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The Male Reproductive System

bullet Spermatogenesis.
bullet The Sexual Act.

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    The formation of spermatozoid begins around the age of thirteen. They are formed in long seminiferous tubules located within the testicle. If laid end to end, these tubules would form a tube, at least 200 meters long.

Section of a seminiferous tubule.
Section of a seminiferous tubule.

Diagram of a sperm cell.
Diagram of a sperm cell.

    The proliferation of spermatogonia (germ cells) occurs at the circumference of the tubules in the epithelial layer, and some of these cells will differentiate into sperm cells (differentiated cells). When sperm cells acquire motility they migrate toward the light of the tubule.

    It is in the middle of the maturation process, during the division of a primary spermatocyte, by meiosis, that the 23 pairs of chromosomes (total 46) separate into two secondary spermatocytes with 23 single chromosomes. Hence, the male cell contains only half the genetic material. For life to occur, the sperm cell will need a female ovum which contains the other half of the genetic material, to form an embryonic cell that will have a full arsenal of 23 pairs of chromosomes.

    In the final stage of differentiation, the sperm acquires a distinct form, with a head, body and tail (flagellum). The genetic material is contained in the sperm head. At the tip of the head there is a structure, named acrosome, which contains enzymes that help the sperm to pierce membrane and penetrate the egg. In the body, there are multitude mitochondria that provide energy to the tail. The tail serves to propel the sperm, so it can reach the egg. This is done by oscillating movements of the tail, much like the motion of a snake, which allows the tail to propel the sperm at speed that can reach 20 cm per hour.

    When formed, spermatozoids are transported to the epididymis and take the vas deferens to reach the ampulla of the vas deferens. At the time of sexual intercourse, the sperm pass into the ejaculatory duct located inside of the prostate and empties into the urethra. In the ejaculatory duct, the seminal fluid secreted by the seminal vesicles mixte with the sperm (semen volume is 60% of sperm and 40% of seminal fluid). The seminal fluid is the last to be ejaculated and this participates to rinse the ejaculatory duct and urethra.

The seminal organs.
The seminal organs.

Example of abnormal sperms.
Example of abnormal sperms.

    There is also the fluids from the prostate and the bulbourethral gland that mix with the sperm. These fluids contain mostly protein and enzymatic agents. The fibrinogen protein is used to coagulate semen at ejaculation and for about fifteen minutes. Then, the enzymatic agents, such as the fibrinolysin protein, liquefy the semen and restore motility to the spermatozoids.

     Although spermatozoids can survive up to 42 days in the ejaculatory duct, their longevity is reduced to 24-72 hours when they are expelled into the vagina of women. This could be due to the presence of protective substances inside the ejaculatory duct, or a depletion of energy inside the vagina because of their increased activity. With each ejaculation, there are about 3.5 ml of semen that is ejaculated. This semen may contain up to 200 to 400 million spermatozoids. A smaller count could reveal a defficient fertility.

    Several other factors can affect male fertility and temperature is probably the most important. Maturation of the sperm cell requires a temperature slightly below body temperature. This is why the testis are located in a scrotum that can contract to bring the testis closer to the body when it's cold, and relax to move them away from the body when it is hot. Dysregulation in the level of testosterone (the main male sexual hormone) may also result in poor fertility. Allthogether, these factors affect either the quantity or the quality of the sperm.

The Sexual Act.

    The penis is the primary sexual organ of the male. It is the elongated shape of the penis that enables the male to leave his semen in the vagina of the female, and thus fertilization may occur.

Lateral view of the penis.
Lateral view of the penis.

External and subcutaneous views of the male genitals.
External and subcutaneous views of the male genitals.

    To facilitate penetration of the penis into the vaginal cavity of the female, the male must be sexually aroused and get an erection of his penis (it become harder and longer).

    The most excitable of the penis is the glans. Indeed, the glans has an extensive network of nerve endings that, when excited, causes sexual sensations (arousal) and erection of the penis. Nerve impulses from these sensory fibers travel through the pudendal nerve to the sacral region of the spinal cord before being relayed to the brain. But the region of the brain reflecting these kinds of sensations is still poorly defined. Many other nerve fibers innervate the sacral region and participate in sexual arousal. Among these regions, there is the scrotum, the epithelium of the anal and pelvic regions, including the buttocks. Some internal regions can also cause sexual stimulation, such irritation of the urethra or the bladder, and prostatic or seminal secretions meeting the genital ducts. Finally, there are psychological factors that may cause sexual arousal, or inhibit it. Just the thought of sexual acts or even erotic dreams may cause sexual arousal.

Internal view of the male genitals.
Internal view of the male genitals.

    Penile erection results from influx of the parasympathetic nervous system. These nerve impulses dilate the arteries and constrict the veins of the penis which causes an increase in blood pressure in the erectile tissues (corpora cavernosa). These impulses also stimulate the bulbourethral and Littre's glands to secrete mucus. The mucus will help for lubrication during intercourse, even though most of the lubricant will come from the woman.

    The climax of the sexual act is ejaculation. When the stimulation is sufficiently intense, it triggers a reflex at the L1 - L2 levels of the spinal cord which causes contraction of the epididymis, the vas deferens and the prostate, causing the expulsion of semen into the urethra. Wave of contractions of the corpora cavernosa causes the final ejaculation.

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