The Reproductive System
The human body is a complex system of bone, tissue, organs and fluids. Chemicals produced by glands – cells or organs that secrete fluids – regulate many bodily functions. The pituitary and the ovaries, glands that secrete hormones related to sex and fertility, play a major role in human reproduction.
The pituitary gland, in both males and females, is responsible for producing hormones that travel through the bloodstream and ultimately regulate the maturation and release of gametes (sperm and eggs respectively). In the female, the ovaries produce estrogen, progesterone and other hormones affecting the sex drive and fertility. Testosterone and other hormones, produced by cells in the male’s genital area, contribute to the production of sperm and his ability to achieve and sustain an erection during intercourse.
The Female Reproductive System
The Menstrual Cycle and the Role of Hormones
Puberty occurs at various ages but, on average, females are around 12 when they experience their first menstrual period and become capable of reproduction. While still in the womb, the female develops some seven million eggs; however, this ovarian reserve has already dropped to approximately one million eggs at birth. By the age of her first period, about 300,000 to 400,000 eggs remain. Typically, only one healthy egg matures in the ovaries each month, before release into the fallopian tube in preparation for fertilization. This process continues for several decades and ends with menopause, which is triggered by depletion of the ovarian reserve.
Although menopause occurs around age 51 on average, it is influenced by many factors. These include a woman’s age at her first period, her mother’s history and her ovarian reserve. Before menopause, most women experience menstrual periods approximately every 28 days. The result is 10 to 11 periods each year, unless a woman is taking “the pill” for birth control, is pregnant or has clinical problems affecting her reproductive system. At age forty, even though a woman may experience regular menses, her chances of pregnancy diminish significantly due largely to the quality of eggs remaining. In fact, at age 19, the likelihood that an egg will be fertilized during unprotected sex is approximately 26%. By age 41, the chances are fewer than 5%.
Significant variation occurs amongst women when it comes to the length of their cycles – the time between the first day of bleeding during one menstrual period and the start of the next. Although the average is 28 days, a “normal” cycle can be as few as 21 days or as many as 35. For most women, bleeding occurs for approximately five days, but even normal periods can last anywhere from three to six days.
The bleeding is caused by the shedding of the lining of the uterus, in cases where an egg was not fertilized during a cycle. Had pregnancy occurred, the uterine lining would have been ready to accept and nurture the fertilized egg (embryo) and, in most cases, the female’s menstrual cycle would stop during gestation and begin again following delivery.
The development of an egg takes approximately 90 days, with maturation occurring during menstrual bleeding, when it is still protected by a fluid-filled sac (follicle) within the ovary. A chemical called follicle stimulating hormone (FSH), produced by the pituitary gland, aides this growth process. During this part of the female’s cycle, two other reproductive hormones – estrogen and progesterone – are at their lowest levels.
The levels of estrogen and androgen secreted by the ovaries will rise as the egg matures, from about the sixth to the fourteenth day of the cycle. Androgen may increase the woman’s desire for sexual intercourse, while estrogen causes the uterine lining to grow and rebuild from the previous cycle, preparing it to accept an embryo.
By about the fourteenth day, the estrogen level is high enough to trigger a surge of luteinizing hormone (LH), another chemical produced by the pituitary. Some 34 to 36 hours after the surge begins – and about 10 to 12 hours after it peaks – ovulation occurs. During ovulation, the egg is released from the follicle and into the fallopian tube, where is can be fertilized by the sperm.
After ovulation, the follicle becomes the corpus luteum and continues to produce androgen, estrogen and progesterone. The progesterone works with the estrogen to change the uterine lining and the mucus that lines the cervix. In some cases, the mucus becomes toxic to sperm, affecting its ability to reach and fertilize the egg.
The window of opportunity for fertilization is limited to the time around release of the egg into the fallopian tube for its journey to the uterus. Whether pregnancy occurs depends on many factors including the availability of a healthy egg and an adequate number of high quality sperm. In addition to being plentiful, the sperm must be strong enough to swim through the vagina, the mucus in the cervical canal and the uterus, before entering the fallopian tube. Once it reaches the egg, it also must penetrate the outer shell (or zona pellucida), which is likely to be hard and impenetrable in older women. The fertilized egg must then travel and implant in the prepared uterine lining in order to develop.
If fertilization does not occur, the corpus luteum breaks apart and levels of estrogen and progesterone drop. Without these hormones to support it, the uterus sheds its lining and menstruation begins about 13 to 15 days after the peak in the LH surge.
The Male Reproductive System
Sperm Production and the Role of Male Hormones
As in women, the male’s pituitary gland produces follicle stimulating hormone (FSH) and luteinizing hormone (LH). LH stimulates cells located in the testes to produce testosterone, the hormone associated with male characteristics and responsible for sperm production and maturation. Testosterone also is responsible for male potency, as it stimulates the sex drive and the male’s ability to achieve and maintain an erection. Male FSH supports the development of sperm in the seminiferous tubules, tiny tubes located in the testes. The sperm carry the male’s genetic traits, which meld with the genetic material in the female’s egg during fertilization.
New, round-shaped sperm cells are produced in a layer along the tubule wall. Still immature, they move into a central passageway to begin their travel through the epididymis, a 10-foot-long tube attached to each testes. From the epididymis, the sperm move through one of two 14-inch tubes called the vas deferens (or vas). Each vas is connected to a small pouch, the seminal vesicle, located behind the bladder. Both lead to the prostate gland, through which the semen enters the urethra. Carried by seminal fluid produced mostly by the prostate and seminal vesicles, the sperm then travel along the urethra to the end of the penis during ejaculation.
Sperm take approximately three months to fully mature, during which the round cell becomes elongated and sprouts a tail-like structure. Following ejaculation into the vagina, the tail provides the thrust the sperm need to swim up and fertilize a waiting egg. Healthy sperm can live up to two days in the female’s body, making fertilization possible not only during, but also around the time of ovulation. Whether this goal is achieved depends in large part on the volume, quantity and quality of sperm available. These factors are assessed during semen analysis, an important part of the male infertility work-up.