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Female hormones

The control of reproduction in females is more complex that in males. The female reproductive cycle is divided into the ovarian cycle and the menstrual cycle. The ovarian cycle    governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle    governs the preparation and maintenance of the uterine lining ( [link] ). These cycles are coordinated over a 22–32 day cycle, with an average length of 28 days.

As with the male, the GnRH from the hypothalamus causes the release of the hormones FSH and LH from the anterior pituitary. In addition, estrogen    and relatively small amounts of progesterone    are released from the developing follicles. As with testosterone in males, estrogen is responsible for the secondary sexual characteristics of females. These include breast development, flaring of the hips, and a shorter period for bone growth.

The ovarian cycle and the menstrual cycle

The ovarian and menstrual cycles are regulated by hormones of the hypothalamus, pituitary, and ovaries ( [link] ). The ebb and flow of the hormones causes the ovarian and menstrual cycles to advance. The ovarian and menstrual cycles occur concurrently. The first half of the ovarian cycle is the follicular phase. Slowly rising levels of FSH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation. As the follicles grow, they begin releasing estrogen. The first few days of this cycle coincide with menstruation or the sloughing off of the functional layer of the endometrium in the uterus. After about five days, estrogen levels rise and the menstrual cycle enters the proliferative phase. The endometrium begins to regrow, replacing the blood vessels and glands that deteriorated during the end of the last cycle.

Art connection

Hormone levels during the follicular phase, ovulation, and the luteal phase are compared. During the follicular phase, LH and FSH secreted from the pituitary stimulate several follicles to grow. The follicles produce low levels of estrogen that inhibit GnRH secretion by the hypothalamus, keeping LH and FSH levels low. Low levels of estrogen also cause the endometrial arteries to constrict, resulting in menstruation. During the time leading up to ovulation, LH and FSH stimulate maturation of one of the follicles. The growing follicle begins to produce high levels of estrogen, which stimulates GnRH secretion by the hypothalamus. As a result, LH and FSH levels rise, resulting in ovulation about a day later. Estrogen also causes the endometrium to thicken. After ovulation, the ovarian cycle enters the luteal phase. LH from the pituitary stimulates growth of the corpus luteum from the ruptured follicle. The corpus luteum secretes estrogen and progesterone that block GnRH production by the hypothalamus and LH and FSH production by the pituitary. Estrogen and progesterone also cause the endometrium to further develop.
The ovarian and menstrual cycles of female reproduction are regulated by hormones produced by the hypothalamus, pituitary, and ovaries.

Just prior to the middle of the cycle (approximately day 14), the high level of estrogen causes FSH and especially LH to rise rapidly then fall. The spike in LH causes the most mature follicle to rupture and release its egg. This is ovulation    . The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.

Following ovulation, the ovarian cycle enters its luteal phase and the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28. The luteal and secretory phases refer to changes in the ruptured follicle. The cells in the follicle undergo physical changes and produce a structure called a corpus luteum. The corpus luteum produces estrogen and larger amounts of progesterone. The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus is being prepared to accept a fertilized egg, should it occur during this cycle. The inhibition of FSH and LH prevents any further eggs and follicles from developing, while the progesterone is elevated. The level of estrogen produced by the corpus luteum increases to a steady level for the next few days.

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Source:  OpenStax, Human biology. OpenStax CNX. Dec 01, 2015 Download for free at http://legacy.cnx.org/content/col11903/1.3
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