13.60: Menstrual Cycle

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What's the most important part of the female menstrual cycle?

A menstrual cycle calendar. A lot of things to keep track of. And for a few very important reasons, it is important to know when a woman is ovulating. But what's the most important part of the female menstrual cycle? That depends on who you ask.

Menstrual Cycle

Ovulation, the release of an egg from an ovary, is part of the menstrual cycle, which typically occurs each month in a sexually mature female unless she is pregnant. Another part of the cycle is the monthly period, or menstruation. Menstruation is the process in which the endometrium of the uterus is shed from the body. The menstrual cycle is controlled by hormones from the hypothalamus, pituitary gland, and ovaries.

Phases of the Menstrual Cycle

As shown in Figure below, the menstrual cycle occurs in several phases. The cycle begins with menstruation. During menstruation, arteries that supply the endometrium of the uterus constrict. As a result, the endometrium breaks down and detaches from the uterus. It passes out of the body through the vagina over a period of several days.

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After menstruation, the endometrium begins to build up again. At the same time, a follicle starts maturing in an ovary. Ovulation occurs around day 14 of the cycle. After it occurs, the endometrium continues to build up in preparation for a fertilized egg. What happens next depends on whether the egg is fertilized.

If the egg is fertilized, the endometrium will be maintained and help nourish the egg. The ruptured follicle, now called the corpus luteum, will secrete the hormone progesterone. This hormone keeps the endometrium from breaking down. If the egg is not fertilized, the corpus luteum will break down and disappear. Without progesterone, the endometrium will also break down and be shed. A new menstrual cycle thus begins.

Menopause

For most women, menstrual cycles continue until their mid- or late- forties. Then women go through menopause, a period during which their menstrual cycles slow down and eventually stop, generally by their early fifties. After menopause, women can no longer reproduce naturally because their ovaries no longer produce eggs.

Summary

The menstrual cycle includes events that take place in the ovary, such as ovulation.
The menstrual cycle also includes changes in the uterus, including menstruation.
Menopause occurs when menstruation stops occurring, usually in middle adulthood.

Review

Define menstruation.
What is menopause? When does it occur?
What is the corpus luteum?
Compare and contrast what happens in the menstrual cycle when the egg is fertilized with what happens when the egg is not fertilized.
Make a cycle diagram to represent the main events of the menstrual cycle in both the ovaries and the uterus, including the days when they occur.

Image	Reference	Attributions
	[Figure 1]	Credit: Image copyright 9nong, 2013;Image copyright GRei, 2014 Source: http://www.shutterstock.com License: CC BY-NC
	[Figure 2]	Credit: Image copyright GRei, 2014 Source: http://www.shutterstock.com License: Used under license from Shutterstock.com