By understanding the reproductive process, you can understand the numerous places where the system may not work properly and thus result in infertility. The three main things needed for reproduction are sperm, eggs, and the uterus/fallopian tubes.
Sperm (and the Male Reproductive System)
The production of sperm is a very complicated process that begins at puberty and lasts, in healthy males, until death. The production of sperm begins in the testicles and is controlled by several hormones. These hormones are regulated by the hypothalamus and the pituitary gland located in the brain.From beginning to end, sperm production takes about 72 days.
The male reproductive system consists of three main components:
- Vas Deferens
The testicles are paired organs, approximately 1½–2 inches long, located within the scrotum (a multi-layered muscular structure that protects the testes and helps with temperature regulation). The testicles have two very important functions that are essential for normal male fertility:
- Production of testosterone
- Production of sperm
Sperm only start their development in the testes. From there, they move on to the epididymis, where they mature and are stored.
The epididymis is a long, narrow tube coiled and contained beneath a fibrous sheath. Uncoiled, the epididymis would be approximately 19 feet long. The epididymis sits behind the testis. It is divided into head, body and tail regions. Here, sperm undergo their final development and maturation, and they are stored until they are ejaculated. From the epididymis, sperm proceed into the vas deferens, also known as the spermatic cord.
The vas deferens is also a long, tube-like structure that connects the epididymis (where the sperm is stored) to the urethra (the tube that expels sperm). Although the urethra also expels urine, a valve controls the flow of sperm versus urine. During ejaculation, the sperm flows out of the testicles, through the vas deferens and into the urethra. Seminal fluid is added by the prostate and other glands as the sperm flow outside the body through the penis.
Eggs (and the Female Reproductive System)
The female reproductive system consists of three main components:
- Fallopian Tubes
The ovaries are 1- to 2-inch oblong organs just below the fallopian tubes on each side of the uterus. The ovaries contain about a half a million immature eggs. All the eggs that a woman will ever have are produced while she is still a fetus. Only a small fraction, about 300–400 eggs, of a woman’s supply will ever ovulate during her lifetime. After puberty, many eggs will start the developmental process each month, however, only one is usually ovulated each month. After the egg is released from the ovary, it is picked up by the fallopian tube.
Fallopian Tubes (Oviducts)
Fallopian tubes are responsible for:
- Picking up a newly released egg
- Sustaining an environment for fertilization
- Moving a fertilized egg into the uterus
The fallopian tube is one of the most complex organs in the body. This is where fertilization takes place. The tubes have a fan-like ends called fimbria that can sweep across the ovary and pick up the ovulated egg. Nearly ½ inch wide at its open end close to the ovary, the tube narrows to the size of a pencil tip near the uterus. Once fertilization occures, it takes several additional days for the fertilized embryo to travel the length of the fallopian tube and enter the uterus.
The uterus is a pear-shaped organ located in the low center of a woman’s pelvis. The uterine lining, known as the endometrium, builds up during the first half or proliferative phase of the menstrual cycle in anticipation of receiving an embryo from the fallopian tube. It protects, develops, and nourishes the fetus until birth. The uterus is also referred to as the womb. If a woman is not pregnant, the uterine lining sheds in the menstrual cycle (i.e., menses).
The cervix is the connection between the vagina and the uterus. For intrauterine insemination or for embryo transfer, a catheter is inserted through the vagina and cervical canal to reach the uterus.
Normal Female Reproduction
Beginning the first day of menstruation, the brain starts to make and release increasing amounts of follicle-stimulating hormone (FSH). This hormone causes the follicles to grow and produce estrogen. The egg that is developing inside the follicle also begins to mature. During the average cycle, at around day 14, the pituitary gland releases a burst of another hormone called luteinizing hormone (LH); this is referred to as the LH surge. LH stimulates the final maturation of the egg and starts the ovulation process, resulting in the release of a mature egg from the follicle. As the egg travels through the fallopian tube, the ovulated follicle forms a cyst known as the corpus luteum, which produces progesterone. Progesterone aids in the preparation of the uterine lining for the soon-to-be fertilized embryo.
If there are sperm present in the fallopian tube, the egg may be fertilized and the resulting embryo will pass through the fallopian tube and enter the uterus (a 5-day journey), where it will implant itself in the uterine wall and grow into a baby. If the egg is not fertilized, it will still travel to the uterus, but will not implant itself. Approximately two weeks after ovulation, the uterus sends a signal to the ovary to decrease progesterone production since no pregnancy occurred. Menstruation, or the shedding of the lining of the uterus, follows. The cycle will begin anew the next month.