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 develop into ovulated eggs during her lifetime. After puberty, many eggs will start the developmental process each month; however, only one is usually released or 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
- Providing nutrients and movement for the 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. The end closest to the ovary contracts to push the egg down to the site of fertilization while the end closest to the uterus contracts to help get the sperm up to the site. The fallopian tubes sustain the female’s egg and male’s sperm throughout the fertilization process. It may take a few days for the egg and the subsequent fertilized embryo to travel the length of the fallopian tube.
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. The cervix produces mucous secretions that change throughout the reproductive cycle. Around the time of ovulation, the cervical mucous is thin and watery to aid in the passage of sperm to the uterus. After ovulation and/or during pregnancy, the mucous is thick to provide a protective barrier to the uterus from infections.
Normal Female Reproduction
Beginning the first day of menstruation, the body starts to make and release increasing amounts of follicle-stimulating hormone (FSH). This hormone, produced by the pituitary gland at the base of the brain, 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 levels of another female hormone, 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.