Male Reproductive Anatomy
The key components of the male reproductive system are shown in the image above. This system consists of a network of tubes that begins in the testes and eventually leads to the urethra, which then exits the body. Along the way are several accessory organs that contribute to the formation of the semen. Let's begin our discussion with the testes, the male gonads. Although they develop in the abdominal cavity, the testes are housed externally in the scrotum (we will explain how this happens later). This is essential for their function since sperm formation (spermatogenesis) requires a temperature slightly lower than normal body temperature. To assist in the regulation of temperature the walls of the scrotum contain a layer of smooth muscle, the dartos muscles, and the spermatic cord that passes through the inguinal canal contains skeletal muscle, the cremaster muscle.
These muscles can contract or relax depending on the outside temperature. In warmer conditions, these muscles relax allowing the testes to move away from the body to lower testicular temperature. In colder conditions, they contract, pulling the testes up against the body to increase temperature. The adult testes are ovoid organs measuring about 4-5 cm in length. Each is housed in the scrotum and is surrounded by a connective tissue sheath called the tunica albuginea. This sheath branches internally, forming septa which divide the testes into numerous smaller compartments called lobules. Each lobule houses a long, highly coiled seminiferous tubule. It is within these tubules that spermatogenesis occurs. The interstitial tissue surrounds the seminiferous tubules and contains the cells that produce testosterone. These cells are the interstitial cells of Leydig or more commonly simply called the Leydig cells. The seminiferous tubules come together and converge into a system of ducts known as the rete testis which further converges into a group of efferent ductules. The efferent ductules exit the testes and unite to form the epididymis.
The epididymis is a single, long, convoluted duct that forms a comma-shaped structure which sits on the surface of the testes. Although the sperm form in the seminiferous tubules of the testes, final maturation takes place in the epididymis. For example, sperm taken directly from the seminiferous tubules are not motile (they can't swim) and would not be able to move through the female system. Only after spending about 10-14 days in the epididymis do they become motile. The sperm remain in the epididymis until ejaculation. The epididymis is continuous with the vas deferens or ductus deferens. This duct exits the scrotum and passes through the inguinal canal into the abdominal cavity. In addition to the vas deferens and the cremaster muscles, the spermatic cord that passes through the inguinal canal also contains the testicular arteries and veins. Upon entering the pelvic cavity, the vas deferens ascends along the anterior surface of the bladder, curves over its superior surface, and passes to the posterior side. As it descends, the tubes enlarge to form the ampulla of the vas deferens. The ampulla joins the ducts of the paired seminal vesicles forming the ejaculatory duct.
The seminal vesicles are the first of the accessory organs that contribute to the semen. They produce approximately 60% of the seminal fluid. The seminal vesicles provide several important components to the sperm. These include:
1) Fructose and citric acid which provide nutrients to fuel the sperm;
2) Fibrinogen which promotes a weak, temporary coagulation of semen after ejaculation. This helps the semen adhere to the vaginal walls near the cervix and offers some protection against the acidic environment of the vagina;
3) Prostaglandins, which thin cervical mucus, allowing the sperm to more easily enter the uterus. Prostaglandins also stimulate uterine contractions, helping propel the sperm up the female tract.
The ejaculatory duct then enters the second of the accessory organs, the prostate gland, where it unites with the urethra. The prostate accounts for approximately 30% of the seminal fluid. Prostatic secretions include clotting factors that participate with the fibrinogen from the seminal vesicles in the coagulation reaction as well as fibrinolysis, which breaks down the clot and frees the sperm. In addition, the prostatic secretion has a high pH, which helps neutralize the acidic environment of both the male urethra and the female tract.
After the urethra exits the prostate it receives secretions from the last of the accessory organs, the bulbourethral glands, also known as Cowper’s gland, which contribute about 5% of the seminal volume. Secretions of the bulbourethral glands contain mucus to lubricate the urethra and buffers to neutralize acids in the urethra and vagina. The final 5% of the seminal volume is the sperm that comes from the epididymis.
The male urethra participates in both the urinary system and the reproductive system. Anatomically, it is divided into three segments; the prostatic urethra exits the bladder and passes through the prostate gland; the membranous urethra passes through the pelvic floor muscles and the spongy or penile urethra passes through the penis and opens to the exterior via the external urethral orifice. The final structure of the male reproductive system to be discussed is the penis. The penis is the male organ of copulation that is inserted into the female vagina during sexual intercourse. In performing this function, it is necessary that the penis become rigid. To make this possible, the penis is composed of three parallel columns of erectile tissue, the two corpora cavernosa that run side by side just above the urethra, and the corpus spongiosum that surrounds the urethra. At the tip of the penis, the corpus spongiosum expands to form the glans penis. We will discuss how these erectile tissues produce an erection later. In uncircumcised boys, the glans penis is covered by the prepuce or foreskin. The prepuce is cut away during circumcision.