Male Reproductive System
Testes
The testes (sing., testis) lie outside the abdominal cavity of the male, within the scrotum. The testes begin to develop inside the abdominal cavity but descend into the scrota: sacs during the last two months of fetal development. If the testes do not descend, a condition called cryptorchidism (krip-tor'kah-dizm) occurs. If the male is not treated or operated on to place the testes in the scrotum, sterility (the inability to produce offspring) usually results. This is because the internal temperature of the body (37°C) is too high to produce viable sperm; the temperature in the scrotum is about 34°C. Wearing tight clothing can increase scrotal temperature and reduce sperm production. When the body is cold, the testes are normally held closer to the body to maintain an optimum temperature.
Seminiferous Tubules
Fibrous connective tissue forms the wall of each testis and divides the testis into lobules. Each lobule contains one to three tightly coiled seminiferous (se"mi-niferus) tubules with a combined length of approximately 1.5 meters (5 feet). A cross section through a tubule when viewed with a microscope shows that each tubule is packed with cells undergoing spermatogenesis (production of sperm). Sperm cells are derived from undifferentiated germ cells called spermatogonia (sing., spermatogonium) that lie just inside the outer wall of a tubule and divide mitotically, producing new spermatogonia. Newly formed spermatogonia move away from the outer wall, increase in size, and become primary spermatocytes that undergo meiosis, which reduces the chromosome number. Secondary spermatocytes divide to produce four spermatids, also with 23 chromosomes. Spermatids then differentiate into spermatozoa (sperm). Also present in the tubules are the sustentacular (Sertoli) cells, which support, nourish, and regulate the spermatogenic cells.
Sperm
The mature sperm has three distinct parts: a tail, a middle piece, and a head. The tail is a flagellum, the middle piece contains energy-producing mitochondria, and the head contains the 23 chromosomes within a nucleus. Adhering to the nucleus is a specialized structure called the acrosome, which contains enzymes that facilitate penetration of the oocyte (egg). The human egg is surrounded by several layers of cells and a thick membrane; the acrosomal enzymes help a sperm digest its way into an egg.
Interstitial Cells
The male sex hormones, the androgens, are secreted by cells that lie between the seminiferous tubules. Therefore, they are called interstitial cells. The most important of the androgens is testosterone.
The testes contain the interstitial cells and seminiferous tubules. in vvhich spermatogenesis occurs. Sperm have a single flagellum and an acrosome-capped head in vvhich 23 chromosomes reside in a nucleus.
Genital Tract
Sperm are produced in the testes, but they mature and are stored in the epididymides (ep"I-did'I-mah-dez). Each epididymis is a tightly coiled tubule 5-6 meters (about 17 feet) in length, located just outside each testis. Each epididymis joins with a vas (ductus) deferens, which ascends through the inguinal canal and enters the abdomen, where it curves around the bladder and empties into the urethra. Sperm are also stored in the first part of a vas deferens.
Spermatic Cords
The testes are suspended in the scrotum by the spermatic cords, each of which consists of fibrous connective tissue and muscle fibers that enclose the vas deferens, the blood vessels, and the nerves. The region of the inguinal canaL where the spermatic cord passes into the abdomen, remains a weak point in the abdominal wall. As such, it is frequently the site of hernias. A hernia is an opening or separation of some part of the abdominal wall through which a portion of an internal organ, usually the intestine, protrudes.
Seminal Fluid
At the time of ejaculation, sperm leave the penis in seminal fluid (semen). Three types of glands add secretions to seminal fluid: the seminal vesicles, the prostate gland, and the bulbourethral (Cowper's) glands. The seminal vesicles are located at the base of the bladder, and each has a duct that joins with a vas deferens. The prostate gland is a single, doughnut-shaped gland that surrounds the upper portion of the urethra, just below the bladder. In older men, the prostate can enlarge and press on the urethra, making urination painful and difficult. The second most common cancer in men is prostate cancer, and detection requires
Each component of seminal fluid seems to have a particular function. Sperm are more viable in a basic solution, and seminal fluid, which is milky in appearance, has a slightly basic pH of about 7.5. Swimming sperm require energy, which is presumably provided by the sugar fructose contained in seminal fluid. Seminal fluid also contains prostaglandins, chemicals that cause the uterus to contract. Some investigators now believe that uterine contraction is necessary to help propel the sperm toward the egg.
Sperm mature in the epididymis and are stored in the vas deferens before entering the urethra just prior to ejaculation. The accessory glands (seminal vesicles, prostate gland, and bulbourethral glands) produce seminal fluid. Semen, vvhich contains sperm and seminal fluid, leaves the penis during ejaculation.
External Genitals
The penis and scrotum are the male external genitals. As discussed earlier, the saotum contains the testes. The penis is the organ of sexual intercourse in males. It has a long shaft and an enlarged tip called the glans penis. At birth, the glans penis is covered by a layer of skin called the foreskin, or prepuce. Sometime near puberty, small glands located in the foreskin and glans begin to produce an oily secretion. This secretion, along with dead skin cells, forms a cheesy substance known as smegma. Circumcision is the surgical removal of the foreskin, usually soon after birth.
When the male is sexually aroused, the penis becomes erect. Erection is achieved because blood sinuses within the erectile tissue of the penis fill with blood. Parasympathetic impulses dilate the arteries of the penis, while the veins are compressed passively so that blood flows into the erectile tissue under pressure. Impotence is a condition in which erection cannot be achieved. Medical and surgical remedies are available to treat impotence.
Orgasm in Males
As sexual stimulation intensifies, sperm enter the urethra from each vas deferens, and the glands add their fluids. Once seminal fluid is in the urethra, rhythmic muscle contractions expel it in spurts from the penis. During ejaculation, a sphincter closes off the bladder so that no urine enters the urethra.
The contractions that expel semen from the penis are a part of male orgasm, the physiological and psychological sensations that occur at the climax of sexual stimulation. The psychological sensation of pleasure is centered in the brain, but the physiological reactions involve the genital (reproductive) organs and associated muscles, as well as the entire body. Marked muscular tension is followed by contraction and relaxation.
Following ejaculation and/or loss of sexual arousaL the penis returns to its normal flaccid state. After ejaculation, a male typically experiences a refractory period, during which stimulation does not bring about an erection. The length of the refractory period increases with age.
In excess of 400 million sperm may be present in the 3.5 milliliters of semen expelled during ejaculation. The sperm count can be much lower than this, however, and fertilization still can take place.
Regulation of Male Hormone levels
The hypothalamus has ultimate control of the testes' sexual functions because it secretes gonadotropic-releasing hormone (GnRH), which stimulates the anterior pituitary to produce the gonadotropic hormones. Two gonadotropic hormones, FSH (follicle-stimulating hormone) and LH (luteinizing hormone), are named for their function in females but exist in both sexes, stimulating the appropriate gonads in each. FSH promotes spermatogenesis in the seminiferous tubules, and LH promotes testosterone production in the interstitial cells. LH in males is also called interstitial cell-stimulating hormone (ICSH).
The hormones mentioned are regulated by negative feedback, a mechanism that maintains testosterone production at a fairly constant level. For example, when the amount of testosterone in the blood rises to a certain level, it causes the anterior pituitary to decrease its secretion of LH. As the level of testosterone begins to fall, the anterior pituitary increases its secretion of LH, and stimulation of the interstitial cells recurs. Only minor fluctuations of the testosterone level occur in the male, and the feedback mechanism in this case acts to maintain testosterone at a normal level. The sustentacular cells in the wall of the seminiferous tubules produce a hormone called inhibin that blocks FSH secretion.
Testosterone
The male sex hormone, testosterone, has many functions. It is essential for normal development and function of the primary sex organs, those structures we have just discussed. It is also necessary for the sperm production. FSH causes spermatogenic cells to take up testosterone, which promotes spermatogenesis.
Greatly increased testosterone secretion at the time of puberty stimulates maturation of the penis and the testes. Testosterone also brings about and maintains the male secondary sexual characteristics, which develop at the time of puberty. Testosterone causes growth of a beard, axillary (underarm) hair, and pubic hair. It prompts the larynx and the vocal cords to enlarge, causing the voice to change. It is responsible for the greater muscular strength of males, which is why some athletes take a supplemental anabolic steroid, which is either testosterone or a related chemical. The disadvantages of anabolic steroid use are discussed in a Medical Focus reading in chapter 10. Testosterone also causes oil and sweat glands in the skin to secrete, thereby contributing to acne and body odor. A side effect of testosterone activity is baldness. Genes for baldness are probably inherited by both sexes, but baldness is seen more often in males because of the presence of testosterone. This makes baldness a sex-influenced trait.
Testosterone is believed to be largely responsible for the sex drive. It may even contribute to the supposed c: gressiveness of males.
In males, FSH promotes spermatogenesis in the seminiferous tubules, and LH promotes testosterone production by the interstitial cells. Testosterone stimula growth of the male genitals during puberty and is necessary for sperm maturation and development of the male secondary sexual characteristics.
