153 18.2 Introduction to the Reproductive System

Created by CK-12 Foundation/Adapted by Christine Miller

17.2.1 Sperm and Egg
Figure 17.2.1 Love at first sight!

It’s All about Sex

A tiny from dad breaks through the surface of a huge egg from mom. Voilà! In nine months, a new son or daughter will be born. Like most other multicellular organisms, human beings reproduce sexually. In human sexual reproduction, males produce sperm and females 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.

What Is the Reproductive System?

The  is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and, in females, the carrying of a fetus. Both male and female reproductive systems have organs called  that produce gametes. A  is a cell that combines with another haploid gamete during , forming a single diploid cell called a . Besides producing gametes, the gonads also produce 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 (shown in Figure 18.2.2) initiates the production of multiple proteins. These proteins cause undifferentiated gonadal tissue to develop into male gonads (testes). The male gonads then secrete hormones — including the male sex hormone 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 female gonads (ovaries) that will produce the female sex hormone estrogen. Estrogen, in turn, will lead to the formation of the other organs of a normal female reproductive system.

18.2.2 Y Chromosome SRY gene
Figure 18.2.2 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 . Structures that arise from the same tissues in males and females are called . The male testes and female ovaries, for example, are homologous structures that develop from the undifferentiated gonads of the embryo. Likewise, the male penis and female 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 and stimulate the testes or ovaries to start producing sex hormones again. The main sex hormones are  in males and  in females. Sex hormones, in turn, lead to the growth and maturation of the reproductive organs, rapid body growth, and the development of secondary sex characteristics.  are traits that are different in mature males and females, but are not directly involved in reproduction. They include facial hair in males and breasts in females.

Male Reproductive System

The main structures of the male reproductive system are external to the body and illustrated in Figure 18.2.3. The two (singular, testis) hang between the thighs in a sac of skin called the . The testes produce both and . Resting atop each testis is a coiled structure called the (plural, epididymes). The function of the epididymes is to mature and store sperm. The 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, the male reproductive system consists of several ducts and glands that are internal to the body. The ducts, which include the (also called the ductus deferens), transport sperm from the to the . The glands, which include the and , produce fluids that become part of 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.

18.2.3 Male Reproductive System
Figure 18.2.3 Most of the major male reproductive organs are located outside of the body.

Female Reproductive System

The main structures of the female reproductive system are internal to the body and shown in the following figure. They include the paired , which are small, ovoid structures that produce and secrete . The two (sometimes called Fallopian tubes or uterine tubes) start near the ovaries and end at the . Their function is to transport ova from the ovaries to the uterus. If an egg is fertilized, it usually occurs while it is traveling through an oviduct. 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 labour to push the baby out of the uterus and into the vagina. The is a tubular tract connecting the uterus to the outside of the body. The vagina is where sperm are usually deposited during and . The vagina is also called the birth canal because a baby travels through the vagina to leave the body during birth.

18.2.4 Female Reproductive System
Figure 18.2.4 The main organs of the female reproductive system lie within the abdominal cavity.

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

18.2 Summary

  • The is the human organ system responsible for the production and of  and, in females, the carrying of a .
  • Both male and female reproductive systems have organs called  ( in males, in females) that produce gametes ( or ova) and sex hormones (such as in males and in females). Sex are endocrine hormones that control the prenatal development of reproductive organs, sexual maturation at puberty, and reproduction after .
  • 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.
  • Structures such as testes and ovaries that arise from the same undifferentiated embryonic tissues in males and females are called .
  • Male and female reproductive systems are different at birth, but at that point, they are immature and nonfunctioning. Maturation of the reproductive system occurs during puberty, when hormones from the and stimulate the gonads to produce sex hormones again. The sex hormones, in turn, cause the changes of puberty.
  • Male reproductive system organs include the , , , , , and .
  • Female reproductive system organs include the , , , , , and .

18.2 Review Questions

  1. What is the reproductive system?
  2. Explain the difference between the vulva and the vagina.

18.2 Explore More

Sex Determination: More Complicated Than You Thought, TED-Ed, 2012.

The evolution of animal genitalia – Menno Schilthuizen, TED-Ed, 2017.

Hormones and Gender Transition, Reactions, 2015.

 

Attributions

Figure 18.2.1

Sperm-egg by Unknown author on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).  

Figure 18.2.2

Y Chromosome by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license. 

Figure 18.2.3

3D_Medical_Animation_Vas_Deferens by https://www.scientificanimations.com on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 18.2.4

Blausen_0399_FemaleReproSystem_01 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.


References
 

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Reactions. (2015, June 8). Hormones and gender transition. YouTube. https://www.youtube.com/watch?v=l5knvmy1Z3s&feature=youtu.be

TED-Ed. (2012, April 23). Sex determination: More complicated than you thought. YouTube. https://www.youtube.com/watch?v=kMWxuF9YW38&feature=youtu.be

TED-Ed. (2017, April 24). The evolution of animal genitalia – Menno Schilthuizen. YouTube. https://www.youtube.com/watch?v=vcPJkz-D5II&feature=youtu.be

 

License

Icon for the Creative Commons Attribution-NonCommercial 4.0 International License

Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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