Female Reproductive System
From the top of the uterus extend the fallopian tubes, which lead backward and downward to the ovaries. These are the two small sacs that contain the eggs. A woman is born with about 400,000 eggs.
Each month during her reproductive years, usually only a single egg, or ovum, matures. The egg matures inside a follicle within an ovary.
At midcycle, the egg is released from the follicle in a process called ovulation.
The egg then enters and travels down one of the fallopian tubes toward the uterus. When the egg reaches the end of the fallopian tube, it is ready for fertilization by the man's sperm.
This sequence of events is controlled by monthly changes in a woman's hormone levels. To help understand the process a little better, it is important to understand the role of the menstrual cycle and the hormones involved.
The Menstrual Cycle
The cycle is driven by two hormones:
- Follicle-stimulating hormone (FSH)
- Luteinizing hormone (LH)
On the first day of the cycle, when menstruation begins, the uterus sheds its lining from the previous cycle. During this phase, the pituitary gland, located at the base of the brain, releases two hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which stimulate the ovary and cause follicular growth. The developing follicle secretes the hormone estrogen, which has several important roles in the body.
One of these roles results in midcycle changes in the cervical mucus about the time of ovulation. During the ovulatory phase, this abundant mucus is prepared to receive and nourish sperm from the man.
The ovulatory phase begins when the level of LH surges, causing the follicle to rupture and the egg to be released from the ovary. The fimbria of the fallopian tube sweep over the ovary and wave the egg into the fallopian tube.
After ovulation occurs, the luteal phase begins. During the luteal phase, the follicle that produced the egg becomes a functioning gland called the corpus luteum. The corpus luteum produces progesterone, which prepares the uterus with the rich lining needed for implantation.
If intercourse has taken place and the egg meets with sperm in the outer end of the tube, conception may occur. The fertilized egg travels through the tube toward the uterus with the help of tiny hairlike projections called cilia, which sweep the egg along. Once inside the uterus, the embryo implants into the lining on about the 20th day of the cycle. The corpus luteum can sense the pregnancy and will continue to produce progesterone, thus preserving the uterine lining and pregnancy.
If fertilization does not occur, the ovum passes through the uterus, and the corpus luteum will cease to function on about Day 26. The uterus will then break down, shed its lining several days later and the next menstrual period begins.
Male Reproductive System
The male reproductive system is also under the influence of hormones and is responsible for producing sperm. The male reproductive system is both internal and external.
The testes are located within the scrotal sac, the pouch of skin located below the man's penis. These are the two organs that produce both sperm and testosterone, the male hormone that helps maintain the male sexual characteristics.
As sperm are produced, they pass from the testes through the coiled channels of the epididymis, an organ that stores and nourishes them as they mature.
Once sperm are completely mature, they move into the vas deferens. This tubal structure connects the epididymis with the seminal vesicles, the two pouchlike glands that provide storage for the mature sperm. The entire process of sperm formation to maturation takes about 72 days.
When a man ejaculates (or expels the fluid from his penis) during intercourse, sperm from the seminal vesicles combine with a thick fluid from the prostate gland to create semen. This fluid (or ejaculate) is deposited into the woman's vagina.
The development of normal, mature sperm is key in establishing male fertility and involves an interplay of several factors:
- An exchange of hormonal messages between the brain and the testicles
- A secretion of necessary reproductive hormones (testosterone) by Leydig cells and normal sperm development within Sertoli cells
- Normal transport of developing sperm from testicles to epididymis to vas deferens
The production of sperm is primarily regulated by three hormones:
- FSH (follicle-stimulating hormone)
- LH (luteinizing hormone)
In the male, pituitary hormones are responsible for maintaining the sperm production process. The pituitary gland, which is located at the base of the brain, secretes FSH and LH, the same hormones necessary for regulating the female's reproductive functions.
FSH is responsible for stimulating sperm production in the testicles. LH stimulates the production of testosterone.
Under the influence of Sertoli cells and specific hormones, immature sperm cells develop through several stages and eventually become mature sperm cells, called spermatozoa. Spermatozoa are not yet motile. They must pass through the epididymis, where after 18–24 hours, they gain motility (movement). After acquiring the ability to move, mature sperm are stored in the vas deferens until ejaculation.