The Ovarian Cycle and the Menstrual Cycle

Hormone levels during the follicular phase, ovulation, and the luteal phase are compared. During the follicular phase, L H and F S H secreted from the pituitary stimulate several follicles to grow. The follicles produce low levels of estradiol that inhibit G n R H secretion by the hypothalamus, keeping L H and F S H levels low. Low levels of estradiol also cause the endometrial arteries to constrict, resulting in menstruation. During the time leading up to ovulation, L H and F S H stimulate maturation of one of the follicles. The growing follicle begins to produce high levels of estradiol, which stimulates G n R H secretion by the hypothalamus. As a result, L H and F S H levels rise, resulting in ovulation about a day later. Estradiol also causes the endometrium to thicken. After ovulation, the ovarian cycle enters the luteal phase. L H from the pituitary stimulates growth of the corpus luteum from the ruptured follicle. The corpus luteum secretes estradiol and progesterone that block G n R H production by the hypothalamus and L H and F S H production by the pituitary. Estradiol 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. Source: OpenStax Biology 2e

OpenStax Biology 2e

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. These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days.

The first half of the ovarian cycle is the follicular phase. Slowly rising levels of FSH and LH 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 estrogens and a low level of progesterone. Progesterone maintains the endometrium to help ensure pregnancy. The trip through the fallopian tube takes about seven days. At this stage of development, called the morula, there are 30-60 cells. If pregnancy implantation does not occur, the lining is sloughed off. 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.

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 ovulation, the most mature follicle, ruptures and releases its egg. The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.

 Micrograph shows a spherical egg growing on the surface of a tissue.
This mature egg follicle may rupture and release an egg. (credit: scale-bar data from Matt Russell)

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 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.

If no fertilized egg is implanted into the uterus, the corpus luteum degenerates and the levels of estrogen and progesterone decrease. The endometrium begins to degenerate as the progesterone levels drop, initiating the next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again. 

The menstrual cycle encompasses both an ovarian cycle and a uterine cycle. The uterine cycle is divided into menstrual flow, the proliferative phase and the secretory phase. The ovarian cycle is separated into follicular and luteal phases. At day zero the uterine cycle enters the menstrual phase and the ovarian cycle enters the follicular phase. Menstruation begins, and the follicle inside the uterus begins to grow. The level of the pituitary hormone F S H rises slightly, while L H levels remain low. The levels of ovarian hormones estradiol and progesterone remain low. After menses the uterine cycle enters the proliferative phase and the follicle continues to grow. The level of the ovarian hormone estradiol begins to rapidly rise. Toward the end of the proliferative phase, levels of the pituitary hormones F S H and L H rise as well. Around day fourteen, just after the levels of estrogen, progesterone and estradiol reach their peak, ovulation occurs. The follicle ruptures, releasing the oocyte. The ovarian cycle enters the luteal phase. The follicle grows into a corpus luteum and then degenerates. The uterus enters the secretory phase. Progesterone levels increase and estradiol levels, which had dropped after ovulation, increase as well. Toward the end of the secretory phase estrogen and progesterone levels decrease, reaching their baseline levels around day 28. At this point menstruation begins.
Rising and falling hormone levels result in progression of the ovarian and menstrual cycles. (credit: modification of work by Mikael Häggström)

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e


Advertisements
Advertisements
Advertisements
Advertisements


0 0 vote
Article Rating
Subscribe
Notify of
guest
0 Comments
Inline Feedbacks
View all comments