estimated by basal body temperature or the characteristics of cervical mucus. In withdrawal, the penis is withdrawn from the vagina before ejaculation occurs. Behavioural methods are the least effective methods of contraception.
  • Sterilization is the most effective contraceptive method, but it requires a surgical procedure and may be irreversible. Male sterility is usually achieved with a vasectomy, in which the vas deferens are clamped or cut to prevent sperm from being ejaculated in semen. Female sterility is usually achieved with a tubal ligation, in which the oviducts are clamped or cut to prevent sperm from reaching and fertilizing eggs.
  • Emergency contraception is any form of contraception that is used after unprotected vaginal intercourse. One method is the “morning after” pill, which is a high-dose birth control pill that can be taken up to five days after unprotected sex. Another method is an IUD, which can be inserted up to five days after unprotected sex.

In this chapter, you learned how the male and female reproductive systems work together to produce a zygote. In the next chapter, you will learn about how the human organism grows and develops throughout life — from a zygote all the way through old age.