13.9: Testes and Male Gamete Production

The testes are the male gonads that produce sperm and secrete the hormones essential for male reproductive function.

Master this section and you'll be able to:

• Explain how the scrotum and its muscles regulate testicular temperature for proper sperm production.
• Identify the major structures of the testes and describe their roles in producing and supporting sperm.
• Summarize the stages of spermatogenesis from spermatogonia to mature sperm and the functions of Leydig and Sertoli cells.

The testes are located in a skin-covered, highly pigmented, muscular sac called the scrotum, which hangs behind the penis. This position is important because sperm production inside the testes works best when they are kept about 2 to 4°C below core body temperature.

The dartos muscle forms the subcutaneous muscle layer of the scrotum and continues inward to create the scrotal septum, which divides the scrotum into two compartments. Each testis is also surrounded by a cremaster muscle, which descends from the internal oblique muscle of the abdominal wall. Working together, the dartos and cremaster muscles can lift the testes in cold conditions, reducing heat loss, or relax in warm conditions, allowing the testes to move farther from the body and release heat. On the outside, the scrotum shows a raised midline ridge called the raphe.

Figure \(\PageIndex{1}\):  The Scrotum and Testes. This anterior view shows the different layers surrounding the scrotum and testes. (Image credit: “Scrotum and Internal Structures” by OpenStax is licensed under CC BY 4.0)

Testes

The testes (singular, testis) are the male gonads, meaning the primary reproductive organs. They produce sperm and secrete androgens such as testosterone, and they remain active throughout the male reproductive lifespan.

Each testis is an oval structure about 4 to 5 centimeters long and is housed within the scrotum (see Figure above). Two protective connective tissue layers surround each testis. Just deep to the cremaster muscle lies the tunica vaginalis, a serous membrane with parietal and thin visceral layers. Beneath this is the tunica albuginea, a tough, white, dense connective tissue layer that not only covers the testis but also folds inward to form septa. These septa divide the interior into 300 to 400 lobules.

Inside the lobules are the seminiferous tubules, where sperm develop. During fetal development, the testes normally descend from the abdomen into the scrotum around the seventh month. This process is called the descent of the testis. When one or both testes fail to descend before birth, the condition is known as cryptorchidism.

Figure \(\PageIndex{2}\): Cross-Section of the Testis and Epididymis. Internal structures of the testis are organized into lobules filled with seminiferous tubules, where sperm are produced before passing into the straight tubules, rete testis, and efferent ductules. The sperm then enter the epididymis for maturation and storage. Surrounding layers, including the tunica albuginea, tunica vaginalis, and cremaster muscle, protect the testis and help regulate its temperature, while the spermatic cord carries vessels and the ductus deferens toward the inguinal canal. (Image credit: “Anatomy of the Testis” by OpenStax is licensed under CC BY 4.0, modifications by Jennifer Lange.)

Seminiferous Tubules for Sperm Production

The seminiferous tubules make up most of each testis. These tightly coiled tubes contain developing sperm cells that mature as they move toward the central lumen. Once released into the lumen as spermatozoa (or simply “sperm”), they enter the testicular duct system. From there, sperm flow into the straight tubules, then into the rete testis, and finally leave the testis through 15 to 20 efferent ductules that pass through the tunica albuginea.

Figure \(\PageIndex{3}\):  Seminiferous Tubules with Developing Sperm Cells. Cross-sections of seminiferous tubules display the layers of developing germ cells arranged from the basal lamina toward the lumen. Spermatocytes lie deeper in the wall, while spermatids appear closer to the lumen as they near maturation and eventually will be released as spermatozoa. Large sustentacular (Sertoli) cells support developing sperm. Between the tubules, interstitial (Leydig) cells occupy the connective tissue and secrete testosterone.
 

The Two Testicular Supporting Cells

Leydig cells and Sertoli cells are the two main supporting cell types inside the testis. They perform distinct, complementary roles that enable male fertility and proper hormone balance.​ Without the coordinated actions of both cell types, normal sperm production and male fertility would not be possible.
 

Interstitial (Leydig) cells

Leydig cells are interstitial cells nestled between the seminiferous tubules in the testis. Their primary function is to produce testosterone, especially in response to luteinizing hormone (LH) from the anterior pituitary gland. Testosterone supports the development of male secondary sexual characteristics and drives spermatogenesis in conjunction with the sustentacular (or Sertoli) cells. These cells are named after Franz Leydig, the German anatomist who first described them in the mid-1800s.
 

Sustentacular (or Sertoli) Cells

Sertoli cells provide structural and nutritional support for developing sperm by creating the scaffold of the seminiferous tubules, regulating spermatogenesis, forming the blood-testis barrier, secreting signals and nutrients, and clearing away leftover cytoplasm via phagocytosis. Because they support and organize every stage of germ cell development, they are often called supporting or sustentacular cells. The name comes from Enrico Sertoli, the Italian physiologist who first described these big cells “nurse cells” in 1865. Note that both cell names start with an "S".

The Main Players Inside the Testes: The Germ Cells

The least mature germ cells, the spermatogonia sit along the basement membrane of the seminiferous tubules. These cells act as stem cells and continuously divide, with some remaining as spermatogonia and others becoming primary spermatocytes. Primary spermatocytes move inward and undergo meiosis to form secondary spermatocytes, which then divide again to produce haploid spermatids. As development continues, spermatids begin transforming their shape and internal structures until they become mature sperm. The full sequence of cell divisions and maturation is called spermatogenesis.  Let us take a closer look at this topic: 
 

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Male Gamete Production: Spermatogenesis

As you just learned, spermatogenesis takes place inside the seminiferous tubules, which make up most of each testis. The process begins at puberty and continues throughout adult life. One full production cycle takes about 64 days, and new cycles begin roughly every 16 days, although different tubules are not synchronized. Sperm counts tend to decline slowly after about age 35, and smoking is associated with lower counts at any age.

The process starts with mitosis of the diploid spermatogonia at the outer edge of the tubule. Each spermatogonium divides to form two primary spermatocytes, which still contain the full diploid set of 46 chromosomes. These primary spermatocytes then enter meiosis. Meiosis I produces secondary spermatocytes, and meiosis II produces spermatids, all of which are haploid cells containing 23 chromosomes. Because mature gametes must be haploid, the second round of cellular division is essential. Spermatids then undergo spermiogenesis, the final maturation that transforms them into streamlined spermatozoa (sperm cells). In a cross section of a seminiferous tubule, the youngest cells lie along the outer basement membrane, while the increasingly mature cells are found closer to the central lumen, where sperm are eventually released.

Figure \(\PageIndex{4}\): Progression of Male Germ Cells from Stem Cell to Mature Spermatozoon. A spermatogonium at the basement membrane divides by mitosis to produce primary spermatocytes. These enter meiosis, forming secondary spermatocytes after meiosis I and then haploid spermatids after meiosis II. Spermatogenesis refers to all stages up to the formation of spermatids. Spermiogenesis is the final differentiation step in which spermatids elongate, condense their nucleus, form an acrosome and a flagellum, and become mature spermatozoa that are eventually released into the lumen for transport to the epididymis.

 

Building a Sperm, One Step at a Time 

Mitosis of a spermatogonium produces two identical diploid cells. One of these new cells stays behind as a spermatogonium so that the process can continue throughout life. The other becomes a primary spermatocyte. This primary spermatocyte is the cell that will begin the long journey through meiosis, the special type of cell division that creates gametes.

Before a primary spermatocyte divides, it copies its DNA, just as cells do before mitosis. It then enters meiosis I, the first stage of meiosis. During this division, the cell separates its pairs of replicated chromosomes and produces two secondary spermatocytes. These two cells still have duplicated chromosomes, but each now holds only one member of each chromosome pair.

Next, both secondary spermatocytes enter meiosis II. This second division separates the duplicated chromosomes into single copies. By the end of meiosis II, each secondary spermatocyte produces two haploid daughter cells, giving a total of four haploid cells. These cells are called spermatids. Even though spermatids have the correct haploid chromosome number, they do not yet look or function like sperm. Early spermatids are round cells with a central nucleus and a fairly large amount of cytoplasm, very different from the streamlined shape of a mature sperm cell.

Spermiogenesis is the remodeling process that turns these round spermatids into recognizable sperm. During spermiogenesis, most of the cytoplasm is removed, the nucleus becomes tightly packed, the acrosome begins to form over the nucleus, mitochondria gather in the midpiece, and a long tail develops for movement. None of this changes the chromosome number; it simply transforms the cell’s structure so that it can function as a gamete.

The final product of this transformation is the spermatozoon (one spermatozoon, two or more spermatozoa) — the formed sperm cell. These mature-shaped sperm cells appear in the region of the seminiferous tubule closest to the lumen. Once they reach this point, the sperm are released into the lumen and carried away through a series of tiny ducts toward the epididymis. There, they will undergo further maturation and eventually gain the ability to swim on their own.

Figure \(\PageIndex{5}\):  Progression of Spermatogenesis in a Seminiferous Tubule. Developing germ cells are shown progressing from the outer edge of the seminiferous tubule toward the central lumen. Spermatogonia line the basement membrane, where stem cells divide to produce primary spermatocytes. As these cells move inward, they undergo meiosis to form secondary spermatocytes and then early spermatids. Late spermatids elongate and begin shaping into spermatozoa during spermiogenesis. Fully formed spermatozoa are released into the lumen for transport toward the epididymis. Sertoli cells support and nourish the developing cells, while interstitial (Leydig) cells in the surrounding tissue produce testosterone, which drives this entire process. (Image credits:  "Slagter - Drawing Human spermatogenesis diagram - English labels" by Ron Slagter, O. Paul Gobée, LUMC, Hope Wicks, LUMC et al, license: CC BY-NC-SA.  "Slagter - Drawing Spermatogenesis in seminiferous tubules of testis - no labels" by Ron Slagter, Jill P.J.M. Hikspoors, MUMC+, Hope Wicks, LUMC et al, license: CC BY-NC-SA; modified by Jennifer Lange.)

 

Figure \(\PageIndex{6}\):  Cross-section of a Seminiferous Tubule Under High Power. Germ cell development progresses from the outer edge of the tubule toward the central lumen. Along the basement membrane lie the spermatogonia, the least mature germ cells. Moving inward, larger primary spermatocytes and then smaller secondary spermatocytes mark the two stages of meiotic division. Closer to the lumen, early spermatids appear as small, round haploid cells beginning their transformation into sperm. Elongated Sertoli cells extend across the entire thickness of the tubule, providing structural and nutritional support to each stage of germ cell development. Image provided by the Regents of the University of Michigan under a BY-SA-NC 4.0.  Illustration by Sofia Elizondo and Jennifer Lange is licensed under CC BY-NC-SA 4.0.)

Sperm Maturation in the Epididymis

After sperm are released into the lumen of the seminiferous tubules, they are carried in testicular fluid toward the epididymis (plural = epididymides). The epididymis is a long, tightly coiled tube attached to each testis, and it is where sperm complete much of their maturation.

Although it appears small from the outside, the epididymis would measure about 6 meters (20 feet) if stretched out. Sperm take an average of 12 days to travel through these coils, although the fastest recorded transit in humans is about one day. Sperm enter at the head of the epididymis and are gently moved forward by rhythmic contractions of smooth muscle in the tube walls.

As they progress, sperm continue to mature, gaining characteristics needed for future motility and fertilization. The most mature sperm are stored in the tail of the epididymis, where they remain until ejaculation.

Sperm Structure

Sperm are much smaller than most cells in the body. Their volume is roughly 85,000 times less than that of the female gamete. Males produce 100 to 300 million sperm per day, whereas females typically ovulate just one oocyte per month. As with many highly specialized cells, the structure of a sperm reflects its job.

The head contains the tightly packed haploid nucleus with almost no cytoplasm, which helps keep the sperm small and streamlined. Covering most of the head is the acrosome, a cap filled with enzymes that help the sperm penetrate the outer layers of the oocyte during fertilization.

The midpiece is packed with mitochondria that produce ATP, the energy needed for movement. This energy powers the flagellum, which extends from the midpiece into the tail and propels the sperm forward. At the core of the flagellum is the axial filament, a structural strand formed from a centriole during the final stages of spermatogenesis.

Figure \(\PageIndex{7}\):  Structural Features of a Sperm Cell. A streamlined spermatozoon is organized into three main regions that support its role in reaching and fertilizing an oocyte. The head contains the tightly packed haploid nucleus and is capped by the acrosome, which stores enzymes that help the sperm penetrate the egg’s outer layers. Just behind the head, the midpiece houses a dense mitochondrial sheath that supplies ATP to power movement. Extending from this region, the long tail contains a central axial filament whose rhythmic beating propels the sperm forward through the reproductive tract. (Image credit: Cropped version of "Slagter - Drawing Penetration of oocyte wall at fertilisation - English labels" by Ron Slagter, Cindy J.M. Hulsman, MUMC+, Jill P.J.M. Hikspoors, MUMC+ et al, license: CC BY-NC-SA.)