reproductive system terms assignment

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High school biology

Course: high school biology   >   unit 8.

• Welcome to the reproductive system
• Egg, sperm, and fertilization

The reproductive system review

• The reproductive system

The female reproductive system

The male reproductive system, common mistakes and misconceptions.

• Fertilization occurs in the fallopian tube (oviduct) of the female reproductive system. Once fertilized, the egg attaches to the lining of the uterus. It becomes a ball of cells over time, then develops in the uterus of the female to become a baby.
• Only females are born with reproductive sex cells. Females are born with immature eggs already in their ovaries. When puberty occurs, the eggs mature and are released by the ovaries. Males only produce sperm after reaching puberty.
• Females do not urinate through the vagina. In men, both semen and urine pass through the urethra, a passageway that terminates at the end of the penis. Females urinate through a urethra as well, but it is not connected to their vaginal opening. The female urethra is located above the vagina and urine may pass over or around the opening, but the two passageways are not connected.

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• Types of Bones
• Axial Skeleton
• Appendicular Skeleton
• Skeletal Pathologies
• Muscle Tissue Types
• Muscle Attachments & Actions
• Muscle Contractions
• Muscular Pathologies
• Functions of Blood
• Blood Vessels
• Circulation
• Circulatory Pathologies
• Respiratory Functions
• Upper Respiratory System
• Lower Respiratory System
• Respiratory Pathologies
• Oral Cavity
• Alimentary Canal
• Accessory Organs
• Absorption & Elimination
• Digestive Pathologies
• Lymphatic Structures
• Immune System
• Urinary System Structures
• Urine Formation
• Urine Storage & Elimination
• Urinary Pathologies
• Female Reproductive System
• Male Reproductive System
• Reproductive Process
• Endocrine Glands

Glossary of the Reproductive System

Gametes, the male and female sex cells, are produced through meiosis in the ovaries and testes.

System: Reproductive

Region: Pelvis

Function: The sperm and the egg are gametes. They each contain half the genetic information necessary for reproduction. When a sperm cell penetrates and fertilizes an egg, that genetic information combines.

The ovaries produce secondary oocytes—the female sex cells. Each month one secondary oocyte is released into the uterine tube. If the oocyte is fertilized, it implants in the uterus.

The ovaries are two almond-shaped structures that sit on either side of the uterus, connected to the uterine tubes. See it in 3D !

System: Endocrine, Reproductive

Function: They produce oocytes (egg cells), as well as estrogen, progesterone, and other hormones.

Pathologies: Infertility, ovarian cysts, pelvic inflammatory disease, polycystic ovary syndrome, premature ovarian failure

Egg cell production, or oogenesis, begins with the primordial follicles. As girls reach puberty, each ovary contains thousands of these follicles, and each follicle contains a primary oocyte. When follicles mature, some primary oocytes become secondary oocytes. By the time of ovulation, there is only one mature follicle remaining. The rest of the follicles deteriorate. During ovulation (about once a month), the dominant follicle bursts and releases its secondary oocyte. The oocyte travels into the uterine tube, where it can be fertilized.

Uterine tube (Fallopian tube)

Tuba uterina

The uterine tubes (also called Fallopian tubes or oviducts) connect the ovaries to the uterus. Each uterine tube can be divided into three parts: The infundibulum is open to the abdomen. A constricted section called the isthmus connects with the uterus. Finally, an intermediate and dilated portion, the ampulla, curves over the ovary.

Function: Egg fertilization usually occurs in the ampulla. The eggs then travel through the isthmus into the uterus.

Pathologies: Ectopic pregnancy, infertility

The uterus is a pear-shaped organ located in the pelvic cavity between the bladder and the rectum. It is a hollow organ with thick, muscular walls. See it in 3D !

Function: During menstruation, the inner lining of the uterus is shed. When a woman becomes pregnant, however, the fertilized egg embeds itself in the uterine wall and menstruation is prevented. The uterus expands dramatically as the egg develops into an embryo and then a growing fetus.

Pathologies: Endometriosis, gonorrhea, infertility, pelvic inflammatory disease, uterine fibroids

Female reproductive cycle

Women of childbearing age go through a cycle about every 28 days that makes it possible to become pregnant. A follicle in the ovary develops and releases a secondary oocyte at the same time that the lining of the uterus thickens to prepare for the possibility of a fertilized egg. These cycles begin at puberty and continue until menopause. During pregnancy the cycles are suspended.

Cervix uteri

The lower part of the uterus constricts into a segment called the cervix , which leads to the vagina. See it in 3D !

Function: The cervix is the passageway for menstrual flow, for entering sperm, and for childbirth. Glands in the mucous membrane of the cervix secrete a clear, viscous, alkaline mucus that changes character at different times during a female’s menstrual cycle.

Pathologies: Endometriosis, genital warts, gonorrhea, HPV, infertility, pelvic inflammatory disease, trichomoniasis, uterine fibroids

The vagina extends down from the cervix, the lower part of the uterus, to the vestibule, which is part of the vulva and the external genitalia. It sits behind the bladder and in front of the rectum. See it in 3D !

Function: The vagina has three core functions: it carries menstrual flow outside the body, it receives the male penis during sexual intercourse, and it serves as a birth canal during labor.

Pathologies: Chlamydia infection, genital herpes, genital warts, HPV, infertility, syphilis, trichomoniasis, yeast infections

The external genitalia (vulva) of the female reproductive system include the mons pubis, the labia majora and labia minora, the clitoris and prepuce (clitoral hood), the vestibule of the vagina, urethral orifice, and the greater vestibular glands.

Pathologies: Genital warts

Vestibulum vaginae

The vestibule encompasses the vaginal orifice, the external urethral orifice, and in some females, the hymen. Lateral to the vaginal orifice are masses of erectile tissue known as the bulbs of the vestibule. When a female is sexually aroused, each bulb engorges with blood and swells around the vaginal opening.

Labia minora

Labia minora pudendi

The labia minora are situated between the labia majora and extend from the clitoris obliquely downward, laterally, and backward for about 4 cm on either side of the orifice of the vagina, enclosing the vestibule. Unlike the labia majora, the labia minora lack pubic hair.

Labia majora

The two labia majora have an outer pigmented surface covered with pubic hair and an inner surface well-endowed with sebaceous (oil) and sudoriferous (sweat) glands.

System: Integumentary, Reproductive

Developmentally homologous to the penis, the clitoris consists of two corpora cavernosa composed of erectile tissue enclosed in a dense layer of fibrous membrane.

Function: The clitoris has an extremely high concentration of nerve endings. It can engorge with blood when stimulated and functions in female sexual arousal.

Spermatozoon

The testes constantly produce sperm—the male sex cells. Sperm are produced in seminiferous tubules inside the testes through a process called spermatogenesis.

Testicle (testis)

The testes (or testicles) are the male gonads and sit below the penis within a sac called the scrotum. See it in 3D !

Function: The testes generate sperm, the male sex cells, as well as testosterone and other sex hormones. The production of sperm is constant and occurs within numerous lobules in each testis.

Pathologies: Infertility

Seminiferous tubules

Inside testes are coiled tubes called seminiferous tubules. Production begins with the seminiferous tubules where stem cells, called spermatogonia, develop into immature sperm. Each 46-chromosome spermatogonium divides through mitosis to produce primary spermatocytes. These cells divide by meiosis to become 23-chromosome cells called secondary spermatocytes that develop into spermatids.

The epididymis is the duct of the male reproductive system that attaches directly to the testis. It is part of the male internal genitalia. The epididymis sits directly on top of each testis.

Function: Sperm from the testis mature as they move through the coiled duct of the epididymis. During sexual intercourse and ejaculation, they are expelled into the vas deferens.

Vas deferens

The vas deferens, also known as the ductus deferens, is one of the ducts of the male reproductive system. The vas deferens serves as the excretory duct of the testis and is the continuation of the epididymis.

Function: The vas deferens pushes the sperm up over the bladder and down toward the prostate gland. There, the vas deferens joins the ends of the seminal vesicles (accessory reproductive glands) to form the ejaculatory ducts.

Ejaculatory duct

Ductus ejaculatorius

The ducts of the male reproductive system include two ejaculatory ducts, one associated with each testis.

Function: The ejaculatory ducts receive seminal fluid from the vesicles, pass through the prostate, and move semen into the urethra.

Semen is a mixture of seminal fluid produced by accessory glands and sperm produced by the testes. Sperm cells depend on seminal fluid to keep them moving and alive. This fluid is produced during ejaculation by accessory glands: the seminal vesicles, the prostate, and the bulbourethral glands.

Seminal vesicle

Glandula vesiculosa

The seminal vesicles, two saclike structures, sit close behind the bladder and extend toward the bladder. There they each join one of the vas deferens to form the ejaculatory ducts.

Function: The vesicles secrete a whitish-brown fluid containing sugars, prostaglandins, and other substances that makes up two-thirds of the semen volume.

The prostate, located under the bladder and above the start of the penis, contains the ejaculatory ducts and the prostatic urethra. See it in 3D !

Function: As semen enters the urethra, the prostate secretes enzymes that help activate the sperm.

Pathologies: Enlarged prostate (BPH), prostate cancer

Bulbourethral glands (Cowper’s glands)

Glandulae bulbourethreales

The bulbourethral glands (or Cowper’s glands) are pea-sized, with single ducts that connect to the urethra where it emerges from the prostate.

Function: These glands add mucus that helps with sperm motility.

Urethra (male)

The male urethra extends from the bladder, through the prostate, to the external orifice at the end of the penis. It receives additional seminal fluids from the prostate before it expels semen out of the body.

System: Urinary, Reproductive

Pathologies: Urinary tract infections

The penis is part of the male external genitalia, suspended from the body at the front and sides of the pubic arch. Internally, the penis consists of three connected columns of tissue.

Function: During sexual arousal, the erectile tissue of the penis fills with blood and the penis stiffens, allowing it to penetrate the vagina during coitus. Ejaculation delivers semen at or near the cervix, the passage to the uterus.

Pathologies: Chlamydia infections, erectile dysfunction, genital herpes, genital warts, gonorrhea, HPV, syphilis, trichomoniasis

Corpus cavernosum

The paired corpora cavernosa extend together from the root of the penis through the body. Together, they form the greater part of the penis.

Function: The corpus spongiosum and corpora cavernosa consist of sponge-like erectile tissue containing spaces that can temporarily fill with blood from the deep and dorsal arteries of the penis. These structures engorge with blood and become erect when a male is sexually aroused.

Corpus spongiosum

The corpus spongiosum runs along the underside of the cavernosa. It contains the spongy urethra and expands past the body of the penis to form the glans penis (the tip).

Glans penis

The glans penis is the tip of the penis, the external genital organ of the male reproductive system. It arises as the anterior end of the corpus spongiosum and is expanded in the form of a flattened cone. In an uncircumcised male, the loose and retractable skin of the prepuce (foreskin) covers a variable amount of the glans.

The testes (or testicles) are the male gonads and sit below the penis within a sac called the scrotum.

Pregnancy is a series of events through which a fertilized egg implants, becomes an embryo, and develops into a fetus.

System: Reproduction

Function: The resulting offspring carries genetic information from a male and female into a new generation.

Fertilization

During sexual intercourse, some sperm ejaculated from the male penis swim up through the female vagina and uterus toward an oocyte (egg cell) floating in one of the uterine tubes. The sperm and the egg are gametes. They each contain half the genetic information necessary for reproduction. When a sperm cell penetrates and fertilizes an egg, that genetic information combines.

Zygote & blastocyst

The 23 chromosomes from the sperm pair with 23 chromosomes in the egg, forming a 46-chromosome cell called a zygote. The zygote starts to divide and multiply. As it travels toward the uterus it divides to become a blastocyst, which will burrow into the uterine wall.

Fifteen days after conception marks the beginning of the embryonic period. The embryo contains a flat embryonic disc that now differentiates into three layers: the endoderm, the mesoderm, and the ectoderm. All organs of the human body derive from these three tissues. They begin to curve and fold and to form an oblong body. By week four, the embryo has a distinct head and tail and a beating heart. Over the next six weeks, limbs, eyes, brain regions, and vertebrae form.

Yolk sac & amnion

At day 15 after conception, the cells that will form the embryo become an embryonic disc. Other cells begin to form support structures. The yolk sac, on one side of the disc, will become part of the digestive tract. On the other side, the amnion fills with fluid and will surround the embryo as it develops.

By the end of week 10, the embryo is a fetus. From week 10 of pregnancy, the fetus grows inside the uterus, fueled by nutrient-rich blood supplied by the umbilical cord. Bones, muscles, skin, and connective tissues form. Body systems develop. Limbs and facial features take shape.

Umbilical cord

Growth is supplied by nutrients diffusing from the maternal blood into the fetal blood vessels in the placenta. The umbilical cord carries this blood, rich in oxygen and nutrients, to the fetus.

The placenta provides oxygen and nutrients to the fetus and removes waste products from the fetus’ blood.

Around week 36 (usually), the process of labor begins. In the first stage, dilation, hormones stimulate downward contractions of the uterine walls. The contractions push the head of the fetus against the cervix at the lower end of the uterus. The cervix dilates.

In the second stage, expulsion, powerful contractions push the head and the rest of the body through the dilated cervix, and out through the vagina and the vulva. The baby is born.

Placental stage

Contractions detach the placenta from the uterus and expel it.

System: Reproductive system

Related Articles

Female Reproductive Structures

Male Reproductive Structures

Reproductive Processes

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Theme 5: How Do We Control Our Fertility?

5.1 Human Reproductive Anatomy

Human reproductive anatomy.

In general, the reproductive structures in humans can be divided into three main categories:  gonads,  internal genitalia and external genitalia.  The gonads are the organs in which gametes , the cells that fuse in fertilization to form new individuals, develop and mature.  All other reproductive structures are called genitalia, or genitals.  Internal genitalia are found inside of the body, while external genitalia are visible from the outside.  The structures seen in adult males and females actually come from the same precursors in embryos, so there are many similarities in both structure and function between males and females. There is also a wide spectrum of structures present in any one individual; many people have structures that resemble a combination of male and female structures, or that resemble neither. In this textbook, we will define “male” and “female” based on individuals who have the most typical structures characteristic of those two sexes; other types of structures are also normal and common. We will describe the functions of these structures during vaginal sexual intercourse, since that is the sexual act used in reproduction; keep in mind that other types of sexual activity are also common and normal.

Male Reproductive Anatomy

In the male reproductive system, the  scrotum  houses the testicles or testes (singular: testis), including providing passage for blood vessels, nerves, and muscles related to testicular function. The  testes are gonads, and they produce sperm (the male gametes) and some reproductive hormones. Each testis is approximately 2.5 by 3.8 cm (1.5 by 1 in) in size and divided into wedge-shaped lobules by connective tissue called septa.

Sperm are immobile at body temperature; therefore, the scrotum and penis are external to the body, as illustrated in  Figure 1 so that a proper temperature is maintained for motility.

The internal genitalia in males are important for the production of sperm, and of other components of . the semen. Sperm mature in seminiferous tubules  that are coiled inside the testes, as illustrated in  Figure 1 . The walls of the seminiferous tubules are made up of the developing sperm cells, with the least developed sperm at the periphery of the tubule and the fully developed sperm in the lumen. The sperm cells are mixed with “nursemaid” cells called Sertoli cells which protect the germ cells and promote their development. Other cells mixed in the wall of the tubules are the interstitial cells of Leydig . These cells produce high levels of testosterone once the male reaches adolescence.

When the sperm have developed flagella and are nearly mature, they leave the testicles and enter the epididymis, shown in  Figure 1 . This structure resembles a comma and lies along the top and posterior portion of the testes; it is the site of sperm maturation. The sperm leave the epididymis and enter the vas deferens (or ductus deferens), which carries the sperm, behind the bladder, and forms the ejaculatory duct with the duct from the seminal vesicles. During a vasectomy, a section of the vas deferens is removed, preventing sperm from being passed out of the body during ejaculation and preventing fertilization.

Semen  is a mixture of sperm and spermatic duct secretions (about 10 percent of the total) and fluids from accessory glands that contribute most of the semen’s volume. Sperm are haploid cells, consisting of a flagellum as a tail, a neck that contains the cell’s energy-producing mitochondria, and a head that contains the genetic material.  Figure 2 shows a micrograph of human sperm as well as a diagram of the parts of the sperm. An acrosome is found at the top of the head of the sperm. This structure contains enzymes that can digest the protective coverings that surround the egg to help the sperm penetrate and fertilize the egg. An ejaculate (a single emission of sperm) will contain from two to five milliliters of fluid with from 50–120 million sperm per milliliter.

The bulk of the semen comes from the accessory glands associated with the male reproductive system. These are the seminal vesicles, the prostate gland, and the bulbourethral gland, all of which are illustrated in  Figure 1 . The  seminal vesicles  are a pair of glands that lie along the posterior border of the urinary bladder. The glands make a solution that is thick, yellowish, and alkaline. As sperm are only motile in an alkaline environment, a basic pH is important to reverse the acidity of the vaginal environment. The solution also contains mucus, fructose (a sperm mitochondrial nutrient), a coagulating enzyme, ascorbic acid, and local-acting hormones called prostaglandins. The seminal vesicle glands account for 60 percent of the bulk of semen.

The  penis , illustrated in  Figure 1 , is an organ that drains urine from the renal bladder and functions as a copulatory organ during intercourse. The penis contains three tubes of erectile tissue running through the length of the organ. These consist of a pair of tubes on the dorsal side, called the corpus cavernosum, and a single tube of tissue on the ventral side, called the corpus spongiosum. This tissue will become engorged with blood, becoming erect and hard, in preparation for sexual intercourse. The organ is inserted into the vagina culminating with an ejaculation. During intercourse, the smooth muscle sphincters at the opening to the renal bladder close and prevent urine from entering the penis. An orgasm is a two-stage process: first, glands and accessory organs connected to the testes contract, then semen (containing sperm) is expelled through the urethra during ejaculation. After intercourse, the blood drains from the erectile tissue and the penis becomes flaccid.

The walnut-shaped  prostate gland  surrounds the urethra, the connection to the urinary bladder. It has a series of short ducts that directly connect to the urethra. The gland is a mixture of smooth muscle and glandular tissue. The muscle provides much of the force needed for ejaculation to occur. The glandular tissue makes a thin, milky fluid that contains citrate (a nutrient), enzymes, and prostate specific antigen (PSA). PSA is a proteolytic enzyme that helps to liquefy the ejaculate several minutes after release from the male. Prostate gland secretions account for about 30 percent of the bulk of semen.

The  bulbourethral gland , or Cowper’s gland, releases its secretion prior to the release of the bulk of the semen. It neutralizes any acid residue in the urethra left over from urine. This usually accounts for a couple of drops of fluid in the total ejaculate and may contain a few sperm. Withdrawal of the penis from the vagina before ejaculation to prevent pregnancy may not work if sperm are present in the bulbourethral gland secretions. The location and functions of the male reproductive organs are summarized in  Table 1 .

Female Reproductive Anatomy

A number of reproductive structures are exterior to the female’s body. These include the breasts and the vulva, which consists of the mons pubis, clitoris, labia majora, labia minora, and the vestibular glands, all illustrated in  Figure 3 . The location and functions of the female reproductive organs are summarized in  Table 2 . The mons pubis is a round, fatty area that overlies the pubic bone. The clitoris  is a structure with erectile tissue that contains a large number of sensory nerves and serves as a source of stimulation during intercourse. The  labia majora  are a pair of elongated folds of tissue that run posterior from the mons pubis and enclose the other components of the vulva. The labia majora derive from the same tissue that produces the scrotum in a male. The  labia minora  are thin folds of tissue centrally located within the labia majora. These labia protect the openings to the vagina and urethra. The mons pubis and the anterior portion of the labia majora become covered with hair during adolescence; the labia minora is hairless. The greater vestibular glands are found at the sides of the vaginal opening and provide lubrication during intercourse. The vulva is the name for the entire set of external genitalia in the inguinal (groin) area of females; in common language this is sometimes referred to as the vagina, but that is not anatomically accurate; the vagina is an entirely internal structure.

The breasts consist of mammary glands and fat. The size of the breast is determined by the amount of fat deposited behind the gland. Each gland consists of 15 to 25 lobes that have ducts that empty at the nipple and that supply the nursing child with nutrient- and antibody-rich milk to aid development and protect the child.

Internal female reproductive structures include ovaries, oviducts, the  uterus , and the vagina, shown in  Figure 3 . The two ovaries (the female gonads) are held in place in the abdominal cavity by a system of ligaments. Ovaries consist of a medulla and cortex: the medulla contains nerves and blood vessels to supply the cortex with nutrients and remove waste. The outer layers of cells of the cortex are the functional parts of the ovaries. The cortex is made up of follicular cells that surround eggs that develop during fetal development in utero . During the menstrual period, a batch of follicular cells develops and prepares the eggs for release. At ovulation, one follicle ruptures and one egg is released, as illustrated in  Figure 4a .

The  oviducts , or fallopian tubes, extend from the uterus in the lower abdominal cavity to the ovaries, but they are not in contact with the ovaries. The lateral ends of the oviducts flare out into a trumpet-like structure and have a fringe of finger-like projections called fimbriae, illustrated in  Figure 4b . When an egg is released at ovulation, the fimbrae help the non-motile egg enter into the tube and passage to the uterus. The walls of the oviducts are ciliated and are made up mostly of smooth muscle. The cilia beat toward the middle, and the smooth muscle contracts in the same direction, moving the egg toward the uterus. Fertilization usually takes place within the oviducts and the developing embryo is moved toward the uterus for development. It usually takes the egg or embryo a week to travel through the oviduct. Sterilization in females is called a tubal ligation; it is analogous to a vasectomy in males in that the oviducts are severed and sealed.

The uterus is a structure about the size of a females’s fist. This is lined with an endometrium rich in blood vessels and mucus glands. The uterus supports the developing embryo and fetus during gestation. The thickest portion of the wall of the uterus is made of smooth muscle. Contractions of the smooth muscle in the uterus aid in passing the baby through the vagina during labor. A portion of the lining of the uterus sloughs off during each menstrual period, and then builds up again in preparation for an implantation. Part of the uterus, called the cervix, protrudes into the top of the vagina. A small opening called the cervical orifice allows menstrual fluid out of the cervix into the vagina, and sperm into the uterus.  During childbirth the cervical orifice is greatly enlarged.

The  vagina is a muscular tube that serves several purposes. It allows menstrual flow to leave the body. It is the receptacle for the penis during intercourse and the vessel for the delivery of offspring. It is lined cells that produce acidic secretions that limit the growth of microbes that could potentially travel into the uterus.

Development of Reproductive Organs in Humans.

The reproductive tissues of male and female humans develop similarly  in utero (in a fetus developing in the mother’s uterus) for the first several weeks of gestation.  The hormone testosterone is typically only released in embryos that have a male sex chromosome (the Y chromosome, discussed in the next chapter), and this hormone controls the generation of reproductive structures. A low level of the hormone testosterone is released from male gonads in the developing embryos, starting at around the second month of gestation. Testosterone causes the undeveloped tissues to differentiate into male sexual organs. When testosterone is absent, the tissues develop into female sexual tissues. Primitive gonads become testes or ovaries. Tissues that produce a penis in males produce a clitoris in females. The tissue that will become the scrotum in a male becomes the labia in a female; that is, they are homologous structures.  Because of this, there are often variations in development resulting in structures that may have characteristics of both sexes, or of neither sex, depending on hormonal levels and other factors present during embryogenesis.  These variations in sexual structures are quite common and normal.

Sexual Response During Intercourse

The sexual response in humans is both psychological and physiological. Both sexes experience sexual arousal through psychological and physical stimulation. There are four phases of the sexual response. During phase one, called excitement, vasodilation leads to vasocongestion in erectile tissues in both males and females. The nipples, clitoris, labia, and penis engorge with blood and become enlarged. Vaginal secretions are released to lubricate the vagina to facilitate intercourse. During the second phase, called the plateau, stimulation continues, the outer third of the vaginal wall enlarges with blood, and breathing and heart rate increase.

During phase three, or orgasm, rhythmic, involuntary contractions of muscles occur in both sexes. In the male, the reproductive accessory glands and tubules constrict placing semen in the urethra, then the urethra contracts expelling the semen through the penis. In females, the uterus and vaginal muscles contract in waves that may last slightly less than a second each. During phase four, or resolution, the processes described in the first three phases reverse themselves and return to their normal state. Males experience a refractory period in which they cannot maintain an erection or ejaculate for a period of time ranging from minutes to hours.

Section Summary

The reproductive structures that evolved in humans allow males and females to mate, fertilize internally, and support the growth and development of offspring. Reproductive structures include gonads, internal and external genitalia. Some male and female reproductive structures have analogous functions and are derived from common precursor structures. Both males and females have four stages of the sexual response.

Human Biology Copyright © by Sarah Malmquist and Kristina Prescott is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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Human Reproductive System

Reproduction can be defined as the biological process of producing a new individual or an offspring identical to the parents. This process ensures the increase in the number of individuals of a species when conditions are favourable. It is one of the fundamental characteristics of living things and an essential life process.

There are two types of reproduction – asexual and sexual.

Sexual Reproduction –This process of reproduction is very complex that involves the formation and transfer of gametes, followed by fertilization, the formation of the zygote, and embryogenesis.

Asexual Reproduction — This process of reproduction involves only one parent and the new offspring produced is genetically similar to the parent.

Also read:  Asexual Reproduction

Reproduction in Human Beings

All human beings undergo a sexual mode of reproduction. In this process, two parents are involved in producing a new individual. Offspring are produced by the fusion of gametes (sex cells) from each parent. Hence, the newly formed individual will be different from parents, both genetically and physically. Human reproduction is an example of sexual reproduction.

In human beings, both males and females have different reproductive systems; hence, they are known to exhibit sexual dimorphism. Males have testes- also called testicles, while the females have a pair of ovaries.

Also read:  Sexual Reproduction

The reproduction in human beings involves the fusion of male and female gametes produced in their reproductive system. The male reproductive system is different from the female reproductive system, both in structure and in function.

Male Reproductive System

The male gametes, i.e., sperms are produced within the male reproductive system. Sperms are small unicellular structures with a head, middle piece, and a tail.

The male reproductive system consists of :

• Testicles (testes): A pair of oval-shaped organs masked in a pouch called the scrotum. They are responsible for the production of sperms and the male hormone testosterone.
• Scrotum:  It is a sac-like organ that hangs below the penis and behind it. It is the houses of the testicles, or testes, and maintains a temperature that is required for the production of sperm by it.
• Vas deferens: The sperms produced in testes are stored in a tube called the epididymis. Here the sperms get matured and pass to urethra through the muscular tube called vas deferens.
• Accessory glands : This includes three glands, namely seminal vesicles, prostate gland, and Cowper’s gland. The secretions from the three glands mix to form a fluid called semen. Semen nourishes the sperm, increases the volume and helps in lubrication.
• Penis : Penis is a cylindrical tube which serves as both reproductive organ and an excretory organ. It delivers sperms into the vagina during sexual intercourse.

Explore more:   Male Reproductive System

Female Reproductive System

The female reproductive system is active before, during and after fertilization as well. It consists of the following parts:

• A pair of ovaries: Ovaries produce and store ovum in them. They also produce a female hormone called estrogen.
• Fallopian tubes (Oviducts): They are the site of fertilization. They connect ovaries with the uterus.
• Uterus: Uterus is the site of development for the embryo.
• Vagina: It is the part which connects the cervix to the external female body parts. It is the route for the penis during coitus as well as a fetus during delivery.

Female reproductive system has two functions –

• Production of female gamete called ovum/egg.
• Providing nutrition and protecting the developing embryo.

During puberty, eggs in the ovaries start to mature. One of the ovaries releases the matured ovum in every 28 to 30 days and is called ovulation.

Also check:

• Female Internal Genitalia
• Vestibule in Female Reproductive System

Reproduction Process in Human Beings

The process of fusion of sperm with egg (ovum) to produce zygote is called fertilization. Fertilization is a crucial stage of reproduction in human beings. The fertilized egg is called the zygote. Zygote starts to divide into many cells and develops into an embryo.

Embryo moves into the uterus and gets attached to its walls. This process is referred to as implantation, and the implanted embryo eventually develops into a fetus.

Learn more about reproduction in human beings, its types, process, significance  and other related topics at  BYJU’S Biology

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Frequently Asked Questions

What is reproduction.

Reproduction is a fundamental biological process of producing young ones or offspring, which are identical to their parents.

What is Fertilization?

Fertilization is the fusion of male and haploid female gametes (egg and sperm) resulting in the formation of a diploid zygote.

What is Cell Differentiation?

Cell Differentiation is the process through which a young and immature cell develops into a specialized and matured cell.

Explain the process of reproduction in human beings?

The process of reproduction in humans usually begins with copulation, followed by the Pre-fertilization, Fertilization, and Post-fertilization. During this fundamental process, both male and female reproductive organs play an important role.

Explain the term trimester system?

In Biology, the trimester system mainly refers to three months. A complete pregnancy period lasts for 38-40 weeks or 9 months from the first day of your last menstrual period to the birth of the baby. This period is divided into three stages, which are collectively called trimesters.

• First trimester (1st 3-months).
• Second trimester (2nd 3 months).
• Third trimester (3rd 3 months).

What is Parturition?

Parturition is the process of delivering the baby after the completion of pregnancy or a fully grown developed fetus and placenta from the uterus to the vagina to the outside world. This process occurs in three stages, which includes:

• Stage 1: Preparatory Stage- 2 to 12 hours.
• Stage 2: Birthing Process –30 to 180 minutes.
• Stage 3: Placenta Expulsion –1 to 12 hours.

How do humans reproduce their young ones?

Humans reproduce their young ones sexually by the interaction between the male and female reproductive organs.

List out the stages of Sexual Reproduction?

Sexual Reproduction is carried out by a set of events and are divided into three stages: Pre-fertilization, Fertilization, and Post-fertilization

What is the significance of human reproduction?

Reproduction is a fundamental biological process carried out by different living organisms to produce their young ones or offspring. In human, reproduction plays a significant role in the continuity of species from one generation to another generation. Without reproduction, there would no life existing on the planet earth.

Difference between sexual and asexual mode of reproduction?

Both sexual and asexual are two different modes of reproduction. Sexual mode reproduction takes place in all multicellular organisms including humans, animals, and higher plants. Asexual mode reproduction occurs only in lower invertebrates and other simpler living species such as amoeba, bacteria, and hydra.

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27.2 Anatomy and Physiology of the Female Reproductive System

Learning objectives.

By the end of this section, you will be able to:

• Describe the structure and function of the organs of the female reproductive system
• List the steps of oogenesis
• Describe the hormonal changes that occur during the ovarian and menstrual cycles
• Trace the path of an oocyte from ovary to fertilization

The female reproductive system functions to produce gametes and reproductive hormones, just like the male reproductive system; however, it also has the additional task of supporting the developing fetus and delivering it to the outside world. Unlike its male counterpart, the female reproductive system is located primarily inside the pelvic cavity ( Figure 27.9 ). Recall that the ovaries are the female gonads. The gamete they produce is called an oocyte . We’ll discuss the production of oocytes in detail shortly. First, let’s look at some of the structures of the female reproductive system.

External Female Genitals

The external female reproductive structures are referred to collectively as the vulva ( Figure 27.10 ). The mons pubis is a pad of fat that is located at the anterior, over the pubic bone. After puberty, it becomes covered in pubic hair. The labia majora (labia = “lips”; majora = “larger”) are folds of hair-covered skin that begin just posterior to the mons pubis. The thinner and more pigmented labia minora (labia = “lips”; minora = “smaller”) extend medial to the labia majora. Although they naturally vary in shape and size from woman to woman, the labia minora serve to protect the female urethra and the entrance to the female reproductive tract.

The superior, anterior portions of the labia minora come together to encircle the clitoris (or glans clitoris), an organ that originates from the same cells as the glans penis and has abundant nerves that make it important in sexual sensation and orgasm. The hymen is a thin membrane that sometimes partially covers the entrance to the vagina. An intact hymen cannot be used as an indication of “virginity”; even at birth, this is only a partial membrane, as menstrual fluid and other secretions must be able to exit the body, regardless of penile–vaginal intercourse. The vaginal opening is located between the opening of the urethra and the anus. It is flanked by outlets to the Bartholin’s glands (or greater vestibular glands).

The vagina , shown at the bottom of Figure 27.9 and Figure 27.10 , is a muscular canal (approximately 10 cm long) that serves as the entrance to the reproductive tract. It also serves as the exit from the uterus during menses and childbirth. The outer walls of the anterior and posterior vagina are formed into longitudinal columns, or ridges, and the superior portion of the vagina—called the fornix—meets the protruding uterine cervix. The walls of the vagina are lined with an outer, fibrous adventitia; a middle layer of smooth muscle; and an inner mucous membrane with transverse folds called rugae . Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. The thin, perforated hymen can partially surround the opening to the vaginal orifice. The hymen can be ruptured with strenuous physical exercise, penile–vaginal intercourse, and childbirth. The Bartholin’s glands and the lesser vestibular glands (located near the clitoris) secrete mucus, which keeps the vestibular area moist.

The vagina is home to a normal population of microorganisms that help to protect against infection by pathogenic bacteria, yeast, or other organisms that can enter the vagina. In a healthy woman, the most predominant type of vaginal bacteria is from the genus Lactobacillus . This family of beneficial bacterial flora secretes lactic acid, and thus protects the vagina by maintaining an acidic pH (below 4.5). Potential pathogens are less likely to survive in these acidic conditions. Lactic acid, in combination with other vaginal secretions, makes the vagina a self-cleansing organ. However, douching—or washing out the vagina with fluid—can disrupt the normal balance of healthy microorganisms, and actually increase a woman’s risk for infections and irritation. Indeed, the American College of Obstetricians and Gynecologists recommend that women do not douche, and that they allow the vagina to maintain its normal healthy population of protective microbial flora.

The ovaries are the female gonads (see Figure 27.9 ). Paired ovals, they are each about 2 to 3 cm in length, about the size of an almond. The ovaries are located within the pelvic cavity, and are supported by the mesovarium, an extension of the peritoneum that connects the ovaries to the broad ligament . Extending from the mesovarium itself is the suspensory ligament that contains the ovarian blood and lymph vessels. Finally, the ovary itself is attached to the uterus via the ovarian ligament.

The ovary comprises an outer covering of cuboidal epithelium called the ovarian surface epithelium that is superficial to a dense connective tissue covering called the tunica albuginea. Beneath the tunica albuginea is the cortex, or outer portion, of the organ. The cortex is composed of a tissue framework called the ovarian stroma that forms the bulk of the adult ovary. Oocytes develop within the outer layer of this stroma, each surrounded by supporting cells. This grouping of an oocyte and its supporting cells is called a follicle . The growth and development of ovarian follicles will be described shortly. Beneath the cortex lies the inner ovarian medulla, the site of blood vessels, lymph vessels, and the nerves of the ovary. You will learn more about the overall anatomy of the female reproductive system at the end of this section.

The Ovarian Cycle

The ovarian cycle is a set of predictable changes in a female’s oocytes and ovarian follicles. During a woman’s reproductive years, it is a roughly 28-day cycle that can be correlated with, but is not the same as, the menstrual cycle (discussed shortly). The cycle includes two interrelated processes: oogenesis (the production of female gametes) and folliculogenesis (the growth and development of ovarian follicles).

Gametogenesis in females is called oogenesis . The process begins with the ovarian stem cells, or oogonia ( Figure 27.11 ). Oogonia are formed during fetal development, and divide via mitosis, much like spermatogonia in the testis. Unlike spermatogonia, however, oogonia form primary oocytes in the fetal ovary prior to birth. These primary oocytes are then arrested in this stage of meiosis I, only to resume it years later, beginning at puberty and continuing until the woman is near menopause (the cessation of a woman’s reproductive functions). The number of primary oocytes present in the ovaries declines from one to two million in an infant, to approximately 400,000 at puberty, to zero by the end of menopause.

The initiation of ovulation —the release of an oocyte from the ovary—marks the transition from puberty into reproductive maturity for women. From then on, throughout a woman’s reproductive years, ovulation occurs approximately once every 28 days. Just prior to ovulation, a surge of luteinizing hormone triggers the resumption of meiosis in a primary oocyte. This initiates the transition from primary to secondary oocyte. However, as you can see in Figure 27.11 , this cell division does not result in two identical cells. Instead, the cytoplasm is divided unequally, and one daughter cell is much larger than the other. This larger cell, the secondary oocyte, eventually leaves the ovary during ovulation. The smaller cell, called the first polar body , may or may not complete meiosis and produce second polar bodies; in either case, it eventually disintegrates. Therefore, even though oogenesis produces up to four cells, only one survives.

How does the diploid secondary oocyte become an ovum —the haploid female gamete? Meiosis of a secondary oocyte is completed only if a sperm succeeds in penetrating its barriers. Meiosis II then resumes, producing one haploid ovum that, at the instant of fertilization by a (haploid) sperm, becomes the first diploid cell of the new offspring (a zygote). Thus, the ovum can be thought of as a brief, transitional, haploid stage between the diploid oocyte and diploid zygote.

The larger amount of cytoplasm contained in the female gamete is used to supply the developing zygote with nutrients during the period between fertilization and implantation into the uterus. Interestingly, sperm contribute only DNA at fertilization —not cytoplasm. Therefore, the cytoplasm and all of the cytoplasmic organelles in the developing embryo are of maternal origin. This includes mitochondria, which contain their own DNA. Scientific research in the 1980s determined that mitochondrial DNA was maternally inherited, meaning that you can trace your mitochondrial DNA directly to your mother, her mother, and so on back through your female ancestors.

Everyday Connection

Mapping human history with mitochondrial dna.

When we talk about human DNA, we’re usually referring to nuclear DNA; that is, the DNA coiled into chromosomal bundles in the nucleus of our cells. We inherit half of our nuclear DNA from our father, and half from our mother. However, mitochondrial DNA (mtDNA) comes only from the mitochondria in the cytoplasm of the fat ovum we inherit from our mother. She received her mtDNA from her mother, who got it from her mother, and so on. Each of our cells contains approximately 1700 mitochondria, with each mitochondrion packed with mtDNA containing approximately 37 genes.

Mutations (changes) in mtDNA occur spontaneously in a somewhat organized pattern at regular intervals in human history. By analyzing these mutational relationships, researchers have been able to determine that we can all trace our ancestry back to one woman who lived in Africa about 200,000 years ago. Scientists have given this woman the biblical name Eve, although she is not, of course, the first Homo sapiens female. More precisely, she is our most recent common ancestor through matrilineal descent.

This doesn’t mean that everyone’s mtDNA today looks exactly like that of our ancestral Eve. Because of the spontaneous mutations in mtDNA that have occurred over the centuries, researchers can map different “branches” off of the “main trunk” of our mtDNA family tree. Your mtDNA might have a pattern of mutations that aligns more closely with one branch, and your neighbor’s may align with another branch. Still, all branches eventually lead back to Eve.

But what happened to the mtDNA of all of the other Homo sapiens females who were living at the time of Eve? Researchers explain that, over the centuries, their female descendants died childless or with only male children, and thus, their maternal line—and its mtDNA—ended.

Folliculogenesis

Again, ovarian follicles are oocytes and their supporting cells. They grow and develop in a process called folliculogenesis , which typically leads to ovulation of one follicle approximately every 28 days, along with death to multiple other follicles. The death of ovarian follicles is called atresia, and can occur at any point during follicular development. Recall that, a female infant at birth will have one to two million oocytes within her ovarian follicles, and that this number declines throughout life until menopause, when no follicles remain. As you’ll see next, follicles progress from primordial, to primary, to secondary and tertiary stages prior to ovulation—with the oocyte inside the follicle remaining as a primary oocyte until right before ovulation.

Folliculogenesis begins with follicles in a resting state. These small primordial follicles are present in newborn females and are the prevailing follicle type in the adult ovary ( Figure 27.12 ). Primordial follicles have only a single flat layer of support cells, called granulosa cells , that surround the oocyte, and they can stay in this resting state for years—some until right before menopause.

After puberty, a few primordial follicles will respond to a recruitment signal each day, and will join a pool of immature growing follicles called primary follicles . Primary follicles start with a single layer of granulosa cells, but the granulosa cells then become active and transition from a flat or squamous shape to a rounded, cuboidal shape as they increase in size and proliferate. As the granulosa cells divide, the follicles—now called secondary follicles (see Figure 27.12 )—increase in diameter, adding a new outer layer of connective tissue, blood vessels, and theca cells —cells that work with the granulosa cells to produce estrogens.

Within the growing secondary follicle, the primary oocyte now secretes a thin acellular membrane called the zona pellucida that will play a critical role in fertilization. A thick fluid, called follicular fluid, that has formed between the granulosa cells also begins to collect into one large pool, or antrum . Follicles in which the antrum has become large and fully formed are considered tertiary follicles (or antral follicles). Several follicles reach the tertiary stage at the same time, and most of these will undergo atresia. The one that does not die will continue to grow and develop until ovulation, when it will expel its secondary oocyte surrounded by several layers of granulosa cells from the ovary. Keep in mind that most follicles don’t make it to this point. In fact, roughly 99 percent of the follicles in the ovary will undergo atresia, which can occur at any stage of folliculogenesis.

Hormonal Control of the Ovarian Cycle

The process of development that we have just described, from primordial follicle to early tertiary follicle, takes approximately two months in humans. The final stages of development of a small cohort of tertiary follicles, ending with ovulation of a secondary oocyte, occur over a course of approximately 28 days. These changes are regulated by many of the same hormones that regulate the male reproductive system, including GnRH, LH, and FSH.

As in men, the hypothalamus produces GnRH, a hormone that signals the anterior pituitary gland to produce the gonadotropins FSH and LH ( Figure 27.13 ). These gonadotropins leave the pituitary and travel through the bloodstream to the ovaries, where they bind to receptors on the granulosa and theca cells of the follicles. FSH stimulates the follicles to grow (hence its name of follicle-stimulating hormone), and the five or six tertiary follicles expand in diameter. The release of LH also stimulates the granulosa and theca cells of the follicles to produce the sex steroid hormone estradiol, a type of estrogen. This phase of the ovarian cycle, when the tertiary follicles are growing and secreting estrogen, is known as the follicular phase.

The more granulosa and theca cells a follicle has (that is, the larger and more developed it is), the more estrogen it will produce in response to LH stimulation. As a result of these large follicles producing large amounts of estrogen, systemic plasma estrogen concentrations increase. Following a classic negative feedback loop, the high concentrations of estrogen will stimulate the hypothalamus and pituitary to reduce the production of GnRH, LH, and FSH. Because the large tertiary follicles require FSH to grow and survive at this point, this decline in FSH caused by negative feedback leads most of them to die (atresia). Typically only one follicle, now called the dominant follicle, will survive this reduction in FSH, and this follicle will be the one that releases an oocyte. Scientists have studied many factors that lead to a particular follicle becoming dominant: size, the number of granulosa cells, and the number of FSH receptors on those granulosa cells all contribute to a follicle becoming the one surviving dominant follicle.

When only the one dominant follicle remains in the ovary, it again begins to secrete estrogen. It produces more estrogen than all of the developing follicles did together before the negative feedback occurred. It produces so much estrogen that the normal negative feedback doesn’t occur. Instead, these extremely high concentrations of systemic plasma estrogen trigger a regulatory switch in the anterior pituitary that responds by secreting large amounts of LH and FSH into the bloodstream (see Figure 27.13 ). The positive feedback loop by which more estrogen triggers release of more LH and FSH only occurs at this point in the cycle.

It is this large burst of LH (called the LH surge) that leads to ovulation of the dominant follicle. The LH surge induces many changes in the dominant follicle, including stimulating the resumption of meiosis of the primary oocyte to a secondary oocyte. As noted earlier, the polar body that results from unequal cell division simply degrades. The LH surge also triggers proteases (enzymes that cleave proteins) to break down structural proteins in the ovary wall on the surface of the bulging dominant follicle. This degradation of the wall, combined with pressure from the large, fluid-filled antrum, results in the expulsion of the oocyte surrounded by granulosa cells into the peritoneal cavity. This release is ovulation.

In the next section, you will follow the ovulated oocyte as it travels toward the uterus, but there is one more important event that occurs in the ovarian cycle. The surge of LH also stimulates a change in the granulosa and theca cells that remain in the follicle after the oocyte has been ovulated. This change is called luteinization (recall that the full name of LH is luteinizing hormone), and it transforms the collapsed follicle into a new endocrine structure called the corpus luteum , a term meaning “yellowish body” (see Figure 27.12 ). Instead of estrogen, the luteinized granulosa and theca cells of the corpus luteum begin to produce large amounts of the sex steroid hormone progesterone, a hormone that is critical for the establishment and maintenance of pregnancy. Progesterone triggers negative feedback at the hypothalamus and pituitary, which keeps GnRH, LH, and FSH secretions low, so no new dominant follicles develop at this time.

The post-ovulatory phase of progesterone secretion is known as the luteal phase of the ovarian cycle. If pregnancy does not occur within 10 to 12 days, the corpus luteum will stop secreting progesterone and degrade into the corpus albicans , a nonfunctional “whitish body” that will disintegrate in the ovary over a period of several months. During this time of reduced progesterone secretion, FSH and LH are once again stimulated, and the follicular phase begins again with a new cohort of early tertiary follicles beginning to grow and secrete estrogen.

The Uterine Tubes

The uterine tubes (also called fallopian tubes or oviducts) serve as the conduit of the oocyte from the ovary to the uterus ( Figure 27.14 ). Each of the two uterine tubes is close to, but not directly connected to, the ovary and divided into sections. The isthmus is the narrow medial end of each uterine tube that is connected to the uterus. The wide distal infundibulum flares out with slender, finger-like projections called fimbriae . The middle region of the tube, called the ampulla , is where fertilization often occurs. The uterine tubes also have three layers: an outer serosa, a middle smooth muscle layer, and an inner mucosal layer. In addition to its mucus-secreting cells, the inner mucosa contains ciliated cells that beat in the direction of the uterus, producing a current that will be critical to move the oocyte.

Following ovulation, the secondary oocyte surrounded by a few granulosa cells is released into the peritoneal cavity. The nearby uterine tube, either left or right, receives the oocyte. Unlike sperm, oocytes lack flagella, and therefore cannot move on their own. So how do they travel into the uterine tube and toward the uterus? High concentrations of estrogen that occur around the time of ovulation induce contractions of the smooth muscle along the length of the uterine tube. These contractions occur every 4 to 8 seconds, and the result is a coordinated movement that sweeps the surface of the ovary and the pelvic cavity. Current flowing toward the uterus is generated by coordinated beating of the cilia that line the outside and lumen of the length of the uterine tube. These cilia beat more strongly in response to the high estrogen concentrations that occur around the time of ovulation. As a result of these mechanisms, the oocyte–granulosa cell complex is pulled into the interior of the tube. Once inside, the muscular contractions and beating cilia move the oocyte slowly toward the uterus. When fertilization does occur, sperm typically meet the egg while it is still moving through the ampulla.

Interactive Link

Watch this video to observe ovulation and its initiation in response to the release of FSH and LH from the pituitary gland. What specialized structures help guide the oocyte from the ovary into the uterine tube?

If the oocyte is successfully fertilized, the resulting zygote will begin to divide into two cells, then four, and so on, as it makes its way through the uterine tube and into the uterus. There, it will implant and continue to grow. If the egg is not fertilized, it will simply degrade—either in the uterine tube or in the uterus, where it may be shed with the next menstrual period.

The open-ended structure of the uterine tubes can have significant health consequences if bacteria or other contagions enter through the vagina and move through the uterus, into the tubes, and then into the pelvic cavity. If this is left unchecked, a bacterial infection (sepsis) could quickly become life-threatening. The spread of an infection in this manner is of special concern when unskilled practitioners perform abortions in non-sterile conditions. Sepsis is also associated with sexually transmitted bacterial infections, especially gonorrhea and chlamydia. These increase a woman’s risk for pelvic inflammatory disease (PID), infection of the uterine tubes or other reproductive organs. Even when resolved, PID can leave scar tissue in the tubes, leading to infertility.

Watch this series of videos to look at the movement of the oocyte through the ovary. The cilia in the uterine tube promote movement of the oocyte. What would likely occur if the cilia were paralyzed at the time of ovulation?

The Uterus and Cervix

The uterus is the muscular organ that nourishes and supports the growing embryo (see Figure 27.14 ). Its average size is approximately 5 cm wide by 7 cm long (approximately 2 in by 3 in) when a female is not pregnant. It has three sections. The portion of the uterus superior to the opening of the uterine tubes is called the fundus . The middle section of the uterus is called the body of uterus (or corpus). The cervix is the narrow inferior portion of the uterus that projects into the vagina. The cervix produces mucus secretions that become thin and stringy under the influence of high systemic plasma estrogen concentrations, and these secretions can facilitate sperm movement through the reproductive tract.

Several ligaments maintain the position of the uterus within the abdominopelvic cavity. The broad ligament is a fold of peritoneum that serves as a primary support for the uterus, extending laterally from both sides of the uterus and attaching it to the pelvic wall. The round ligament attaches to the uterus near the uterine tubes, and extends to the labia majora. Finally, the uterosacral ligament stabilizes the uterus posteriorly by its connection from the cervix to the pelvic wall.

The wall of the uterus is made up of three layers. The most superficial layer is the serous membrane, or perimetrium , which consists of epithelial tissue that covers the exterior portion of the uterus. The middle layer, or myometrium , is a thick layer of smooth muscle responsible for uterine contractions. Most of the uterus is myometrial tissue, and the muscle fibers run horizontally, vertically, and diagonally, allowing the powerful contractions that occur during labor and the less powerful contractions (or cramps) that help to expel menstrual blood during a woman’s period. Anteriorly directed myometrial contractions also occur near the time of ovulation, and are thought to possibly facilitate the transport of sperm through the female reproductive tract.

The innermost layer of the uterus is called the endometrium . The endometrium contains a connective tissue lining, the lamina propria, which is covered by epithelial tissue that lines the lumen. Structurally, the endometrium consists of two layers: the stratum basalis and the stratum functionalis (the basal and functional layers). The stratum basalis layer is part of the lamina propria and is adjacent to the myometrium; this layer does not shed during menses. In contrast, the thicker stratum functionalis layer contains the glandular portion of the lamina propria and the endothelial tissue that lines the uterine lumen. It is the stratum functionalis that grows and thickens in response to increased levels of estrogen and progesterone. In the luteal phase of the menstrual cycle, special branches off of the uterine artery called spiral arteries supply the thickened stratum functionalis. This inner functional layer provides the proper site of implantation for the fertilized egg, and—should fertilization not occur—it is only the stratum functionalis layer of the endometrium that sheds during menstruation.

Recall that during the follicular phase of the ovarian cycle, the tertiary follicles are growing and secreting estrogen. At the same time, the stratum functionalis of the endometrium is thickening to prepare for a potential implantation. The post-ovulatory increase in progesterone, which characterizes the luteal phase, is key for maintaining a thick stratum functionalis. As long as a functional corpus luteum is present in the ovary, the endometrial lining is prepared for implantation. Indeed, if an embryo implants, signals are sent to the corpus luteum to continue secreting progesterone to maintain the endometrium, and thus maintain the pregnancy. If an embryo does not implant, no signal is sent to the corpus luteum and it degrades, ceasing progesterone production and ending the luteal phase. Without progesterone, the endometrium thins and, under the influence of prostaglandins, the spiral arteries of the endometrium constrict and rupture, preventing oxygenated blood from reaching the endometrial tissue. As a result, endometrial tissue dies and blood, pieces of the endometrial tissue, and white blood cells are shed through the vagina during menstruation, or the menses . The first menses after puberty, called menarche , can occur either before or after the first ovulation.

The Menstrual Cycle

Now that we have discussed the maturation of the cohort of tertiary follicles in the ovary, the build-up and then shedding of the endometrial lining in the uterus, and the function of the uterine tubes and vagina, we can put everything together to talk about the three phases of the menstrual cycle —the series of changes in which the uterine lining is shed, rebuilds, and prepares for implantation.

The timing of the menstrual cycle starts with the first day of menses, referred to as day one of a woman’s period. Cycle length is determined by counting the days between the onset of bleeding in two subsequent cycles. Because the average length of a woman’s menstrual cycle is 28 days, this is the time period used to identify the timing of events in the cycle. However, the length of the menstrual cycle varies among women, and even in the same woman from one cycle to the next, typically from 21 to 32 days.

Just as the hormones produced by the granulosa and theca cells of the ovary “drive” the follicular and luteal phases of the ovarian cycle, they also control the three distinct phases of the menstrual cycle. These are the menses phase, the proliferative phase, and the secretory phase.

Menses Phase

The menses phase of the menstrual cycle is the phase during which the lining is shed; that is, the days that the woman menstruates. Although it averages approximately five days, the menses phase can last from 2 to 7 days, or longer. As shown in Figure 27.15 , the menses phase occurs during the early days of the follicular phase of the ovarian cycle, when progesterone, FSH, and LH levels are low. Recall that progesterone concentrations decline as a result of the degradation of the corpus luteum, marking the end of the luteal phase. This decline in progesterone triggers the shedding of the stratum functionalis of the endometrium.

Proliferative Phase

Once menstrual flow ceases, the endometrium begins to proliferate again, marking the beginning of the proliferative phase of the menstrual cycle (see Figure 27.15 ). It occurs when the granulosa and theca cells of the tertiary follicles begin to produce increased amounts of estrogen. These rising estrogen concentrations stimulate the endometrial lining to rebuild.

Recall that the high estrogen concentrations will eventually lead to a decrease in FSH as a result of negative feedback, resulting in atresia of all but one of the developing tertiary follicles. The switch to positive feedback—which occurs with the elevated estrogen production from the dominant follicle—then stimulates the LH surge that will trigger ovulation. In a typical 28-day menstrual cycle, ovulation occurs on day 14. Ovulation marks the end of the proliferative phase as well as the end of the follicular phase.

Secretory Phase

In addition to prompting the LH surge, high estrogen levels increase the uterine tube contractions that facilitate the pick-up and transfer of the ovulated oocyte. High estrogen levels also slightly decrease the acidity of the vagina, making it more hospitable to sperm. In the ovary, the luteinization of the granulosa cells of the collapsed follicle forms the progesterone-producing corpus luteum, marking the beginning of the luteal phase of the ovarian cycle. In the uterus, progesterone from the corpus luteum begins the secretory phase of the menstrual cycle, in which the endometrial lining prepares for implantation (see Figure 27.15 ). Over the next 10 to 12 days, the endometrial glands secrete a fluid rich in glycogen. If fertilization has occurred, this fluid will nourish the ball of cells now developing from the zygote. At the same time, the spiral arteries develop to provide blood to the thickened stratum functionalis.

If no pregnancy occurs within approximately 10 to 12 days, the corpus luteum will degrade into the corpus albicans. Levels of both estrogen and progesterone will fall, and the endometrium will grow thinner. Prostaglandins will be secreted that cause constriction of the spiral arteries, reducing oxygen supply. The endometrial tissue will die, resulting in menses—or the first day of the next cycle.

Disorders of the...

Female reproductive system.

Research over many years has confirmed that cervical cancer is most often caused by a sexually transmitted infection with human papillomavirus (HPV). There are over 100 related viruses in the HPV family, and the characteristics of each strain determine the outcome of the infection. In all cases, the virus enters body cells and uses its own genetic material to take over the host cell’s metabolic machinery and produce more virus particles.

HPV infections are common in both men and women. Indeed, a recent study determined that 42.5 percent of females had HPV at the time of testing. These women ranged in age from 14 to 59 years and differed in race, ethnicity, and number of sexual partners. Of note, the prevalence of HPV infection was 53.8 percent among women aged 20 to 24 years, the age group with the highest infection rate.

HPV strains are classified as high or low risk according to their potential to cause cancer. Though most HPV infections do not cause disease, the disruption of normal cellular functions in the low-risk forms of HPV can cause the male or female human host to develop genital warts. Often, the body is able to clear an HPV infection by normal immune responses within 2 years. However, the more serious, high-risk infection by certain types of HPV can result in cancer of the cervix ( Figure 27.16 ). Infection with either of the cancer-causing variants HPV 16 or HPV 18 has been linked to more than 70 percent of all cervical cancer diagnoses. Although even these high-risk HPV strains can be cleared from the body over time, infections persist in some individuals. If this happens, the HPV infection can influence the cells of the cervix to develop precancerous changes.

Risk factors for cervical cancer include having unprotected sex; having multiple sexual partners; a first sexual experience at a younger age, when the cells of the cervix are not fully mature; failure to receive the HPV vaccine; a compromised immune system; and smoking. The risk of developing cervical cancer is doubled with cigarette smoking.

When the high-risk types of HPV enter a cell, two viral proteins are used to neutralize proteins that the host cells use as checkpoints in the cell cycle. The best studied of these proteins is p53. In a normal cell, p53 detects DNA damage in the cell’s genome and either halts the progression of the cell cycle—allowing time for DNA repair to occur—or initiates apoptosis. Both of these processes prevent the accumulation of mutations in a cell’s genome. High-risk HPV can neutralize p53, keeping the cell in a state in which fast growth is possible and impairing apoptosis, allowing mutations to accumulate in the cellular DNA.

The prevalence of cervical cancer in the United States is very low because of regular screening exams called pap smears. Pap smears sample cells of the cervix, allowing the detection of abnormal cells. If pre-cancerous cells are detected, there are several highly effective techniques that are currently in use to remove them before they pose a danger. However, women in developing countries often do not have access to regular pap smears. As a result, these women account for as many as 80 percent of the cases of cervical cancer worldwide.

In 2006, the first vaccine against the high-risk types of HPV was approved. There are now two HPV vaccines available: Gardasil ® and Cervarix ® . Whereas these vaccines were initially only targeted for women, because HPV is sexually transmitted, both men and women require vaccination for this approach to achieve its maximum efficacy. A recent study suggests that the HPV vaccine has cut the rates of HPV infection by the four targeted strains at least in half. Unfortunately, the high cost of manufacturing the vaccine is currently limiting access to many women worldwide.

The Breasts

Whereas the breasts are located far from the other female reproductive organs, they are considered accessory organs of the female reproductive system. The function of the breasts is to supply milk to an infant in a process called lactation. The external features of the breast include a nipple surrounded by a pigmented areola ( Figure 27.17 ), whose coloration may deepen during pregnancy. The areola is typically circular and can vary in size from 25 to 100 mm in diameter. The areolar region is characterized by small, raised areolar glands that secrete lubricating fluid during lactation to protect the nipple from chafing. When a baby nurses, or draws milk from the breast, the entire areolar region is taken into the mouth.

Breast milk is produced by the mammary glands , which are modified sweat glands. The milk itself exits the breast through the nipple via 15 to 20 lactiferous ducts that open on the surface of the nipple. These lactiferous ducts each extend to a lactiferous sinus that connects to a glandular lobe within the breast itself that contains groups of milk-secreting cells in clusters called alveoli (see Figure 27.17 ). The clusters can change in size depending on the amount of milk in the alveolar lumen. Once milk is made in the alveoli, stimulated myoepithelial cells that surround the alveoli contract to push the milk to the lactiferous sinuses. From here, the baby can draw milk through the lactiferous ducts by suckling. The lobes themselves are surrounded by fat tissue, which determines the size of the breast; breast size differs between individuals and does not affect the amount of milk produced. Supporting the breasts are multiple bands of connective tissue called suspensory ligaments that connect the breast tissue to the dermis of the overlying skin.

During the normal hormonal fluctuations in the menstrual cycle, breast tissue responds to changing levels of estrogen and progesterone, which can lead to swelling and breast tenderness in some individuals, especially during the secretory phase. If pregnancy occurs, the increase in hormones leads to further development of the mammary tissue and enlargement of the breasts.

Hormonal Birth Control

Birth control pills take advantage of the negative feedback system that regulates the ovarian and menstrual cycles to stop ovulation and prevent pregnancy. Typically they work by providing a constant level of both estrogen and progesterone, which negatively feeds back onto the hypothalamus and pituitary, thus preventing the release of FSH and LH. Without FSH, the follicles do not mature, and without the LH surge, ovulation does not occur. Although the estrogen in birth control pills does stimulate some thickening of the endometrial wall, it is reduced compared with a normal cycle and is less likely to support implantation.

Some birth control pills contain 21 active pills containing hormones, and 7 inactive pills (placebos). The decline in hormones during the week that the woman takes the placebo pills triggers menses, although it is typically lighter than a normal menstrual flow because of the reduced endometrial thickening. Newer types of birth control pills have been developed that deliver low-dose estrogens and progesterone for the entire cycle (these are meant to be taken 365 days a year), and menses never occurs. While some women prefer to have the proof of a lack of pregnancy that a monthly period provides, menstruation every 28 days is not required for health reasons, and there are no reported adverse effects of not having a menstrual period in an otherwise healthy individual.

Because birth control pills function by providing constant estrogen and progesterone levels and disrupting negative feedback, skipping even just one or two pills at certain points of the cycle (or even being several hours late taking the pill) can lead to an increase in FSH and LH and result in ovulation. It is important, therefore, that the woman follow the directions on the birth control pill package to successfully prevent pregnancy.

Aging and the...

Female fertility (the ability to conceive) peaks when women are in their twenties, and is slowly reduced until a women reaches 35 years of age. After that time, fertility declines more rapidly, until it ends completely at the end of menopause. Menopause is the cessation of the menstrual cycle that occurs as a result of the loss of ovarian follicles and the hormones that they produce. A woman is considered to have completed menopause if she has not menstruated in a full year. After that point, she is considered postmenopausal. The average age for this change is consistent worldwide at between 50 and 52 years of age, but it can normally occur in a woman’s forties, or later in her fifties. Poor health, including smoking, can lead to earlier loss of fertility and earlier menopause.

As a woman reaches the age of menopause, depletion of the number of viable follicles in the ovaries due to atresia affects the hormonal regulation of the menstrual cycle. During the years leading up to menopause, there is a decrease in the levels of the hormone inhibin, which normally participates in a negative feedback loop to the pituitary to control the production of FSH. The menopausal decrease in inhibin leads to an increase in FSH. The presence of FSH stimulates more follicles to grow and secrete estrogen. Because small, secondary follicles also respond to increases in FSH levels, larger numbers of follicles are stimulated to grow; however, most undergo atresia and die. Eventually, this process leads to the depletion of all follicles in the ovaries, and the production of estrogen falls off dramatically. It is primarily the lack of estrogens that leads to the symptoms of menopause.

The earliest changes occur during the menopausal transition, often referred to as peri-menopause, when a women’s cycle becomes irregular but does not stop entirely. Although the levels of estrogen are still nearly the same as before the transition, the level of progesterone produced by the corpus luteum is reduced. This decline in progesterone can lead to abnormal growth, or hyperplasia, of the endometrium. This condition is a concern because it increases the risk of developing endometrial cancer. Two harmless conditions that can develop during the transition are uterine fibroids, which are benign masses of cells, and irregular bleeding. As estrogen levels change, other symptoms that occur are hot flashes and night sweats, trouble sleeping, vaginal dryness, mood swings, difficulty focusing, and thinning of hair on the head along with the growth of more hair on the face. Depending on the individual, these symptoms can be entirely absent, moderate, or severe.

After menopause, lower amounts of estrogens can lead to other changes. Cardiovascular disease becomes as prevalent in women as in men, possibly because estrogens reduce the amount of cholesterol in the blood vessels. When estrogen is lacking, many women find that they suddenly have problems with high cholesterol and the cardiovascular issues that accompany it. Osteoporosis is another problem because bone density decreases rapidly in the first years after menopause. The reduction in bone density leads to a higher incidence of fractures.

Hormone therapy (HT), which employs medication (synthetic estrogens and progestins) to increase estrogen and progestin levels, can alleviate some of the symptoms of menopause. In 2002, the Women’s Health Initiative began a study to observe women for the long-term outcomes of hormone replacement therapy over 8.5 years. However, the study was prematurely terminated after 5.2 years because of evidence of a higher than normal risk of breast cancer in patients taking estrogen-only HT. The potential positive effects on cardiovascular disease were also not realized in the estrogen-only patients. The results of other hormone replacement studies over the last 50 years, including a 2012 study that followed over 1,000 menopausal women for 10 years, have shown cardiovascular benefits from estrogen and no increased risk for cancer. Some researchers believe that the age group tested in the 2002 trial may have been too old to benefit from the therapy, thus skewing the results. In the meantime, intense debate and study of the benefits and risks of replacement therapy is ongoing. Current guidelines approve HT for the reduction of hot flashes or flushes, but this treatment is generally only considered when women first start showing signs of menopausal changes, is used in the lowest dose possible for the shortest time possible (5 years or less), and it is suggested that women on HT have regular pelvic and breast exams.

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• Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix
• Publisher/website: OpenStax
• Book title: Anatomy and Physiology
• Publication date: Apr 25, 2013
• Location: Houston, Texas
• Book URL: https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
• Section URL: https://openstax.org/books/anatomy-and-physiology/pages/27-2-anatomy-and-physiology-of-the-female-reproductive-system

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13: Module 11- The Reproductive System

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• Page ID 34671
• 13.1: Introduction to the Reproductive System
• 13.2: Development of the Male and Female Reproductive Systems
• 13.3: Anatomy and Physiology of the Male Reproductive System
• 13.4: Anatomy and Physiology of the Female Reproductive System