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  • The menstrual cycle refers to the regular changes in the activity of the ovaries and

  • the endometrium that make reproduction possible.

  • The endometrium is the layer of tissue lining the inside of the uterus.

  • This lining consists of a functional layer, which is subject to hormonal changes and is

  • shed during menstruation, and a thin basal layer which feeds the overlying functional

  • layer.

  • The menstrual cycle actually consists of two interconnected and synchronized processes:

  • the ovarian cycle, which centers on the development of the ovarian follicles and ovulation, and

  • the uterine or endometrial cycle, which centers on the way in which the functional endometrium

  • thickens and sheds in response to ovarian activity.

  • Menarche, which refers to the onset of the first menstrual period, usually occurs during

  • early adolescence as part of puberty.

  • Following menarche, the menstrual cycle recurs on a monthly basis, pausing only during pregnancy,

  • until a person reaches menopause, when her ovarian function declines and she stops having

  • menstrual periods.

  • The monthly menstrual cycle can vary in duration from 20 to 35 days, with an average of 28

  • days.

  • Each menstrual cycle begins on the first day of menstruation, and this is referred to as

  • day one of the cycle.

  • Ovulation, or the release of the oocyte from the ovary, usually occurs 14 days before the

  • first day of menstruation (i.e., 14 days before the next cycle begins).

  • So, for an average 28-day menstrual cycle, this means that there are usually 14 days

  • leading up to ovulation (i.e., the preovulatory phase) and 14 days following ovulation (i.e.,

  • the postovulatory phase).

  • During these two phases, the ovaries and the endometrium each undergo their own set of

  • changes, which are separate but related.

  • As a result, each phase of the menstrual cycle has two different names to describe these

  • two different parallel processes.

  • For the ovary, the two weeks leading up to ovulation is called the the ovarian follicular

  • phase, and this corresponds to the menstrual and proliferative phases of the endometrium.

  • Similarly, the two weeks following ovulation is referred to as the ovarian luteal phase,

  • which also corresponds to the secretory phase of the endometrium.

  • So, let's first focus on the preovulatory period, starting with the ovarian follicular

  • phase.

  • This phase starts on the first day of menstruation and represents weeks one and two of a four-week

  • cycle.

  • The whole menstrual cycle is controlled by the hypothalamus and the pituitary gland,

  • which are like the masterminds of reproduction.

  • The hypothalamus is a part of the brain that secretes gonadotropin-releasing hormone, or

  • GnRH, which causes the nearby anterior pituitary gland to release follicle stimulating hormone,

  • or FSH, and luteinizing hormone, or LH.

  • Before puberty, gonadotropin-releasing hormone is released at a steady rate, but once puberty

  • hits, gonadotropin-releasing hormone is released in pulses, sometimes more and sometimes less.

  • The frequency and magnitude of the gonadotropin-releasing hormone pulses determine how much follicle

  • stimulating hormone and luteinizing hormone will be produced by the pituitary.

  • These pituitary hormones control the maturation of the ovarian follicles, each of which is

  • initially made up of an immature sex cell, or primary oocyte, surrounded by layers of

  • theca and granulosa cells, the hormone-secreting cells of the ovary.

  • Over the course of the follicular phase, these oocyte-containing groups of cells, or follicles,

  • grow and compete for a chance at ovulation.

  • During the first ten days, theca cells develop receptors and bind luteinizing hormone, and

  • in response secrete large amounts of the hormone androstenedione, an androgen hormone.

  • Similarly, granulosa cells develop receptors and bind follicle stimulating hormone, and

  • in response produce the enzyme aromatase.

  • Aromatase converts androstenedione from the theca cells into 17β-estradiol, which is

  • a member of the estrogen family.

  • During days 10 through 14 of this phase, granulosa cells also begin to develop luteinizing hormone

  • receptors, in addition to the follicle stimulating hormone receptors they already have.

  • As the follicles grow and estrogen is released into the bloodstream, increased estrogen levels

  • act as a negative feedback signal, telling the pituitary to secrete less follicle stimulating

  • hormone.

  • As a result of decreased follicle stimulating hormone production, some of the developing

  • follicles in the ovary will stop growing, regress and die off.

  • The follicle that has the most follicle stimulating hormone receptors, however, will continue

  • to grow, becoming the dominant follicle that will eventually undergo ovulation.

  • This dominant follicle continues to secrete estrogen, and the rising estrogen levels make

  • the pituitary more responsive to the pulsatile action of gonadotropin-releasing hormone from

  • the hypothalamus.

  • As blood estrogen levels start to steadily climb higher and higher, the estrogen from

  • the dominant follicle now becomes a positive feedback signalthat is, it makes the

  • pituitary secrete a whole lot of follicle stimulating hormone and luteinizing hormone

  • in response to gonadotropin-releasing hormone.

  • This surge of follicle stimulating hormone and luteinizing hormone usually happens a

  • day or two before ovulation and is responsible for stimulating the rupture of the ovarian

  • follicle and the release of the oocyte.

  • You can think of it this way: for most of the follicular phase, the pituitary saves

  • its energy, then when it senses that the dominant follicle ready for release, the pituitary

  • uses all its energy to secrete enough follicle stimulating hormone and luteinizing hormone

  • to induce ovulation.

  • While the ovary is busy preparing an egg for ovulation, the uterus, meanwhile, is preparing

  • the endometrium for implantation and maintenance of pregnancy.

  • This process begins with the menstrual phase, which is when the old endometrial lining,

  • or functional layer, from the previous cycle is shed and eliminated through the vagina,

  • producing the bleeding pattern known as the menstrual period.

  • The menstrual phase lasts an average of five days and is followed by the proliferative

  • phase, during which high estrogen levels stimulate thickening of the endometrium, growth of endometrial

  • glands, and emergence of spiral arteries from the basal layer to feed the growing functional

  • endometrium.

  • Rising estrogen levels also help change the consistency of the cervical mucus, making

  • it more hospitable to incoming sperm.

  • The combined effects of this spike in estrogen on the uterus and cervix help to optimize

  • the chance of fertilization, which is highest between day 11 and day 15 of an average 28-day

  • cycle.

  • Following ovulation, the remnant of the ovarian follicle becomes the corpus luteum, which

  • is made up of luteinized theca and granulosa cells, meaning that these cells have been

  • exposed to the high luteinizing hormone levels that occur just before ovulation.

  • Luteinized theca cells keep secreting androstenedione, and the luteinized granulosa cells keep converting

  • it to 17β-estradiol, as before.

  • However, luteinized granulosa cells also respond to the low luteinizing hormone concentrations

  • that are present after ovulation by increasing the activity of cholesterol side-chain cleavage

  • enzyme, or P450scc for short.

  • This enzyme converts more cholesterol to pregnenolone, a progesterone precursor.

  • So luteinized granulosa cells secrete more progesterone than estrogen during the luteal

  • phase.

  • Progesterone acts as a negative feedback signal on the pituitary, decreasing release of follicle

  • stimulating hormone and luteinizing hormone.

  • At the same time, luteinized granulosa cells begin secreting inhibin, which similarly inhibits

  • the pituitary gland from making follicle stimulating hormone.

  • Both of these processes result in a decline in estrogen levels, meaning that progesterone

  • becomes the dominant hormone present during this phase of the cycle.

  • Together with the decreased level of estrogen, the rising progesterone level signals that

  • ovulation has occurred and helps make the endometrium receptive to the implantation

  • of a fertilized gamete.

  • Under the influence of progesterone, the uterus enters into the secretory phase of the endometrial

  • cycle.

  • During this time spiral arteries continue to grow, and the uterine glands begin to secrete

  • more mucus.

  • After day 15 of the cycle, the optimal window for fertilization begins to close.

  • The cervical mucus starts to thicken and becomes less hospitable to the sperm.

  • Over time, the corpus luteum gradually degenerates into the nonfunctional corpus albicans.

  • The corpus albicans doesn't make hormones, so estrogen and progesterone levels slowly

  • decrease.

  • When progesterone reaches its lowest level, the spiral arteries collapse, and the functional

  • layer of the endometrium prepares to shed through menstruation.

  • This shedding marks the beginning of a new menstrual cycle and another opportunity for

  • fertilization.

  • All right, so as a quick recap - the menstrual cycle begins on the first day of menstruation.

  • For an average 28-day menstrual cycle, the changes which occur in the ovary during the

  • first 14 days are called the follicular phase.

  • Ovulation usually occurs at day 14, as a result of the estrogen-induced surge in luteinizing

  • hormone.

  • The last 14 days of the cycle are the luteal phase, during which progesterone becomes the

  • dominant hormone.

  • While the length of the follicular phase can vary, the luteal phase almost always precedes

  • the onset of menses by 14 days.

  • The uterus also goes through its own set of changes.

  • During the first 14 days of the cycle, the endometrium goes through the menstrual phase

  • and the proliferative phase, and during the last 14 days it goes through the secretory

  • phase.

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C1 US hormone follicle menstrual phase cycle estrogen

The menstrual cycle

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    JillHuang posted on 2021/04/12
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