Info

Sperm are produced in the testes.

4 Seminal fluids are produced by the testes, epididymus, seminal vesicles, the prostate gland, and the bulbourethral glands.

3} Sperm are delivered to the urethra through the vas deferens, which joins the urethra behind the bladder.

Sperm are stored and [ mature in the epididymis.

Sperm are produced in the testes.

Physiology Male Penis

Semen is ejaculated through the male copulatory organ, the penis.

Glans penis

Foreskin-

erectile tissue

Urethra

Glans penis

Foreskin-

Basement Membrane Sperm

Scrotum

'Testis

Scrotum

'Testis

Ureter

(from kidney) Colon

Seminal vesicle Ejaculatory duct Rectum Prostate gland

Bulbourethral gland

Vas deferens Epididymis

Semen is ejaculated through the male copulatory organ, the penis.

Basement membrane

Spermatogonium (2n)

Mitosis Primary spermatocytes (2n)

First meiotic division

Basement membrane

Site Spermatogenesis

Sperm cells develop continuously over the great length of the seminiferous tubules.

43.9 Seminiferous Tubules Are the Site of Spermatogenesis Seminiferous tubules fill the testes of the human male, continuously producing millions of sperm. As sperm mature, they move from the outer layer of the tubule toward the center, where they are shed into the lumen of the tubule.

Secondary spermatocytes (n)

Second meiotic division

Spermatids (n)

Differentiation and maturation

Sperm cells develop continuously over the great length of the seminiferous tubules.

43.9 Seminiferous Tubules Are the Site of Spermatogenesis Seminiferous tubules fill the testes of the human male, continuously producing millions of sperm. As sperm mature, they move from the outer layer of the tubule toward the center, where they are shed into the lumen of the tubule.

Spermatogonia reside in the outer layers of this epithelium. Moving inward from these outer layers toward the lumen of the tubule, we find germ cells in successive stages of sper-matogenesis (Figure 43.9). These germ cells are intimately associated with Sertoli cells, which protect them by providing a barrier between them and any noxious substances that might be circulating in the blood. The Sertoli cells also provide nutrients for the developing sperm and are involved in the hormonal control of spermatogenesis. Between the seminiferous tubules are clusters of Leydig cells, or interstitial cells, which produce male sex hormones.

With completion of the second meiotic division, each primary spermatocyte has given rise to four spermatids (see Figure 43.3a), which develop into sperm cells as they continue to migrate toward the lumen of the seminiferous tubule. The nucleus in what will become the head of the mammalian sperm becomes compact, and the surrounding cytoplasm is lost (see Figure 43.9). A flagellum, or tail, develops. The mitochondria, which will provide energy for tail motility, become condensed into a midpiece between the head and the tail. An acrosome forms over the nucleus in the head of the sperm. Fully differentiated sperm are shed into the seminiferous tubule.

From the seminiferous tubules, sperm move into a storage structure called the epididymis, where they mature and be

Acrosome^jT

Acrosome^jT

Foley Tubing Enlarging Tip Penis

>Head

Midpiece

Tail

Sperm cell (n)

>Head

Midpiece

Tail

Sperm cell (n)

come motile. The epididymis connects to the urethra via a tube called the vas deferens (plural, vasa deferentia). The urethra originates in the bladder, runs through the penis, and opens to the outside of the body at the tip of the penis. It serves as the common duct for the urinary and reproductive systems (see Figure 43.8).

The components of the semen other than sperm come from several accessory glands. The bulbourethral glands produce a small volume of an alkaline, mucoid secretion that neutralizes acidity in the urethra and lubricates the tip of the penis. About two-thirds of the volume of semen is seminal fluid from the paired seminal vesicles. Seminal fluid is thick because it contains mucus and protein. It also contains fructose, an energy source for the sperm, which are too small to carry much of their own fuel.

The prostate gland completely surrounds the urethra as it leaves the bladder. One-fourth to one-third of the volume of semen is a thin, milky fluid that comes from the prostate gland. Prostate fluid makes the uterine environment more hospitable to sperm. The prostate also secretes a clotting enzyme that works on the protein in seminal fluid to convert semen into a gelatinous mass.

The penis and the scrotum are the male genitalia. The shaft of the penis is covered with normal skin, but the highly sensitive tip, or glans penis, is covered with thinner, more sensitive skin that is especially responsive to sexual stimulation. A fold of skin called the foreskin covers the glans of the human penis. The cultural practice of circumcision removes a portion of the foreskin.

Sexual arousal triggers responses in the nervous system that result in penile erection. Nerve endings release a gaseous neurotransmitter, nitric oxide (NO), onto blood vessels leading into the penis, The presence of NO stimulates production of the second messenger cGMP, which causes these vessels to dilate (see Chapter 15). The increased blood flow that results fills and swells shafts of spongy, erectile tissue located along the length of the penis. The enlargement of these blood-filled cavities compresses the vessels that normally carry blood out of the penis. As a result, the erectile tissue becomes more and more engorged with blood. The penis becomes hard and erect, facilitating its insertion into the female's vagina. Many species of mammals, but not humans, have a bone in the penis, but these species still depend on erectile tissue for copulation.

At the climax of copulation, semen is propelled through the vasa deferentia and the urethra in two steps, emission and ejaculation. During emission, rhythmic contractions of smooth muscles in the vas deferentia and accessory glands move sperm and the various seminal secretions into the urethra at the base of the penis. Ejaculation, which follows emission, is caused by contractions of other muscles at the base of the penis surrounding the urethra. The rigidity of the erect penis allows these contractions to force the gelatinous mass of semen through the urethra and out of the penis. The muscle contractions of ejaculation are accompanied by feelings of intense pleasure known as orgasm. They are also accompanied by transient increases in heart rate, blood pressure, breathing, and skeletal muscle contractions throughout the body.

Once ejaculation has been achieved, the autonomic nervous system switches signaling. NO release decreases, and enzymes break down cGMP, causing the blood vessels flowing into the penis to constrict. This constriction causes a decrease in blood pressure in the erectile tissue, thus relieving the compression of the blood vessels leaving the penis, and the erection declines.

Erectile dysfunction, or impotence, is the inability to achieve or sustain an erection. Viagra, a drug used to treat erectile dysfunction, is one of the largest selling drugs today. Viagra inhibits the breakdown of cGMP and therefore enhances the effect of NO released in the penis, thus improving the ability to achieve and maintain an erection.

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Anti-Aging Report

When people generally think about anti-aging, they tend to think about the visible signs of wear and tear, those tell-tale wrinkles, age spots and their developing jowls. No-one wants to get old, let alone feel and look older than their years and anti-aging treatments are becoming so sought after by both men and women that the skincare market is colossal, but what really works?

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