The Ovarian Cycle: Anatomy & Processes

Instructor: Meredith Mikell
The process of ovulation has many different components leading to the release of an egg. Here, we will examine the anatomy and steps of the ovarian cycle in humans. At the end, you can test your knowledge with a brief quiz.

Ongoing Cycles

A woman's monthly cycle is a dynamic and complex process. We typically just consider the menstruation, or period, when we think of the female reproductive cycle, but there are constant fluctuations in hormones and subtle physical changes involved in a healthy reproductive system.

One part of the female reproductive cycle involves the endometrium, a nutrient-rich layer of mucous built up in the uterus to support a possible developing embryo. The other part of the cycle involves ovulation, which involves the development of ovary follicles and culminates in the release of an egg. Both parts are controlled by specific hormones. If an egg goes unfertilized and no pregnancy ensues, the uterine mucous and egg are shed away during menses. If the egg is fertilized and pregnancy results, a whole new set of hormonal cycles take charge! But let's not get ahead of ourselves. In this lesson, we will focus on the ovarian cycle.

Anatomy of the Ovarian Cycle

Women have two ovaries, one on either side of the uterus, to which they are connected by way of fallopian tubes. The ovaries contain thousands of follicles, tiny egg housings that secrete hormones and enlarge, or ripen, during the ovarian cycle in preparation of egg release. They are embedded in a cortex of tissue that comprises the rest of the ovary, through the center of which is the medulla, a region of blood vessels that deliver nutrients to the rest of the ovarian cells.

Each follicle has one egg, which, at any given time, is in a different stage of development. For this reason, an ultrasound of an ovary would show many asymmetrical lumps of different sizes, each a follicle in a different phase of the cycle. The entirety of the female menstrual cycle is usually 28 days (though it varies slightly from woman to woman) and begins with the ripening of the follicles.

The parts and phases of the ovarian cycle.

The Follicular Phase

Day 1-14 is all about the growth of the follicles and the release of an egg from one of the ovaries. Two hormones are involved with this growth: the follicle stimulating hormone (FSH) and the luteinizing hormone (LH). Both FSH and LH are released by the anterior pituitary gland at the direction of the hypothalamus, like most other hormonal functions of the body. In the first couple of days several follicles within the ovary are still quite small in their primary follicle state. Meanwhile in the uterus, menstruation occurs from about day 1 to day 5 as the woman experiences her period and the release of mucous and of the unfertilized egg from the last cycle. But, back to the ovary.

LH and FSH hormones trigger the follicle's growth in size, as fluid is retained within it, until it becomes a secondary follicle, and then a tertiary follicle. By this time, the follicle itself begins to release the hormone estradiol, which triggers an increase it uterine lining and works in cohort with several other reproductive hormones, as we will see. When the follicle is fully mature and ready to release the egg, it is considered to be a Graafian follicle, and has reached peak estradiol levels. This occurs around day 12.5, which is also when the FSH and LH hormones spike. At this time, there is also a spike in body temperature, which will remain elevated throughout the remainder of the cycle. On day 14, the egg is released and ovulation has occurred!

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