Men have two testicles (testes) that produce male sex cells called sperm. They are held in a bag called the scrotum. During puberty, germ cells line the seminiferous tubules and start to change into sperm cells. 

Seminiferous Tubules

The testis is lined by coiled tubules, known as seminiferous tubules. The tubules are the specific site of meiosis and the creation of male gametes, called sperm cells.  The seminiferous tubules are surrounded by a type of sustentacular cell known as Sertoli cells. Sertoli cells nourish the developing sperm cells and secrete androgens to stimulate them. During the first phase of spermatogenesis, spermatogenic stem cells (spermatocytes) proliferate and differentiate into dark type (Adark) and pale type (Apale) spermatogonia. Throughout adult life, these spermatogonia periodically reproduce through mitosis to give rise to B spermatocytes.

As sperm cell proliferation and meiosis progress, they become larger and elongate. Once spermatids reach the third stage of development, they are known as mature sperm cells or spermatozoa. This stage is characterized by the formation of a midpiece joining the head and tail of each sperm cell. This structure is essential for sperm motility. The sperm cells also develop a tail, known as the flagellum.

Epididymis

The epididymis is a long, coiled tube that stores sperm until the time of ejaculation. It interfaces the testicles to the vas deferens. It is separated into three parts: head, body, and tail. During  maturation in the epididymis, sperm acquires the factors necessary for sperm-oocyte binding. These are gained through carbohydrate-protein interactions between oligosaccharides on the sperm and receptor proteins on the zona pellucida of an oocyte. This process is known as the acrosome reaction and is required for sperm penetration into an oocyte. 

Sperm enters the epididymis from the testes and passes through the caput, or head, of the epididymis. At this point, the sperm is not motile and is very dilute. It then moves through the body of the epididymis and begins to gain motility. After passing through the body of the epididymis, sperm is stored in the cauda, or tail, of the epididymis. The sperm is then transported to the vas deferens, which carries it to the urethra in the form of semen.

Vas Deferens

The paired vas deferens (ductus deferens) are tubes that carry the sperm from the epididymis to the prostate and urethra. They are made of thick fibrous connective tissue and muscle with blood vessels, nerves, and lymph vessels. The vas deferens is a tortuously coiled tube that receives immature sperm from the epididymis and stores them until they are ready to exit the body through ejaculation. It then carries the sperm to the urethra, which is joined by the duct of the seminal vesicles.

The ductus deferens is lined by pseudostratified columnar epithelium with long microvilli, called stereocilia. It is surrounded by a thick layer of circular, inner, and outer longitudinal smooth muscle layers with adventitia. The ductus deferens is innervated by sympathetic nervous fibers and, when stimulated, the muscles contract to propel the sperm to the urethra. The contractions produce the fluids and lubricants that make up semen. The ductus deferens also secretes a substance that helps to coagulate the semen after it is expelled from the body.

Sperm Cells

The sperm cells that pass through the testis and epididymis become mature, haploid male gametes called spermatozoa. These contain half the genetic information of a full cell and can only reproduce by uniting with an egg cell (egg – or ovum) in sexual reproduction to form a diploid zygote. The head of a sperm contains the nucleus that holds 23 chromosomes and the acrosome, which has inactive enzymes that react with diffusible molecules on the surface of an egg. When this reaction occurs, it causes the sperm to change its motility: Its whip-like tail starts to bend more forcefully and swim faster. The midpiece of a sperm, which takes up 10% of its length, contains tightly concentrated mitochondria that generate energy for movement. The tail, which accounts for 80% of its length, is studded with channels that allow the passage of calcium ions. This sudden influx causes the tail to extend deeper into seminal fluid and propels the sperm toward its target.