Ovaries: Anatomy, Hormones, Function and Menstrual Cycle

Ovulation, or the release of the egg down the uterine tube and into the uterus, is completely hormone-driven. About a week or so into the menstrual cycle, the hypothalamus releases GnRH, which instructs the pituitary gland to produce FSH and LH, which together makes the oocyte develop, mature, and migrate to the uterus. A hormone-stimulated oocyte matures when a follicle starts to develop around it.

When FSH increases, many follicles develop, but the FSH also tells the ovaries to increase their production of estrogen. As estrogen goes up, FSH goes down and follicle development slows to a halt. At the midpoint of the menstrual cycle at about 14 days, when estrogen is very high, a surge of LH is produced, which fully matures the egg and causes ovulation to begin. Luteinizing hormone is essential for ovulation to occur.

The luteal phase

Ovaries location at the end of the uterine tubes gives mature ova a few days journey down the uterine tubes to the uterus. During this time, progesterone increases to thicken the uterine lining, which is called the endometrium, in expectation of pregnancy. Progesterone is produced by the leftover follicles called the corpus luteum. Progesterone continues to remain high for about two weeks, and if the egg is not fertilized by a sperm cell, progesterone levels drop, the lining of the uterus sloughs off, and menstrual bleeding begins, only for the cycle to repeat again.

Meiosis and fertilization

If a sperm cell is able to enter and fertilize the egg, the sperm, and ova fuse, and begin dividing in a sex cell reproduction process called meiosis. In meiosis, chromosomes from each parent are mixed up, so that the new organism produced is unique. Meiosis begins as a clump of rapidly dividing, undifferentiated cells called a blastocyst and eventually develops into a fetus.

Estrogen and Progesterone

Estrogen increases during the first half of the menstrual cycle, called ovulation, while progesterone increases during the second half of the menstrual cycle called the luteal phase. Testosterone is also produced in small amounts by the ovaries and plays a major role in libido.

Lesson Summary

Ovaries are sex organs, called gonads, present in the female reproductive system of most animals. Ovaries produce sex hormones and ova (eggs) called gametes or sex cells. Ovaries are not necessary organs for life, although lacking them causes infertility. Ovaries are one of many parts of the female reproductive system, which also includes the uterus, uterine tubes, vulva, and vagina. The ovaries house millions of immature ova, and only a few hundred will fully mature in a human lifetime. The process where mature eggs are produced is called oogenesis, which occurs during the menstrual cycle.

If a mature egg and sperm are to fuse, the egg must first move through the uterine tubes. The menstrual cycle includes two phases called ovulation and the luteal phase. In ovulation, eggs are released into the uterine tubes and travel down to the uterus, awaiting fertilization. Many hormones drive the process, including luteinizing hormone, which is necessary for ovulation to occur, estrogen, which rises during ovulation, and progesterone which thickens the uterus and rises during the luteal phase. An additional hormone produced by the ovaries is testosterone, which stimulates libido (sex drive). If fertilization does not occur by the end of the luteal phase, progesterone levels decrease and the uterine lining sloughs to cause bleeding. However, if fertilization occurs, sex cell reproduction starts in a process known as meiosis. Eventually, a fetus is created from the rapidly dividing cells.