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Male Reproductive System

Structure and Function of the Male Reproductive System

The primary male reproductive organs are the testes, which produce male gametes and testosterone, and the accessory organs including the penis, glands, ducts, and tubules.

The male reproductive system's primary functions are to produce sperm and deliver them to the female reproductive tract for sexual reproduction. Secondary functions include the production of hormones to support reproductive processes and promote the development of the male sexual characteristics.

The external structures of the male reproductive system are called the genitals or genitalia. The genitals include the penis, urethra, and scrotum, and the internal structures include the testes, the extratesticular duct system, and accessory glands. Sperm are cells that are used to fertilize eggs during sexual reproduction. They are produced in the testes and are transported by ducts such as the epididymis through the male reproductive system to the exterior opening of the male reproductive tract. Accessory glands, such as the seminal vesicles and the prostate, secrete fluids called semen that function to provide support and protection to sperm as well as aid in sperm movement through the reproductive tract. The male reproductive system shares the urethra with the urinary system, as it is the tube that carries both semen and urine through the penis and out of the body. Despite each having a unique function, all organs of the male reproductive system work together to create sperm and deliver them to the female reproductive tract where fertilization occurs.
The male reproductive system includes the penis, testes, prostate, and several internal tubes, such as the seminal vesicle and epididymis. These structures produce and transport sperm and semen out of the body through the urethra.

Scrotum and Testes

The scrotum is a sac-like structure that hangs from the root of the penis and contains the testes, which are reproductive organs that function to produce male gametes and testosterone.

The scrotum is a sack of skin and fat that is located outside the body. It protects and provides a supporting structure for each testicle (plural, testes), which is one of a pair of reproductive organs that produces sperm cells and testosterone. Externally the scrotum resembles a single pouch of skin. Internally there is a scrotal septum that divides it into two compartments. This septum is connected to a middle ridge called the raphe. Each compartment contains one oval-shaped gland called a testis.

Because the scrotum is found outside the body, the testes are exposed to external body temperature changes. This is beneficial for sperm production by the testes, as it requires a temperature of about 2–3°C below the core body temperature of 37°C. When external body temperatures get too cold, the cremaster and dartos muscles contract. The dartos muscle is composed of smooth muscle fibers while the cremaster muscles are composed of skeletal muscle fibers. During muscular contraction the testes move closer to the body, absorbing heat from the wall of the scrotum. The scrotum also tightens, increasing scrotal thickness and reducing heat loss. When the scrotum is exposed to high body temperatures, these actions occur in reverse by the relaxation of the cremaster and dartos muscles.

The tunica vaginalis and tunica albuginea surround the testes. The outer tunic is the tunica vaginalis, which is double layered and partially covers a testis. Internal to the tunica vaginalis is the tunica albuginea. Septa extend from the tunica albuginea to divide the testis into several internal compartments called lobules. Each lobule in the testis contains several tightly coiled channels, each called a seminiferous tubule, where sperm are produced, stored, and transported. Layers of contractile cells called myoid cells resembling smooth muscle surround these seminiferous tubules. When myoid cells contract, sperm and fluids flow through these tubules and out of both testes. Sperm then travels through tubes within the spermatic cord, the cord-like structure that includes the ductus deferens and surrounding tissue, to reach the urethra so the sperm can be expelled. The spermatic cord contains sperm-carrying ducts, blood vessels, nerves, and other tissues that carry messages and materials to and from the testes and scrotum. A sustentocyte (Sertoli cell), found within the walls of the seminiferous tubules, plays a part in the production and transport of sperm. Between the spaces of seminiferous tubules are cell clusters called interstitial endocrine cells (Leydig cells). An interstitial endocrine cell is one of several cells found in the seminiferous tubules that secretes the sex hormone testosterone, a hormone that promotes sperm production and the development of the male sex organs and traits.

The epididymis flanks the testes posteriorly (lies behind the testes) and is the site for sperm storage and functional maturation of sperm. Sperm leave the scrotal region during ejaculation via the ductus deferens (vas deferens).
The scrotum is a sac-like structure that contains the testes. The scrotum protects and regulates the temperature required by the testes to manufacture sperm.

Sperm Travel through the Ducts

Sperm cells are produced in the seminiferous tubules of the testes before traveling in seminal fluid through a network of ducts that lead to outside the body.

In the testes, sperm cells are produced in the seminiferous tubules. As sustentocytes in the walls of the tubules secrete fluid, pressure is generated in the lumen (the inside of a tubular organ) to propel the sperm and the fluid from the seminiferous tubules to the straight tubules. From the straight tubules the sperm and fluid travel to a network of ducts, first the rete testis and then the efferent ductules that lead to the epididymis. The epididymis is part of the duct system that functions as a storage site for sperm and is where sperm acquire motility.

The epididymis is found along the posterior border of each testis. Within each epididymis is a single tube made of tightly coiled ducts called ductus epididymis. This is where sperm and fluid empty into after traveling from the seminiferous tubules. The epididymis is segmented into a head, body, and tail. The head of the epididymis is the posterior region and is the site where efferent ducts from the testis join the ductus epididymis. The body is the narrow mid-portion of the epididymis. Sperm remain in the epididymis until they are propelled through a series of ducts that lead out of the body by way of ejaculation. Sperm are viable while stored in the epididymis for several months. If stored sperm are not ejaculated, they are eventually reabsorbed.

Upon exiting the epididymis through the tail region, the first duct that sperm travel through is the ductus deferens (vas deferens). This duct extends through the spermatic cord, or supporting structure that ascends out of the scrotum, where it eventually enters the pelvic cavity. There is an extension of the ductus deferens through the inguinal canal, a passageway through the abdominal wall that travels behind the bladder. The ductus deferens moves sperm from the epididymis to the urethra during sexual arousal. It also functions as a storage site for sperm.

In order to reach the urethra, the sperm must pass through the ejaculatory duct, which runs through the prostate gland. The urethra is a muscular tube that serves as the terminal portion of the duct system. The urethra is used by both the reproductive and urinary systems in males. This means both seminal fluid and urine passes through the male urethra, but not simultaneously. After reaching the urethra, sperm are mixed with other secretions and travel through the penis for release out of the body.

In summary, the route of sperm is:

1. Seminiferous tubule

2. Straight tubule

3. Rete testis

4. Efferent ductules

5. Epididymis

6. Ductus deferens

7. Ejaculatory duct

8. Urethra

Accessory Glands and Semen

Semen is a white, viscous fluid produced by male accessory glands that carries sperm to the female reproductive tract following male ejaculation.

The thick, whitish fluid that contains mature sperm cells in suspension is called semen. Semen is ejaculated as a gelatinous material but then liquifies to aid in sperm motility. Each ejaculation contains 2–5 mL of semen containing between 20 million and 150 million sperm cells. In addition to sperm, semen also contains secretions from three types of accessory glands and the seminiferous tubules.

  • Seminal vesicle: each of a pair of glands that releases secretions into the ejaculatory duct. They lie posterior to the base of the urinary bladder and anterior to the rectum. Fluid released from these glands is alkaline and viscous. It contains the sugar fructose, prostaglandins, and clotting proteins, including semenogelin. Semenogelin is a protein that creates a gel matrix around the sperm. The alkalinity of seminal vesicle secretions neutralizes the acidic environment of the male urethra and female reproductive tract to promote sperm survival. Fructose aids in the production of ATP for sperm motility. Prostaglandins contribute to sperm viability and motility by reducing the effect of the female immune system acting against the sperm.
  • Prostate gland: this gland is roughly the size of a golf ball. It surrounds the urethra and is inferior to (below) the urinary bladder. Composed of tubuloalveolar glands, the prostate is enclosed by a layer of thick connective tissue. One function of the prostate is to secrete a milky, slightly acidic fluid that contains several different substances including citric acid, proteolytic enzymes, and prostate-specific antigen. Citric acid is used to help produce ATP, which provides energy to the sperm. Prostate-specific antigen is a protein that liquefies semen by breaking down semenogelin, allowing sperm to more effectively swim. Collectively the prostatic fluid enters the urethra by way of several ducts. This secretion happens as the prostate's smooth muscle fibers contract during ejaculation.
  • Bulbourethral gland: this is one of a pair of pea-sized glands that are inferior to (under) the prostate. They secrete thick, clear alkaline mucus into the urethra during sexual arousal and neutralize acids from urine. These glands also secrete lubricating mucus at the end of the penis and in the lining of the urethra. This lubricant helps reduce the amount of sperm damaged during travel from the reproductive tract to outside the body.
Anterior to the urinary bladder are three types of accessory glands of the male reproductive system-the prostate gland, bulbourethral (Cowper's) glands, and seminal vesicles. These are responsible for secreting fluids that create semen, a thick, viscous fluid that transports sperm.

Anatomy of the Penis

The penis is the organ that propels seminal fluid containing sperm outside the body.

In order to exit the body, seminal fluid and sperm must travel through the urethra in the penis, a passageway for both semen ejaculation and urinary excretion. The penis delivers sperm and other substances outside the body. It is cylindrical in shape and consists of a root, body, and enlarged tip. Along with the scrotum, the bulb of the penis, the most proximal part of the root of the penis, attaches to the perineum, the inferior (lower) region of the pelvis between the pubic symphysis and the coccyx.

Three masses make up the body of the penis and run longitudinally through the shaft. Each mass consists of tissue that is surrounded by the tunica albuginea. There are two dorsolateral masses called the corpora cavernosa penis. The smaller midventral mass is called the corpus spongiosum penis. The corpus spongiosum penis contains the urethra and keeps it open so urine and semen can exit. The enlarged tip at the end of the body, or shaft, of the penis is called the glans penis. The glans penis is found at the distal end of the corpus spongiosum. When the urethra enlarges inside the glans penis, an external opening called the external urethral orifice forms. A loose skin covering the glans penis is the prepuce, or foreskin. This skin is present in an uncircumcised penis.

Erectile tissues that are part of a skin layer enclose all three masses. Erectile tissue consists of a spongy network of connective tissue and smooth muscle. When the male experiences sexual arousal, vascular spaces within the corpus cavernosa fill with blood. This causes the flaccid penis to enlarge and stiffen, creating an event called an erection. An erect penis is able to serve as a penetrating organ, delivering sperm and seminal fluid to the female reproductive tract.

Penis Erection

For the penis to ejaculate semen, it must be erect. During arousal, arterioles (blood vessels) dilate to allow more blood to flow into the penis and fill the vascular spaces in the corpora cavernosa. This engorgement compresses the veins so blood flow out of the penis slows, keeping the penis erect.