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much more. Try it free today! 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 signal – that 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.
