The reproductive system is the structural and physiological network whose purpose is the creation of a new life to continue the species. It is the only body system that is not concerned with supporting the life of its host. Human reproduction is sexual—meaning that both a male and a female are required to produce a life. Gender is determined at conception by the sex chromosome in the sperm that fertilizes an egg. The developing male or female has a reproductive system characteristic of its sex. However, boys and girls can not reproduce until sexual maturation occurs at puberty. The male reproductive system is designed specifically to produce and deliver sperm to the egg in the female. The female reproductive system is designed to develop ova (eggs) and prepare for egg fertilization by a sperm. The male and female systems are both anatomically and biochemically designed to join and make a new life. However, the reproductive system is unique among body systems in that a person may choose not to use it to its full capacity—to procreate. Individuals can decide not to reproduce.

The male reproductive system

The main tasks of the male reproductive system are to provide sex hormones, to produce sperm, and to transport sperm from the male to a female. The first two tasks are performed by the testes, while the third job is carried out by a series of ejaculatory ducts and the penis. The two testes are contained within the scrotum, which hangs below the body between the legs. Each testis is attached at its top to an epididymis, which contains numerous sperm ducts. The epidiymides (plural) send sperm through the vas deferens to the penis. However, the seminal vesicles, prostate, and bulbo-urethral glands each contribute to the seminal fluid, which carries the sperm to the penis. The epididymides and part of the vas deferens are within the scrotum, but the glands creating the seminal fluid are in the abdomen.

Testes

Each of the testes is divided into lobes, or septae, containing coiled seminiferous tubules lined with spermatozoa-producing cells. Between the tubules are hormone-producing cells called interstitial cells, or cells of Leydig. Testosterone is produced by the interstitial cells. Since the testes-containing scrotum hangs below the body, it has a temperature around 89°F (32°C)— ideal for sperm production, which requires a low temperature. When the scrotum is held too close to the body by restrictive clothing, sterility can result.

The seminiferous tubules are the site of sperm maturation from original germ cells (spermatogonia) to mature sperm (spermatoza). This process begins in puberty and is called spermatogenesis. If a small section of a tubule was removed for observation, the wall would appear thick with a hole, or lumen, in the middle. The outer-most layer of this life saver-shaped cut-out is called the basal lamina. Primitive

spermatogonia line the basal lamina and move through the inner layers of the tubule towards the lumen as they mature. Sertoli cells surround the maturing sperm and form tight junctions with one another to closely regulate what nutrients enter the developing sperm. Sertoli cells supply the spermatogenic cells with important ions such as potassium. They also form a blood-testes barrier, which prevents some harmful substances from entering the tubule and spermatogenic cells and entering the man’s blood. The unique genetic composition of individual sperm cells would cause an immune system attack on the circulating sperm. Sperm genetic diversity is created in the seminiferous tubule during spermatogenesis.

Spermatogenesis processes spermatogonia to spermatozoa in stages. Spermatogonia undergo mitotic divisions to yield primary spermatocytes that have 46 chromosomes identical to other cells in the male’s body. Primary spermatocytes then go through two more divisions—this time meiotic—to form secondary spermatocytes and spermatids. Each final spermatid contains 23 randomly-assorted chromosomes that contain all necessary genetic information.

The final phase of spermatogenesis involves structural change. The sperm cell elongates, forming the long flagellum, or tail, which propels it toward an egg. Chromosomes are tightly packed into the sperm head, and an acrosomal tip appears on top of the head that contains enzymes that help the sperm burrow into an egg. In addition, mitochondria are wound around the flagellum’s base to fuel the sperm’s journey through the female reproductive tract. This shape change completes maturation of spermatids into spermatozoa, or sperm. However, they are still immotile. Sperm enter the lumen of the seminferous tubules and travel in a very concentrated form to the epididymis. The sperm become mobile after about two weeks in the epididymis and are sent to the vas deferens for storage.

The full maturation of a single sperm takes about 70 to 80 days. Hence, substances a male is exposed to during that period of time may effect the health of his sperm at the end of that time period. Sperm are always available in healthy males after puberty, because spermatogenesis is an ongoing process with cells in all stages of development existing in different layers of the seminiferous tubules. Up to several hundred million sperm can be produced each day. In addition, one man has approximately one-fourth mile of coiled seminiferous tubules that produce all these sperm.

Late spermatogenic stages are dependent on testosterone secreted by the interstitial cells of the testes. At puberty, male levels of luteinizing hormone (LH) are elevated due to increased secretion by the anterior pituitary (AP) gland. LH has also been called interstitial-cell-stimulating hormone (ICSH) in men, because it stimulates Leydig cells to secrete testosterone. Follicle-stimulating hormone (FSH) is also secreted by the AP and directs early stages of spermatogenesis. Testosterone from the testes is also necessary for secondary sexual characteristics such as facial and body hair growth, voice deepening, and pubertal genital growth.

The spermatic ducts and glands

The vas deferens carries concentrated sperm from the scrotum into the abdominal cavity to the ejacula-tory duct. Sperm that remain in the ejaculatory duct longer than a couple of weeks will degenerate; and are disposed of. The prostate surrounds the ejaculatory duct and contains a sphincter that closes off the bladder during ejaculation. Seminal fluid from the seminal vesicles, the prostate, and the bulbo-urethral glands (or Cowper’s glands) is added to the sperm. The seminal fluid plus the sperm is called semen.

Seminal fluid is designed to carry and nourish sperm. Seminal vesicles are located on either side of the bladder and contribute about 60% of the fluid. Seminal vesicle fluid is rich in essential sperm nutrients such as fructose that sustains sperm for up to 72 hours after ejaculation. Seminal vesicle fluid also supplies prostaglandins that cause uterine contractions in the female reproductive tract to facilitate sperm movement to an egg. The prostate gland provides an alkaline mixture of calcium, enzymes, and other components that make up about 30% of the seminal fluid. The alkaline fluid functions to neutralize the acidic vaginal environment that can kill sperm. Additional fluid is provided by the Cowper’s glands (below the prostate), which secrete a pre-ejaculatory urethral lubricant that may contain some sperm. For this reason, withdrawal is not a foolproof contraceptive method. At ejaculation, additional Cowper secretions combine with the remaining seminal fluid and sperm. This semen is sent through the urethra in the penis.

The penis

The penis provides the route for transmitting sperm to an egg for reproduction. However, in its relaxed state, it cannot effectively deliver sperm. In order for the sperm to have the best chance of fertilizing an egg, the penis must become erect and ejaculate semen close to an egg in the female reproductive tract.

The penis is part of the male’s external reproductive system, which becomes longer, thicker, and stiff during erection. It comprises a shaft region that is the cylindrical body of the penis and the glans, or head region. The glans and the shaft are separated at the coronal ridge, which is a rim of tissue that is very sensitive to touch. The skin covering the penis is loose and allows for expansion during erection. Some males have a prepuce or foreskin, which is a movable skin that covers the penile glans. Circumcised males have had this foreskin removed. Uncircumcised males must carefully clean the foreskin daily to prevent bacteria and foul-smelling secretions (called smegma) from accumulating.

Three cylinders of spongy erectile tissue make up the internal portion of the penis. Two cylinders run along the inner roof of the penis and are called the corpora cavernosa. The third cylinder runs along the lower side of the penis; it contains the urethra and is called the corpus spongiosum, or spongy body. The spongy body includes the penile tip and is more sensitive to touch than the rest of the penis. Several nerves and blood vessels run through the spongy body. An erection occurs when blood flow to the spongy tissue vessels increases. An average erect penis is 6.25 in (15.9 cm) long and 1.5 in (3.8 cm) wide at its base.

Sexual arousal

Sexual intercourse does not necessarily lead to reproduction, but the physiology of reproductive versus non-reproductive sexual arousal is indistinguishable. Masters and Johnson have divided sexual arousal into four stages. These stages are the same whether the arousal results from physical stimulation (such as touch) or mental stimulation (such as reading an arousing book). Hence, arousal can be influenced by personal beliefs, desires, or values. The stages of arousal are: excitement, plateau, orgasm, and resolution.

The male stage of sexual excitement is marked by increased blood flow to the pelvic area and penis. Increased parasympathetic nerve activity causes the blood vessels in the penis to dilate, allowing for vasocongestion, which leads to an erection. This may happen in a matter of seconds. Testes size also increases.

The amount of time spent in the plateau phase varies considerably. In this stage, the head of the penis enlarges and darkens from blood pooling. Testes darken, enlarge from vasocongestion, and are lifted back away from the penis. At this point, preejaculatory secretion from the bulbo-urethral gland occurs, and respiration, heartrate, and blood pressure increase.

Male orgasm results from both emission and ejaculation. Emission is the release of the ejaculatory fluid into the urethra. Emission is caused by increased sympathetic nerve stimulation in the ejaculatory ducts and glands, which leads to rhythmic contractions that force the fluid out. For ejaculation, rhythmic contractions of the urethra expel the semen (usually 3 to 5 ml) while the prostate gland closes off the bladder.

In the resolution phase, blood exits the penis and testes, and the penis relaxes. Respiration, blood pressure, and heart rate return to normal, and sexual arousal enters a refractory period. During the refractory period, erection cannot occur. The length of refractory period varies from a couple of minutes to several hours and increases with fatigue and age.

The female reproductive system

The main tasks of the female reproductive system are to produce hormones, develop ova, receive sperm, and promote fertilization and the growth of a newly conceived life. These events occur internally. Ova mature in the ovaries. Sperm are received in the vagina and cervix. Fertilization takes place usually in the fallopian tubes and less often in the uterus, with the newly formed life developing in the endometrial lining of the uterus. The female reproductive tract can be pictured as a capital Y with the upper arms forming the fallopian tubes. The ovaries would be at the end of these arms. The uterus would be the upper half of the supporting stalk, and the vagina would be the lower half. External female genitals are involved in female sexual arousal.

The ovaries

The ovaries are oval-shaped and about 1.0 to 1.5 in (2.5 to 3.8 cm) long. They are connected to the body of the uterus by an ovarian ligament that tethers the ovaries in place. The ovaries parallel the testes in that they release sex hormones and develop gametes (ova or sperm). However, the job of the ovaries differs from that of the testes: while sperm are created daily through a man’s life after puberty, all of a female fetus’s eggs have been created by the sixth gestational month. Several million primordial follicles capable of forming ova are formed. About one million primor-dial follicles mature into primary follicles that still exist at birth. (The rest have degenerated.) When puberty begins, about 400,000 follicles remain. Mature eggs leave alternating ovaries monthly beginning in puberty in a process called ovulation. Unfertilized eggs are lost through menstruation, when the uterine lining is shed. Women typically menstruate for 30 to

40 years losing 360 to 480 eggs in a lifetime. Ovulation is hormonally suppressed during pregnancy and shortly after childbirth.

The formation of mature ova in the ovaries is called oogenesis. The anterior pituitary (AP) hormones LH (luteinizing hormone) and FSH (follicle stimulating hormone), which regulate spermatogenesis, also orchestrate oogenesis. However, unlike spermatogenesis that occurs daily, oogenesis is on an average 28 day (or monthly) cycle. During embryonic development, primordial follicles are formed, each of which contains an oocyte surrounded by a layer of spindle-shaped cells. These spindle cells multiple during the mid-fetal stage of development and become granulosa cells, which surround the egg. Granulosa cells function much like the Sertoli cells in men: they prevent destructive drugs from getting to the egg while also providing essential nutrients for its development. Granulosa cells also secrete a rich substance that forms a follicular coating called the zona pellucida. Before birth, the cellular layers surrounding the follicle differentiate into a layer of cells called the theca interna. At birth, a baby girl’s ova are suspended at the first meiotic division inside the primary follicles. After the onset of puberty, a new follicle enters the next phase of follicular growth monthly.

The first two weeks of the menstrual cycle are called the follicular phase because of the follicular development that occurs during that time. High FSH levels trigger this development. Although more than one follicle begins to mature each month, one follicle outgrows the others, and slow-growing follicles stay in the ovary to degenerate by a process called atresia. The granulosa cells of the dominant follicle secrete estrogens into the fluid bathing the oocyte inside the follicle. The highly vascular theca interna layer, which is outside the granulosa cells, releases estrogens which enter the female circulation. A build up of circulating estrogen will signal release of additional FSH and LH that initiate the second half of the menstrual cycle. Around day 14 of the cycle, LH and FSH surge to initiate ovulation. Ovulation entails the release of the mature oocyte from the ovarian follicle as it ruptures from the surface of the ovary into the abdominal cavity. Once released, the ovum is caught by the fimbria, which are finger-like projections off the ends of the fallopian tubes. The follicle that housed the growing egg remains in the ovary and is transformed into the corpus luteum. The corpus luteum secretes high levels of progesterone and some estrogen. The corpus luteum secures a position near the ovarian blood vessels to supply these hormones, which prevent another follicle from beginning maturation. If the ovum is fertilized, then these hormone levels continue into pregnancy to prevent another cycle from beginning. However, if fertilization does not occur, then the corpus luteum degenerates allowing the next cycle to start. The second 14 days of the menstrual cycle are called the luteal phase because of the corpus luteum’s hormonal control over this half of the cycle.

The fallopian tubes

The optimal time for an oocyte to be fertilized is when it enters a fallopian tube. The fallopian tubes are fluid-filled, cilia-lined channels from about 4 to 6 in (10 to 15 cm) long that carry the oocyte to the uterus. At ovulation, the primary oocyte completes its suspended meiosis and divides in two. A secondary oocyte and a small polar body result. If the secondary oocyte is fertilized, then it will go through another division which forms another polar body.

As the ripening egg travels along the fallopian tube, it is washed along by cilia which knock away residual nutrient cells on the outside of the egg. This array of cells leaving the cell forms a radiant cluster called the corona radiata. If sperm have made their way to the fallopian tube, then they have already been capacitated. Capacitation is the modification of a sperm’s acrosomal tip that enables it to burrow into the egg. Fertilization blocks the ability of additional sperm to enter the egg. Once the nuclei of the egg and sperm cells have fused, the new cell is called a zygote. The zygote contains all the genetic information required to become a complete human being. This new life signifies the beginning of successful reproduction. As the zygotic cell divides into more cells, it travels from the fallopian tube to the uterus.

The uterus

The uterus, or womb, is a muscular, inverted pear-shaped organ in the female pelvis that is specifically designed to protect and nurture a growing baby. It averages 3 in (7.6 cm) long by 2 in (5 cm) wide. Pregnancy allows it to expand with the growing embryo and fetus. Embryo is a term used to describe a human in the first eight weeks of development. After that, the human is called a fetus.

During the follicular phase of the menstrual cycle, the lining (or endometrium) of the uterus becomes thick and filled with many blood vessels in preparation for supporting an embryo. If fertilization does not occur within about eight days of ovulation, then this lining is shed in menstrual blood through the cervix. This cycle continues until menopause, when menstruation becomes less frequent and eventually stops altogether.

The cervix is the base of the uterus that extends into the vagina. The narrow passageway of the cervix is just large enough to allow sperm to enter and menstrual blood to exit. During childbirth, it becomes dilated (open) to allow the baby to move into the vagina, or birth canal. However, for most of the pregnancy, the cervix becomes plugged with thick mucous to isolate the developing baby from vaginal events. For this reason, non-reproductive sexual intercourse is usually safe during pregnancy.

The uterus is required for reproduction. With all the male and female aspects contributing to reproduction, a number of diseases, genetic disorders, and other variables can cause infertility, which afflicts 10–15% of couples trying to conceive. Technologies such as in vitro fertilization exist for some couples with infertility due to ovarian, fallopian tube, or sperm problems. However, without a uterus, a human baby cannot grow. The uterus plays an integral hormonal and physical role in housing and nourishing the baby.

The vagina

The vagina is a muscular tube about 5 in (12.7 cm) long. A thin layer of tissue called the hymen may cover the vaginal opening, but it is usually gone in physically or sexually active females. A mucous membrane lines and moistens the vagina. During sexual intercourse, the vagina is lubricated further and functions to direct the penis toward the cervix to optimize fertilization. During childbirth, the vagina stretches to accommodate the passage of the baby. Both the uterus and the vagina contract to relatively original sizes some time after delivery.

Some contraceptive devices act as a barrier between semen and the vagina or semen and the cervix. A condom placed correctly on a man’s penis can prevent sperm from entering the vagina. A diaphragm is a rubber, cup-shaped contraceptive inserted into the vagina prior to intercourse that acts as a physical barrier between semen and the cervix; it is usually used along with a spermicidal jelly to chemically kill sperm. Other contraceptives, such as the birth control pill and depo-provera usually inhibit the function of progesterone to prevent ovulation.

External genitals and sexual arousal

External female genitals include the mons veneris, labia majora, labia minora, clitoris, and vestibule. They differ in size and color from female to female, but their location and function are consistent. The mons is a pad of fatty tissue filled with many nerve

KEY TERMS

Androgens— Male sex hormones including testosterone and androstenedione.

Meiosis— In meiosis, a cell’s 46 chromosomes duplicate and go through two successive cellular divisions to create germ cells (sperm and eggs) each containing 23 chromosomes.

Mitosis— In mitosis, the 46 human chromosomes double and divide into two daughter cells each containing 46 chromosomes. Oogenesis—The formation of mature eggs in the female ovaries after the onset of puberty.

Seminiferous tubules— Tubes lining the testes that produce sperm.

Spermatogenesis— The formation of mature sperm in the male testes after the onset of puberty.

Spermatozoa— Mature sperm capable of fertilizing an egg.

endings, which becomes covered with pubic hair in puberty. The labia majora are two folds of skin, which protect the opening to the urethra and internal genitals. Pubic hair grows on their outer surface in puberty. These fat padded folds of skin contain sweat glands, nerve endings, and numerous blood vessels. Inside these outer skin folds are the labia minora, which are hairless. The labia minora form a spongy covering for the vaginal entrance. These smaller skin folds meet at the top of the genitals to form the clitoral hood. The hood houses the clitoris, a very sensitive organ, which has a spongy shaft and a nerve-rich glans (tip). Between the labia minora and the vagina is the area called the vestibule. Within the vestibule are the two Bartholin’s glands, which lubricate the vagina.

Sexual arousal in females parallels the arousal stages in males. Female sexual arousal is not required to reproduce, but it does facilitate reproduction. In the excitement phase, blood flow to the vagina increases which, in turn, pushes fluid into the vaginal canal. This lubricating process is called transudation and allows for comfortable penile insertion. During this phase, blood infiltrates the spongy clitoris and labia, and the cervix and uterus are lifted up away from the vagina. Respiration, heart rate, and blood pressure increase.

During the plateau stage, the vagina expands, forming a pocket near the cervix, which is an ideal deposit site for sperm; this is called tenting. The increased sensitivity of the clitoris causes it to retract in the clitoral hood, and breasts sometimes become flushed. In the orgasmic phase, the vaginal opening contracts rhythmically for about 15 seconds. Unlike the lengthy refractory period, which males experience in the resolution stage, females are more likely to be multi-orgasmic and capable of more closely spaced orgasms. In the resolution stage, genital blood flow returns to normal. Respiration, heart rate, and blood pressure also return to normal. Within 72 hours of sexual intercourse reproduction will either have successfully begun or not succeeded.