The testes (gonads) are the primary reproductive organ in the male. They produce sperm (exocrine function) and testosterone (endocrine function). The accessory reproductive structures are ducts or glands that aid in the delivery of sperm to the body exterior or to the female reproductive tract.
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Testes
Testes are approx. 4cm long and 2.5cm wide. A fibrous connective tissue capsule, the tunica albuginea (white coat) surrounds each testis. Extensions of this capsule (septa) plunge into the testis and divide it into a large number of lobules. Each lobule contains one to four tightly coiled seminiferous tubules, the actual ‘sperm producing site’. Seminiferous tubules of each lobe empty sperm into another set of tubules, the rete testis, located at one side of the testis. Sperm travel through the rete testis to enter the first part of the duct system, the epididymis, which hugs the external surface of the testis.
Lying in the soft connective tissue surrounding the seminiferous tubules are the interstitial cells, functionally distinct cells that produce androgens, most importantly testosterone.
Duct System
The parts that transport sperm from the body are the epididymis, ductus deferens, and urethra.
Epididymis
The comma-shaped epididymis is a highly coiled tube about 6 meters long that caps the superior part of the testis and then runs down its posterolateral side. The epididymis is the first part of the male duct system and provides a temporary storage site for the immature sperm that enter it from the testis. While the sperm make their way along the twisted course of the epididymis (about 20 days), they mature, gaining the ability to swim. When a male is sexually stimulated, the walls of the epididymis contract to expel the sperm into the next part of the duct system, the ductus deferens.
Ductus Deferens
The ductus deferens, or, vas deferens, runs upward from the epididymis through the inguinal canal into the pelvic cavity and arches over the superior aspect of the bladder. This tube is enclosed, along with blood vessels and nerves, in a connective tissue sheath called the spermatic cord. The end of the vas deferens empties into the ejaculatory duct, which passes through the prostate gland to merge with the urethra. The main function of the ductus deferens is to propel live sperm from their storage sites, the epididymis and distal part of the ductus deferens, into the urethra. At the moment of ejaculation, the thick layers of smooth muscle in its walls create peristaltic waves that rapidly squeeze the sperm forward.
Part of the ductus deferens is in the scrotal sac which hangs outside the body cavity. Some men voluntarily have a vasectomy. This entales a surgeon making a small incision into the scrotum and then cutting or cauterizing the vas deferens. Sperm is still produced, but they can no longer reach the outside of the body, they eventually deteriorate and are reabsorbed. The man is then sterile. Testosterone is still produced so libido and secondary sex characteristics are retained.
Urethra
The urethra, which extends from the base of the bladder to the tip of the penis, is the terminal part of the male duct system. It has 3 main regions, the prostatic urethra (surrounded by the prostate gland), the membranous urethra (from the prostate to the penis), and the spongy urethra (runs the length of the penis). The urethra carries both urine and sperm to the body exterior, thus it is part of both the reproductive and urinary systems. However, urine and sperm never pass at the same time. When ejaculation occurs and sperm enter the prostatic urethra from the ejaculatory ducts, the bladder sphincter constricts. This event not only prevents the passage of urine into the urethra, but also prevents sperm from entering the urinary bladder.
Accessory Glands
The accessory glands include the seminal vesicles, the prostate gland, and the bulbourethral glands. These produce most of the semen, the fluid that contains the sperm within that is propelled out of the male’s reproductive tract during ejaculation.
Seminal Vesicles
These are located at the base of the bladder. They produce about 60% of the fluid volume of semen. Their thick, yellowish secretion is rich in fructose, Vitamin C, prostaglandins, and other substances which nourish and activate the sperm passing through the tract. The duct of each seminal vesicle joins that of the vas deferens on the same side to form the ejaculatory duct. Sperm and seminal fluid enter the urethra together during ejaculation.
Prostate Gland
This is a single gland about the size and shape of a chestnut. It encircles the upper part of the urethra just below the bladder. Prostate gland secretion is a milky fluid that plays a role in activating sperm. During ejaculation it enters the urethra through several small ducts. Since the prostate is located immediately anterior to the rectum, its size and texture can be palpated by digital examination through the anterior rectal wall.
Bulbourethral Glands
These are tiny pea shaped glands inferior to the prostate gland. They produce a thick, clear mucus that drains into the penile urethra. This secretion is the first to pass down the urethra when a man becomes sexually excited. It is believed to cleanse the urethra of traces of acidic urine, and it serves as a lubricant during sexual intercourse.
Semen
Semen is a milky white somewhat sticky mixture of sperm and accessory gland secretions. The liquid provides a transport medium and nutrients and contains chemicals that protect the sperm and aid their movement. Mature sperm cells are streamlined and contain little cytoplasm or stored nutrients. The fructose in the seminal vesicle secretion provides essentially all of their energy fuel. The relative alkalinity of semen as a whole (pH 7.2-7.6) helps neutralize the acid environment (pH 3.5-4) of the female’s vagina, protecting the delicate sperm. Sperm are very sluggish under acidic conditions (below pH 6). Semen also contains seminal plasmin, a chemical that inhibits bacterial multiplication, the hormone relaxin, and certain enzymes that enhance sperm motility.
Semen also dilutes sperm, without such dilution, sperm motility is severely impaired. The amount of semen propelled out of the male duct system during ejaculation is relatively small, only 2-5ml (teaspoonish), but there are between 50 and 130 million sperm in each milliliter.
Male infertility may be caused by obstructions of the duct system, hormonal imbalances, environmental estrogens, pesticides, excessive alcohol, and many other factors. One of the first series of tests done when a couple has been unable to conceive is semen analysis. Factors analyzed include sperm count, motility, morphology (shape and maturity), semen volume, pH, and fructose content. A sperm count lower than 20 million per milliliter makes impregnation improbable.
External Genitalia
This includes the scrotum and the penis.
Scrotum
The scrotum is a divided sac of skin that hangs outside the abdominal cavity, between the legs and at the root of the penis. Under normal conditions, the scrotum hangs loosely from its attachments, providing the testes with a temperature that is below body temperature. This is a rather exposed location for a man’s testes, which contain his entire genetic heritage, but apparently viable sperm cannot be produced at normal body temperature. The scrotum, which provides a temperature about 3 degrees centigrade lower, is necessary for the production of healthy sperm. When the external temperature is very cold, the scrotum becomes heavily wrinkled as it pulls the testes closer to the warmth of the body wall. Thus, changes in scrotal surface area can maintain a temperature that favors viable sperm production.
Penis
The penis is designed to deliver sperm into the female reproductive tract. The skin covered penis consists of a shaft which extends in an enlarged tip (the glans penis). The skin covering the penis is loose, and it folds downward to form a cuff of skin called the ‘prepuce’ or foreskin around the proximal end of the glans. Frequently the foreskin is surgically removed shortly after birth, by a procedure called circumcision.
Internally, the spongy urethra is surrounded by three elongated areas of erectile tissue, a spongy tissue that fills with blood during sexual excitement. This causes the penis to enlarge and become rigid. This event is called an erection, which helps the penis serve as a penetrating organ to deliver the semen into the female’s reproductive tract.
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