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22.2: Introduction to the Reproductive System

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  • It’s All About Sex

    A tiny sperm from dad breaks through the surface of a huge egg from mom. Voilà! In nine months, a new baby will be born. Like most other multicellular organisms, human beings reproduce sexually. In human sexual reproduction, individuals with testes produce sperm, and individuals with ovaries produce eggs, and a new offspring forms when a sperm unites with an egg. How do sperm and eggs form? And how do they arrive together at the right place and time so they can unite to form a new offspring? These are functions of the reproductive system.

    sperm and egg fertilization
    Figure \(\PageIndex{1}\): A sperm cell fertilizing an egg cell

    What Is the Reproductive System?

    The reproductive system is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and carrying of a fetus. Both both sexes gonads produce gametes. A gamete is a haploid cell that combines with another haploid gamete during fertilization, forming a single diploid cell called a zygote. Besides producing gametes, the gonads also produce sex hormones. Sex hormones are endocrine hormones that control the development of sex organs before birth, sexual maturation at puberty, and reproduction once sexual maturation has occurred. Other reproductive system organs have various functions, such as maturing gametes, delivering gametes to the site of fertilization, and providing an environment for the development and growth of an offspring.

    Sex Differences in the Reproductive System

    The reproductive system is the only human organ system that is significantly different between males and females. Embryonic structures that will develop into the reproductive system start out the same in males and females, but by birth, the reproductive systems have differentiated. How does this happen?

    Sex Differentiation

    Starting around the seventh week after conception in genetically male (XY) embryos, a gene called SRY on the Y chromosome (Figure \(\PageIndex{2}\)) initiates the production of multiple proteins. These proteins cause undifferentiated gonadal tissue to develop into testes. Testes secrete hormones — including testosterone — that trigger other changes in the developing offspring (now called a fetus), causing it to develop a complete male reproductive system. Without a Y chromosome, an embryo will develop ovaries, that will produce estrogen. Estrogen, in turn, will lead to the formation of the other organs of a female reproductive system.

    Human Y chromosome
    Figure \(\PageIndex{2}\): The SRY gene on the short arm of the Y chromosome causes the undifferentiated gonads of an embryo to develop into testes. Otherwise, the gonads develop into ovaries.

    Homologous Structures

    Undifferentiated embryonic tissues develop into different structures in male and female fetuses. Structures that arise from the same tissues in males and females are called homologous structures. The testes and ovaries, for example, are homologous structures that develop from the undifferentiated gonads of the embryo. Likewise, the penis and clitoris are homologous structures that develop from the same embryonic tissues.

    Sex Hormones and Maturation

    Male and female reproductive systems are different at birth, but they are immature and incapable of producing gametes or sex hormones. Maturation of the reproductive system occurs during puberty when hormones from the hypothalamus and pituitary gland stimulate the testes or ovaries to start producing sex hormones again. The main sex hormones are testosterone and estrogen. Sex hormones, in turn, lead to the growth and maturation of the reproductive organs, rapid body growth, and the development of secondary sex characteristics, such as body and facial hair and breasts.

    Role of Sex Hormones in Transgender Treatment

    Feminizing or masculinizing hormone therapy is the administration of exogenous endocrine agents to induce changes in physical appearance. Since hormone therapy is inexpensive relative to surgery and highly effective in the development of secondary sex characteristics (e.g., facial and body hair in female-to-male [FTM] individuals or breast tissue in male-to-females [MTFs]), hormone therapy is often the first, and sometimes only, medical gender affirmation intervention accessed by transgender individuals looking to develop masculine or feminine characteristics consistent with their gender identity. In some cases, hormone therapy may be required before surgical interventions can be conducted. Trans-females are prescribed estrogen and anti-testosterone medication, such as cyproterone acetate and spironolactone. Trans-men are prescribed testosterone. 

    Male Reproductive System

    Male reproductive system
    Figure \(\PageIndex{3}\): The main organs of the male reproductive system.

    The main structures of the male reproductive system are external to the body and illustrated in Figure \(\PageIndex{3}\). The two testes (singular, testis) hang between the thighs in a sac of skin called the scrotum. The testes produce both sperm and testosterone. Resting atop each testis is a coiled structure called the epididymis (plural, epididymes). The function of the epididymes is to mature and store sperm. The penis is a tubular organ that contains the urethra and has the ability to stiffen during sexual arousal. Sperm passes out of the body through the urethra during a sexual climax (orgasm). This release of sperm is called ejaculation.

    In addition to these organs, there are several ducts and glands that are internal to the body. The ducts, which include the vas deferens (also called the ductus deferens), transport sperm from the epididymis to the urethra. The glands, which include the prostate gland and seminal vesicles, produce fluids that become part of semen. Semen is the fluid that carries sperm through the urethra and out of the body. It contains substances that control pH and provide sperm with nutrients for energy.

    Female Reproductive System

    female reproductive system
    Figure \(\PageIndex{4}\): The main organs of the female reproductive system lie within the abdominal cavity.

    The main structures of the female reproductive system are internal to the body and shown in Figure \(\PageIndex{4}\). They include the paired ovaries, which are small, ovoid structures that produce eggs and secrete estrogen. The two Fallopian tubes start near the ovaries and end at the uterus. Their function is to transport eggs from the ovaries to the uterus. If an egg is fertilized, it usually occurs while it is traveling through a Fallopian tube. The uterus is a pear-shaped muscular organ that functions to carry a fetus until birth. It can expand greatly to accommodate a growing fetus, and its muscular walls can contract forcefully during labor to push the baby out of the uterus and into the vagina. The vagina is a tubular tract connecting the uterus to the outside of the body. The vagina is where sperm are usually deposited during sexual intercourse and ejaculation. The vagina is also called the birth canal because a baby travels through the vagina to leave the body during birth.

    The external structures of the female reproductive system are referred to collectively as the vulva. They include the clitoris, which is homologous to the male penis. They also include two pairs of labia (singular, labium), which surround and protect the openings of the urethra and vagina.


    • The reproductive system is the human organ system responsible for the production and fertilization of gametes and, in females, the carrying of a fetus.
    • Both male and female reproductive systems have organs called gonads (testes in males, ovaries in females) that produce gametes (sperm or eggs) and sex hormones (such as testosterone in males and estrogen in females). Sex hormones are endocrine hormones that control the prenatal development of reproductive organs, sexual maturation at puberty, and reproduction after puberty.
    • The reproductive system is the only organ system that is significantly different between males and females. A Y-chromosome gene called SRY is responsible for undifferentiated embryonic tissues developing into a male reproductive system. Without a Y chromosome, the undifferentiated embryonic tissues develop into a female reproductive system.
    • Male reproductive system organs include the testes, epididymis, penis, vas deferens, prostate gland, and seminal vesicles.
    • Female reproductive system organs include the ovaries, Fallopian tubes, uterus, vagina, clitoris, and labia.


    1. What is the reproductive system?

    2. Define gonad.

    3. What are sex hormones? What are their general functions?

    4. Distinguish between male and female sex hormones.

    5. How does the differentiation of the reproductive system occur in males and females?

    6. In the context of the human male and female reproductive systems, what are homologous structures?

    7. When and how does the human reproductive system mature?

    8. List the organs of the male reproductive system.

    9. List the organs of the female reproductive system.

    10. Female gametes are called _________ and male gametes are called _________ .

    11. True or False: The vagina is the homologous structure to the penis.

    12. True or False: In the absence of a Y chromosome in humans, ovaries will develop.

    13. Which are secondary sex characteristics?

    A. Fallopian tubes

    B. ovaries

    C. breasts

    D. all of the above

    14. Fertilization usually occurs in the _________________.

    A. ovary

    B. Fallopian tube

    C. uterus

    D. vagina

    15. Explain the difference between the vulva and the vagina.

    Explore More

    People's sense of gender identity does not always match their anatomy. Some people do not identify as either male or female, and instead, they identify as non-binary, or genderqueer. Others may identify as a gender that is the opposite of what is typically associated with their chromosomes or reproductive organs. These people are called transgender, and they may choose to transition to the opposite gender, a process that may or may not involve physical modifications. Watch the video below to learn about the use of hormones in gender transitioning.

    Sex determination may be more complicated than originally thought. Check out this video to learn more:


    1. Sperm-Egg; public domain via Wikimedia Commons
    2. Human Y chromosome by National Center for Biotechnology Information (NCBI); public domain via Wikimedia Commons
    3. Male Reproductive System by Charles Molnar and Jane Gair; CC BY 4.0 from Concepts of Biology - 1st Canadian edition
    4. Female Reproductive System by staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
    5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
    6. Some text is adapted from White Hughto JM, Reisner SL..  A systematic review of the effects of hormone therapy on psychological functioning and quality of life in transgender individuals. Transgend Health. 2016;1(1):21-31 CC BY 4.0