18.3 Structures of the Male Reproductive System

Created by CK-12 Foundation/Adapted by Christine Miller

18.3.1 Rocky Mountain Oysters
Figure 18.3.1 Those are some odd looking oysters…

Rocky Mountain Oysters

First, they are peeled and pounded flat. Then, they are coated in flour, seasoned with salt and pepper, and deep fried. What are they? They are often called Rocky Mountain oysters, but they don’t come from the sea. They may also be known as Montana tendergroin, cowboy caviar, or swinging beef — all names that hint at their origins. Here’s another hint: they are harvested only from male animals, such as bulls or sheep. What are they? In a word: testes.

Testes and Scrotum

The two  (singular, testis) are – and -producing gonads in male mammals, including male humans. These and other organs of the human male reproductive system are shown in Figure 18.3.2. The testes are contained within the , a pouch made of skin and smooth muscle that hangs down behind the .

18.3.2 The Male Reproductive System
Figure 18.3.2 The male reproductive system includes external organs (such as the penis and testes), and internal organs (such as the prostate gland and seminal vesicles). This view shows the organs from the side, so only one of each paired organ (such as the testes and seminal vesicles) is pictured.

Testes Structure

The testes are filled with hundreds of tiny tubes, called , which are the functional units of the testes. As shown in the longitudinal-section drawing of a testis in Figure 18.3.3, the seminiferous tubules are coiled and tightly packed within divisions of the testis called lobules. Lobules are separated from one another by internal walls (or septa).

18.3.3 Testicle
Figure 18.3.3 This longitudinal-section drawing includes a testis on the left, its corresponding epididymis in the centre, and its related vas (or ductus) deferens on the right. The three structures are connected to create a tract through which sperm can travel.


The multi-layered covering of each testis, called the tunica, protects the organ, ensures its blood supply, and separates the testis into lobules. There are three layers of the tunica: the tunica vasculosa, tunica albuginea, and tunica vaginalis. The latter two layers are labeled in the drawing above (Figure 18.3.3).

  • The  is the innermost layer of the tunica. It consists of connective tissue and contains arteries and veins that carry blood to and from the testis.
  • The  is the middle layer of the tunica. It is a dense layer of fibrous tissue that encases the testis. It also extends into the testis, creating the septa between lobules.
  • The  is the outmost layer of the tunica. It actually consists of two layers of tissue separated by a thin fluid layer. The fluid reduces friction between the testes and the scrotum.

Seminiferous Tubules

One or more are tightly coiled within each of the hundreds of lobules in the testis. A single testis normally contains a total of about 30 metres of these tightly packed tubules! As shown in the cross-sectional drawing of a seminiferous tubule in Figure 18.3.4, the tubule contains sperm in several different stages of development (spermatogonia, spermatocytes, spermatids, and spermatozoa). The seminiferous tubule is also lined with epithelial cells called . These cells release a hormone () that helps regulate sperm production. Adjacent Sertoli cells are closely spaced so large molecules cannot pass from the blood into the tubules. This prevents the male’s immune system from reacting against the developing sperm, which may be antigenically different from his own self tissues. Cells of another type, called , are found between the seminiferous tubules. Leydig cells produce and secrete testosterone.

18.3.4 Testes Cross Section
Figure 18.3.4 A cross-sectional drawing of a testis and seminiferous tubule shows the lining of Sertoli cells and sperm in different stages of development within the tubule, and Leydig cells surrounding the tubule.

Other Scrotal Structures

Besides the two testes, the scrotum also contains a pair of organs called epididymes (singular, ) and part of each of the paired (or ducti deferens). Both structures play important functions in the production or transport of sperm.


The within each testis join together to form ducts (called efferent ducts) that transport immature sperm to the epididymis associated with that testis. Each  (plural, epididymes) consists of a tightly coiled tubule with a total length of about 6 metres. As shown in Figure 18.3.5, the epididymis is generally divided into three parts: the head (which rests on top of the testis), the body (which drapes down the side of the testis), and the tail (which joins with the vas deferens near the bottom of the testis). The functions of the two epididymes are to mature sperm, and then to store that mature sperm until they leave the body during an ejaculation, when they pass the sperm on to the vas deferens.

18.3.5 Epididymis
Figure 18.3.5 Each epididymis consists of a (a) head, (b) body, and (c) tail. The latter is directly connected to the (d) vas deferens. The gray egg-shaped structure in the drawing is the testis.

Vas Deferens

The , also known as sperm ducts, are a pair of thin tubes, each about 30 cm (almost 12 in) long, which begin at the epididymes in the , and continue up into the . They are composed of ciliated epithelium and . These structures help the vas deferens fulfill their function of transporting sperm from the epididymes to the , which are accessory structures of the male reproductive system.

Accessory Structures

In addition to the structures within the scrotum, the male reproductive system includes several internal accessory structures that are shown in the detailed drawing in Figure 18.3.6. They include the ejaculatory ducts, seminal vesicles, and the prostate and bulbourethral (Cowper’s) glands.

18.3.6 Accessory Glands
Figure 18.3.6 This detailed cross-sectional drawing of the male reproductive system clearly shows the accessory organs of reproduction, including the seminal vesicles, prostate gland, and Cowper’s (bulbourethral) glands. Secretions from these structures help to form semen.

Seminal Vesicles

The  are a pair of exocrine glands that each consist of a single tube, which is folded and coiled upon itself. Each vesicle is about 5 cm (almost 2 in) long and has an excretory duct that merges with the to form one of the two ejaculatory ducts. Fluid secreted by the seminal vesicles into the ducts makes up about 70% of the total volume of , which is the sperm-containing fluid that leaves the during an . The fluid from the seminal vesicles is alkaline, so it gives semen a basic  that helps prolong the lifespan of sperm after it enters the acidic secretions inside the female . Fluid from the seminal vesicles also contains proteins, fructose (a simple sugar), and other substances that help nourish sperm.

Ejaculatory Ducts

The  form where the vas deferens join with the ducts of the seminal vesicles in the . They connect the vas deferens with the . The ejaculatory ducts carry sperm from the vas deferens, as well as secretions from the seminal vesicles and prostate gland that together form semen. The substances secreted into semen by the glands as it passes through the ejaculatory ducts control its pH and provide nutrients to sperm, among other functions. The fluid itself provides sperm with a medium in which to “swim.”

Prostate Gland

The  is located just below the seminal vesicles. It is a walnut-sized organ that surrounds the urethra and its junction with the two ejaculatory ducts. The function of the prostate gland is to secrete a slightly alkaline fluid that constitutes close to 30% of the total volume of semen. Prostate fluid contains small quantities of proteins, such as enzymes. In addition, it has a very high concentration of zinc, which is an important nutrient for maintaining sperm quality and motility.

Bulbourethral Glands

Also called Cowper’s glands, the two  are each about the size of a pea and located just below the prostate gland. The bulbourethral glands secrete a clear, alkaline fluid that is rich in proteins. Each of the glands has a short duct that carries the secretions into the urethra, where they make up a tiny percentage of the total volume of semen. The function of the bulbourethral secretions is to help lubricate the urethra and neutralize any urine (which is acidic) that may remain in the urethra.

Figure 18.3.7 Male reproductive system.


The  is the external male organ that has the reproductive function of delivering sperm to the female reproductive tract. This function is called intromission. The penis also serves as the organ that excretes urine.

Structure of the Penis

The structure of the penis and its location relative to other reproductive organs are shown in Figure 18.3.8. The part of the penis that is located inside the body and out of sight is called the root of the penis. The shaft of the penis is the part of the penis that is outside the body. The enlarged, bulbous end of the shaft is called the glans penis.

18.3.7 Penis Structures
Figure 18.3.8 This cross section shows the internal anatomy of the penis and related structures. The corpus spongiosum is the column of erectile tissue that contains the urethra. It is sometimes referred to simply as corpus cavernosum, like the other two columns of spongy tissue in the penis.


The passes through the penis to carry urine from the bladder — or from the ejaculatory ducts — through the penis and out of the body. After leaving the urinary bladder, the urethra passes through the , where the urethra is joined by the . From there, the urethra passes through the to its external opening at the tip of the glans penis. Called the external urethral orifice, this opening provides a way for urine or semen to leave the body.

Tissues of the Penis

The penis is covered with skin (epithelium) that is unattached and free to move over the body of the penis. In an uncircumcised male, the glans penis is also mainly covered by epithelium, which (in this location) is called the , and below which is a layer of . The foreskin is attached to the penis at an area on the underside of the penis called the .

As shown in the Figure 18.3.9, the interior of the penis consists of three columns of spongy tissue that can fill with blood and swell in size, allowing the penis to become erect. This spongy tissue is called (plural, corpora cavernosa). Two columns of this tissue run side by side along the top of the shaft, and one column runs along the bottom of the shaft. The runs through this bottom column of spongy tissue, which is sometimes called . The also consists mostly of spongy erectile tissue.  and run along the top of the penis, allowing blood circulation through the spongy tissues.

18.3.8 Penis Cross-section
Figure 18.3.9 The penis consists mostly of spongy tissues that can fill with blood, stiffening the organ. The corpus cavernosum urethrae is now usually called corpus spongiosum.

Feature: Human Biology in the News

Lung, heart, kidney, and other organ transplants have become relatively commonplace, so when they occur, they are unlikely to make the news. However, when the nation’s first penis transplant took place, it was considered very newsworthy.

In 2016, Massachusetts General Hospital in Boston announced that a team of its surgeons had performed the first penis transplant in the United States. The patient who received the donated penis was a 64-year-old cancer patient. During the 15-hour procedure, the intricate network of nerves and blood vessels of the donor penis were connected with those of the penis recipient. The surgery went well, but doctors reported it would be a few weeks until they would know if normal urination would be possible, and even longer before they would know if sexual functioning would be possible. At the time that news of the surgery was reported in the media, the patient had not shown any signs of rejecting the donated organ. Within 6 months, the patient was able to urinate properly and was beginning to regain sexual function.  The surgeons also reported they were hopeful that such transplants would become relatively common, and that patient populations would expand to include wounded warriors and transgender males seeking to transition.

The 2016 Massachusetts operation was not the first penis transplant ever undertaken. The world’s first successful penis transplant was actually performed in 2014 in Cape Town, South Africa. A young man who had lost his penis from complications of a botched circumcision at age 18 was given a donor penis three years later. That surgery lasted nine hours and was highly successful. The young man made a full recovery and regained both urinary and sexual functions in the transplanted organ.

In 2005, a man in China also received a donated penis in a technically successful operation. However, the patient asked doctors to reverse the procedure just two weeks later, because of psychological problems associated with the transplanted organ for both himself and his wife.

18.3 Summary

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

18.3 Review Questions

    1. Describe the structure of a testis.
    2. Which parts of the male reproductive system are connected by the ejaculatory ducts? What fluids enter and leave the ejaculatory ducts?
    3. A vasectomy is a form of birth control for men that is performed by surgically cutting or blocking the vas deferens so that sperm cannot be ejaculated out of the body. Do you think men who have a vasectomy emit semen when they ejaculate? Why or why not?

18.3 Explore More

Human Physiology – Functional Anatomy of the Male Reproductive System (Updated), Janux, 2015.

The Science of ‘Morning Wood’, AsapSCIENCE, 2012.

I Had One Of The World’s First Penis Transplants – Thomas Manning | This Morning, 2016.



Figure 18.3.1

Lamb_fries by Paul Lowry on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 18.3.2

Human_reproductive_system_(Male) by Baresh25 on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 18.3.3

Testicle by Unknown Illustrator from National Cancer Institute, of the National Institutes of Health, Visuals Online, ID 1769 is in the public domain (https://en.wikipedia.org/wiki/en:public_domain).

Figure 18.3.4

Testis-cross-section by Laura Guerin from CK-12 Foundation is used under a 
CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 18.3.5

Epididymis-KDS by KDS444 on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 18.3.6

3D_Medical_Animation_Vas_Deferens by https://www.scientificanimations.com/wiki-images (image 26 of 191) on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 18.3.7

Male anatomy blank [adapted] by Tsaitgaist on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license. (Original: Male anatomy.png)

Figure 18.3.8

Penile-Clitoral_Structure by Esseh on Wikimedia Commons is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

Figure 18.3.9

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


AsapSCIENCE, (2012, November 14). The science of ‘morning wood’. YouTube. https://www.youtube.com/watch?v=D1et5NgT6bQ&feature=youtu.be

Associated Press. (2016, May 17). Man receives new penis in 15-hour operation, the first transplant of its kind in U.S. history [online article]. Canada.com. http://www.canada.com/health/receives+penis+hour+operation+first+transplant+kind+history/11922832/story.html

Brainard, J/ CK-12 Foundation. (2012). Figure 3 Cross section of a testis and seminiferous tubules [digital image]. In CK-12 Biology (Section 25.1) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-biology/section/25.1/

Gallagher, J. (2015, March 13). South Africans perform first ‘successful’ penis transplant (online article). BBC News. https://www.bbc.com/news/health-31876219

Grady, D. (2016, May 16).  Cancer survivor receives first penis transplant in the United States [online article]. New York Times. https://www.nytimes.com/2016/05/17/health/thomas-manning-first-penis-transplant-in-us.html

Janux. (2015, August 16). Human physiology – Functional anatomy of the male reproductive system (Updated). YouTube. https://www.youtube.com/watch?v=k60M1h-DKVY&feature=youtu.be

This Morning. (2016, June 15). I had one of the world’s first penis transplants – Thomas Manning | This Morning. YouTube. https://www.youtube.com/watch?v=Ot7CYjm9B7U&feature=youtu.be


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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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