The female reproductive system includes all of internal and external organs that help
with reproduction.
The internal sex organs are the ovaries, which are the female gonads, the fallopian tubes,
two muscular tubes that connect the ovaries to the uterus, and the uterus, which is the
strong muscular sack that a fetus can develop in.
The neck of the uterus is called the cervix, and it protrudes into the vagina.
At the opening of the vagina are the external sex organs, and these are usually just called
the genitals and they're in the vulva region.
They include the labia, the clitoris, and the mons pubis.
The ovaries are a pair of white-ish organs about the size of walnuts.
They're held in place, slightly above and on either side of the uterus and fallopian
tubes by ligaments.
Specifically, there's the broad ligament, the ovarian ligament, and the suspensory ligament.
And the suspensory ligament is particularly important because the ovarian artery, ovarian
vein, and ovarian nerve plexus pass through it to reach the ovary.
If you slice the ovary open and look at it (don't try this at home) there's an outer
layer called the cortex, which has ovarian follicles scattered throughout it, and an
inner layer called the medulla, which contains most of the blood vessels and nerves.
At birth, the ovarian cortex has around two million follicles - that's roughly the population
of Paris - and they're called primordial follicles.
Each primordial follicle has a single immature sex cell called the primary oocyte at the
core, and a layer of follicular cells surrounds this.
The primary oocyte has 46 chromosomes, but eventually it has to turn into a gamete with
only 23 chromosomes.
To do this, the primary oocytes have to complete meiosis 1, and in a person's lifetime only
about 400 successfully do that.
This process of oocyte development follows that of follicular development, which can
be broken into three stages.
The first stage lasts from infancy to puberty, and during this stage the primary oocyte remains
stuck in the prophase step of meiosis 1.
So, in other words, the cell is living, but not dividing.
Meanwhile, the primordial follicle turns into a primary follicle, meaning that the follicular
cells that surrounding the primary oocyte develop into granulosa cells.
The second stage of follicular development begins for a few lucky primary follicles with
the first menstrual cycle in puberty, and a few more primary follicles go into the second
stage with each subsequent menstrual cycle.
In the second stage, the primary follicles develop into secondary and eventually tertiary,
or graafian follicles.
In a secondary follicle, the primary oocyte is still in the prophase step of meiosis 1,
but now the follicle has additional layers of granulosa cells, as well as theca cells.
Theca cells make androstenedione, a sex hormone precursor, and granulosa cells use the enzyme
aromatase to convert it into estradiol - a member of the estrogen family.
In a graafian follicle, a central cavity called the antrum forms within the follicle, and
the granulosa cells secrete a nourishing fluid for the primary oocyte directly into that
antrum.
The second stage takes roughly 70 to 85 days and results in a few fast-growing graafian
follicles.
The third stage of follicular development starts when the graafian follicles are ready
and occurs during the follicular phase of the menstrual cycle.
So let's briefly switch gears and go over the highlights of the menstrual cycle to put
that follicular phase into context.
The menstrual cycle starts on the first day of menstrual bleeding, lasts 28 days on average
from then.
Assuming a 28-day cycle, the follicular phase makes up the first two weeks of the menstrual
cycle, and the luteal phase, the last two weeks.
These two phases are separated by ovulation, which is when the follicle ruptures and releases
an oocyte that is ready to be fertilized.
This usually occurs on day 14 of a 28 day cycle.
The menstrual cycle is ultimately controlled by the hypothalamus, which is at the base
of the brain.
Before puberty, the hypothalamus constantly secretes small amounts of a hormone called
gonadotropin-releasing hormone, or GnRH.
That GnRH travels to the nearby pituitary, which secretes two hormones of its own - follicle
stimulating hormone, or FSH, and luteinizing hormone, or LH.
Once puberty hits, the hypothalamus starts to secrete GnRH in pulses, sometimes more
and sometimes less, and pituitary FSH and LH make the ovarian follicles develop.
The amount of GnRH can be mapped out like a wave over time, and the frequency and amplitude
of the waves of GnRH determine how much FSH and LH get produced by the pituitary.
LH binds to LH receptors on theca cells and they make progesterone and androstenedione.
FSH binds to FSH receptors on granulosa cells and they make aromatase and, as a consequence,
estrogen.
Serum levels of estrogen and progesterone act as feedback for the command center in
the brain, which adjusts its hormone production according to the phases of the menstrual cycle.
During the follicular phase of each menstrual cycle, the few fast-growing graafian follicles
enter the third stage of development.
Pituitary FSH makes the follicles grow and the granulosa cells produce more estrogen.
In addition to estrogen, the granulosa cells also secrete a hormone called activin, which
stimulates FSH production, as well as binding to FSH receptors, and the activity of granulosa
cell aromatase as well.
So early in the follicular phase, a small rise in FSH, leads to a large increase in
estrogen.
However, estrogen acts a negative feedback signal – that is, it tells the pituitary
to secrete less FSH and LH.
Less FSH means that there is only enough left to stimulate one follicle.
The follicle that has the most FSH receptors hoards most of this hormone, and becomes the
dominant follicle.
It usually takes about a week for a dominant follicle to get selected, and after that happens,
the rest of the follicles regress and die off.
The dominant follicle keeps secreting estrogen for the rest of the follicular phase.
The steady increase in estrogen makes the pituitary more responsive to the pulsatile
action of hypothalamic GnRH.
When blood estrogen levels reach 200 picograms/milliliter, dominant follicle estrogen becomes a positive
feedback signal – that is, it makes the pituitary secrete a whole lot of FSH and LH
in response to GnRH.
This triggers the primary oocyte within the dominant follicle to finally complete meiosis
1, and turn into a secondary oocyte, which has 23 chromosomes.
The dominant follicle completes its third stage of development in a blaze of glory called
ovulation.
That's when the nearly 2 centimeter sized follicle ruptures and releases the tiny secondary
oocyte into the fallopian tube.
The secondary oocyte stops in metaphase of meiosis 2, and waits for fertilization as
the menstrual cycle transitions into the luteal phase.
The luteal phase makes up the second half of the menstrual cycle - week 3 and week 4,
of the 4-week cycle.
Right after ovulation, while the LH levels are still high, the remains of the follicle
turn into the corpus luteum, which is made up of luteinized granulosa and theca cells.
Luteinized granulosa cells secrete inhibin, which inhibits the pituitary gland from making
FSH.
Without FSH, estrogen levels fall, and the amount of LH goes back to the level before
ovulation.
Luteinized theca cells respond to the low LH concentrations after ovulation by producing
more progesterone.
So, overall, this means that progesterone surpasses estrogen as the dominant hormone
during the luteal phase of the menstrual cycle.If fertilization occurs - meaning if a sperm
meets the secondary oocyte - then the corpus luteum continues making progesterone until
the placenta forms.
If fertilization doesn't happen, then the corpus luteum stops making hormones after
around 10 days, becomes fibrotic, and is called the corpus albicans.
After ovulation, the secondary oocyte makes a very quick journey through the peritoneal
space and lands in the fallopian tube.
The first part is the fimbriae which are the finger-like projections that surround the
ovary and guide the secondary oocyte into the fallopian tube.
Next is the infundibulum where fertilization happens between the secondary oocyte and the
sperm - this is the magical spot where they meet.
Then there's the ampullar region, which curves around the ovary, and finally the isthmus
region, which opens into the uterine cavity.
On the outside, the fallopian tubes are covered by peritoneum, and supported by the mesosalpinx,
which is part of the broad ligament.
On the inside, the fallopian tubes have smooth muscle with an inner lining that has ciliated
cells that slowly sweep the secondary oocyte or zygote towards the uterus.
The uterus is a hollow organ that sits behind the urinary bladder and in front of the rectum.
The top of the uterus above the openings of the fallopian tubes is called the fundus,
and the region below the openings is called the uterine body.
The uterus tapers down into the uterine isthmus and finally the cervix, which protrudes into
the vagina.
The cervix has a superior opening up top, and an inferior opening down below, both of
which have mucus plugs to keep the uterus closed off except during menstruation and
right before ovulation to allow sperm to reach the secondary oocyte.
The uterus is anchored to the sacrum by utero-sacral ligaments, to the anterior body wall by round
ligaments, and it's supported laterally by cardinal ligaments as well as the mesometrium,
which is part of the broad ligament.
The wall of the uterus has three layers: the perimetrium, which is a layer continuous with
the lining of the peritoneal cavity, the myometrium, which is made of smooth muscle that contracts
during childbirth to help push the baby out, and the endometrium, a mucosal layer, that
undergoes monthly cyclic changes.
During the follicular phase of the menstrual cycle, the endometrium thickens in case fertilization
occurs, during the luteal phase, spiral arteries emerge to bring more nutrients to support
the thick endometrium.
If fertilization doesn't occur, the spiral arteries collapse, and the superficial layers
of the endometrium die.
During menstruation or menstrual bleeding, that dead tissue is removed or sloughed off
of the uterus through the vagina.
The vagina has a muscular wall and is covered by an inner mucosa with ridges that run along
it.
The vagina is the passageway for the baby during childbirth, and it opens up into the
vulva.
In childhood, a thin sheet of vaginal mucosa called the hymen partially covers the vaginal
opening, and it can break because of exercise, the use of tampons, or sexual intercourse.
The external sex organs, together referred to as the vulva, are the labia majora which
are called "the two greater lips", labia minora, or "the two smaller lips", the
mons pubis, or "the mountain of the pubis", and the clitoris, a small erectile organ that
develops from the same embryonic tissue as the male penis.
It is hooded by a skin fold called the clitoral hood.
Both the labia majora and the mons pubis become covered in pubic hair during puberty.
The labia majora cover the labia minora, and between the two labia minora there is a space
called the vulvar vestibule that includes, the opening of the vagina and the the urethral
opening.
All right, so as a quick recap: the female reproductive system comprises of internal
sex organs, such as the ovaries, the fallopian tubes, the uterus and the vagina, as well
as external sex organs such as the labia, the mons pubis and the clitoris.
The ovaries are the female gonads, and they produce the ova, as well as the female sex
hormones.
Both the ovaries and the uterus are subject to the pulsatile hormonal activity of the
hypothalamus and pituitary glands.The uterus is where pregnancy develops, and the baby
comes into the world through the birth canal, or the vagina, that connects the internal
and external sex organs.
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