May 12, 2013

Reproductive organs produce and transport gametes: focus on humans

Female Reproductive Anatomy

The female′s external reproductive structures are the clitoris and two sets of labia, which surround the clitoris and vaginal opening. The internal organs are a pair of gonads and a system of ducts and chambers that carry gametes and house the embryo and foetus.

The female gonads, the ovaries, lie in the abdominal cavity, flanking, and attached by a mesentery to, the uterus. Each ovary is enclosed in a tough protective capsule and contains many follicles. A follicle consists of one egg cell surrounded by one or more layers of follicle cells, which nourish and protect the developing egg cell. Most or all of the 400,000 follicles a woman will ever have are thought to be formed before her birth. Only several hundred follicles will release egg cells during a woman′s reproductive years. Starting at puberty and continuing until menopause, usually one follicle matures and releases its egg cell during each menstrual cycle. The cells of the follicle also produce the primary female sex hormones, the estrogens. The egg cell is expelled from the follicle in the process of ovulation. The remaining follicular tissue then grows within the ovary to form a solid mass called the corpus luteum (“yellow body”). The corpus luteum secretes additional estrogens and progesterone, a hormone that helps maintain the uterine lining during pregnancy. If the egg cell is not fertilised, the corpus luteum disintegrates, and a new follicle matures during the next cycle.

Oviducts and Uterus
The female reproductive system is not completely closed, and the egg cell is released into the abdominal cavity near the opening of the oviduct, or fallopian tube. The oviduct has a funnel–like opening, and cilia on the epithelium lining the duct help collect the egg cell by drawing fluid from the body cavity into the duct. The cilia also convey the egg cell down the duct to the uterus, also known as the womb. The uterus is a thick, muscular organ that can expand during pregnancy to accommodate a 4–kg foetus. The inner lining of the uterus, the endometrium, is richly supplied with blood vessels. The neck of the uterus is the cervix, which opens into the vagina.

Vagina and Vulva
The vagina is a thin–walled chamber that is the repository for sperm during copulation and that serves as the birth canal through which a baby is born. It opens to the outside at the vulva, the collective term for the external female genitalia.

At birth, and usually until sexual intercourse or vigorous physical activity ruptures it, a thin piece of tissue called the hymen partly covers the vaginal opening in humans. The vaginal opening and the separate urethral opening are located within a recess called the vestibule, bordered by a pair of slender skin folds, the labia minora. A pair of thick, fatty ridges, the labia majora, encloses and protects the labia minora and vestibule. Located at the front edge of the vestibule, the clitoris consists of a short shaft supporting a rounded glans, or head, covered by a small hood of skin, the prepuce. During sexual arousal, the clitoris, vagina, and labia minora all engorge with blood and enlarge. The clitoris consists largely of erectile tissue. Richly supplied with nerve endings, it is one of the most sensitive points of sexual stimulation. During sexual arousal, Bartholin′s glands, located near the vaginal opening, secrete mucus into the vestibule, keeping it lubricated and facilitating intercourse.

Mammary Glands
Mammary glands are present in both sexes but normally function only in women. They are not part of the reproductive system but are important to mammalian reproduction. Within the glands, small sacs of epithelial tissue secrete milk, which drains into a series of ducts opening at the nipple. Fatty (adipose) tissue forms the main mass of the mammary gland of a non-lactating mammal. The low level of estrogen in males prevents the development of both the secretory apparatus and the fat deposits, so male breasts remain small, and the nipples are not connected to the ducts.

Male Reproductive Anatomy
In most mammalian species, including humans, the male′s external reproductive organs are the scrotum and penis. The internal reproductive organs consist of gonads that produce gametes (sperm cells) and hormones, accessory glands that secrete products essential to sperm movement, and ducts that carry the sperm and glandular secretions.

The male gonads, or testes (singular, testis), consist of many highly coiled tubes surrounded by several layers of connective tissue. These tubes are the seminiferous tubules, where sperm form. The Leydig cells that are scattered between the seminiferous tubules produce testosterone and other androgens.

Production of normal sperm cannot occur at the body temperatures of most mammals, and the testes of humans and many other mammals are held outside the abdominal cavity in the scrotum, which is a fold of the body wall. The temperature in a scrotum is about 2°C below that in the abdominal cavity. The testes develop high in the abdominal cavity and descend into the scrotum just before birth. In many rodents, the testes are drawn back into the abdominal cavity between breeding seasons, interrupting sperm maturation. Some mammals whose body temperature is low enough to allow sperm maturation, such as monotremes, whales, and elephants, retain the testes within the abdominal cavity permanently.

From the seminiferous tubules of a testis, the sperm pass into the coiled tubules of the epididymis. It takes about 20 days for sperm to pass through the 6–m–long tubules of each epididymis of a human male. During this passage, the sperm become motile and gain the ability to fertilise an egg. During ejaculation, the sperm are propelled from the epididymis through the muscular vas deferens. These two ducts (one from each epididymis) run from the scrotum around and behind the urinary bladder, where each joins a duct from the seminal vesicle, forming a short ejaculatory duct. The ejaculatory ducts open into the urethra, the tube that drains both the excretory system and the reproductive system. The urethra runs through the penis and opens to the outside at the tip of the penis.

Three sets of accessory glands—the seminal vesicles, prostate gland, and bulbourethral glands—add secretions to the semen, the fluid that is ejaculated. A pair of seminal vesicles contributes about 60% of the total volume of semen. The fluid from the seminal vesicles is thick, yellowish, and alkaline. It contains mucus, the sugar fructose (which provides most of the energy used by the sperm), a coagulating enzyme, ascorbic acid, and prostaglandins, local regulators discussed in Chapter 45.

The prostate gland is the largest of the semen–secreting glands. It secretes its products directly into the urethra through several small ducts. Prostatic fluid is thin and milky; it contains anticoagulant enzymes and citrate (a sperm nutrient). The prostate gland is the source of some of the most common medical problems of men over age 40. Benign (noncancerous) enlargement of the prostate occurs in more than half of all men in this age–group and in virtually all men over 70. Prostate cancer is one of the most common cancers in men. It is treated surgically or with drugs that inhibit gonadotropins, resulting in reduced prostate activity and size.

The bulbourethral glands are a pair of small glands along the urethra below the prostate. Before ejaculation, they secrete a clear mucus that neutralizes any acidic urine remaining in the urethra. Bulbourethral fluid also carries some sperm released before ejaculation, which is one reason for the high failure rate of the withdrawal method of birth control.

Semen in the Female Reproductive Tract
A man usually ejaculates 2–5 mL of semen, and each milliliter may contain 50–130 million sperm. Once in the female reproductive tract, prostaglandins in the semen cause thinning of the mucus at the opening of the uterus and stimulate contractions of the uterine muscles, which help move the semen up the uterus. The alkalinity of the semen helps neutralize the acidic environment of the vagina, protecting the sperm and increasing their motility. When first ejaculated, the semen coagulates, making it easier for uterine contractions to move it along; then anticoagulants liquefy the semen, and the sperm begin swimming through the female tract.

The human penis is composed of three cylinders of spongy erectile tissue derived from modified veins and capillaries. During sexual arousal, the erectile tissue fills with blood from the arteries. As this tissue fills, the increasing pressure seals off the veins that drain the penis, causing it to engorge with blood. The resulting erection is essential to insertion of the penis into the vagina. Rodents, raccoons, walruses, whales, and several other mammals also possess a baculum, a bone that is contained in, and helps stiffen, the penis. Temporary impotence, a reversible inability to achieve an erection, can result from alcohol consumption, certain drugs, and emotional problems. Several drugs and penile implant devices are available for men with nonreversible impotence due to nervous system or circulatory problems. The oral drug Viagra promotes the action of the local regulator nitric oxide (NO), enhancing relaxation of smooth muscles in the blood vessels of the penis. This allows blood to enter the erectile tissue and sustain an erection.

The main shaft of the penis is covered by relatively thick skin. The head, or glans penis, has a much thinner covering and is consequently more sensitive to stimulation. The human glans is covered by a fold of skin called the foreskin, or prepuce, which may be removed by circumcision.