Maybe you’ve noticed that every time we talk about a new system, we highlight its
importance by saying how you’d die without it.
Like, without your muscular and skeletal systems you’d collapse into an inert bag of goo.
Or how if we magically removed your respiratory or circulatory system, you’d die in a couple
of minutes ‘cause your cells wouldn’t have oxygen.
That’s because most of our bodies’ systems are just trying to keep us alive, minute to minute.
But one of those systems doesn’t really care if you live or die. At least, not until it’s done its job.
It’s how every living thing gets its start, but it doesn’t really kick in until puberty,
and even then it’s more concerned about investing in the future than keeping you alive.
When it comes to your reproductive system, it’s not concerned about you, so much as
it is about your alleles, your genetic code, and the future of the human species.
Which are no small stakes.
This system includes our primary, internal sex organs, the gonads -- like testes and
ovaries -- the various sex hormones they secrete, and the gametes -- the sperm and eggs -- they produce.
It also includes the glands, ducts, external genitalia, and particular brain parts that help the
gonads and gametes do what they need to do, which basically is mate, combine alleles, and make babies.
Now, all animals have their own particular and fascinating anatomical methods for getting
their gametes together, and we could do a whole course just on that, and never run out
of material, and let me tell you, I would like to do that.
But while we may seem kinda tame compared to animals that turn bright red, bite off
penises, or starve themselves for a chance to breed, our systems are still plenty complex.
In fact, it’s gonna take most of the month to get through all our various anatomical
parts and hormones, and explain how sexy time, fertilization, pregnancy, and development
work, starting today with the female anatomy.
And remember, this is nothing to be shy about -- when we’re talking about sex, we’re
talking about the future of humanity.
So, when we talk about sex, we’re talking about spreading our alleles around. But when
we visualize sex, what most of what we picture are our anatomies.
Who has what. And what goes where.
In an anatomical female, that involves the vulva, which includes the mons pubis over
the pubic bone, and labia majora and labia minora — the elongated skin folds that surround
the vestibule, which contains both the urethral and vaginal openings.
Beyond that is the vagina, which I’m sure you know is how menstrual blood and babies
leave the body, and how sperm gets in.
But, as much as we tend to put all the focus on the bathing-suit parts, those are only
the genitalia -- the external sexual organs.
And they’re really just a means of getting gametes together. Reproductively speaking,
they’re the least important parts of the system.
The ground control of the female reproductive anatomy -- the place where the orders are
given -- are of course the ovaries. Their main job is
to produce and release female gametes and sex hormones like estrogen and progesterone.
You’ll remember from biology that gametes are haploid cells, meaning that they only
have one set of chromosomes, and are formed by meiosis.
When a sperm fuses with an egg, they make a diploid cell, which has all the genetic
instructions required to make a baby. And pretty much everything about how our reproductive
systems work is designed to make that happen.
Each ovary lives inside a fibrous sac that consists of a layer of connective tissue called
the tunica albuginea, and another layer of cuboidal epithelial cells called the germinal
epithelium, which is actually part of the peritoneum that lines the abdominal cavity.
The ovary itself contains a cortex that houses developing eggs, and a medulla that contains
most of the ovary’s blood vessels and nerves.
But the business of passing on alleles and saving humanity really begins in the basic
reproductive units in the cortex -- the ovarian follicles.
These are tiny-sac-like structures that each hold a single primary oocyte — a sort of
incomplete proto-egg — along with a bunch of supporting follicle cells around it.
Females are born with essentially all of these early versions of eggs in all of the primordial
follicles they will ever have -- around 1 million at the time of birth.
But right around birth, the oocytes stop developing -- they get stuck in the first stage of meiosis.
And they stay that way for years, sometimes forever.
The actual process of egg creation, or oogenesis, is delayed until puberty, when the rest of
the body is physically ready to reproduce.
Now, this works differently for us than it does for some other animals. Like, if you’re
a salmon or a mayfly, then all of your eggs will mature at once, and then you’ll mate,
reproduce, and usually die, in quick succession.
I mean, people talk about living fast and dying young, but that -- that’s too fast.
So human eggs mature one-by-one, almost constantly, doled out so that every month or so, a mature
egg is either fertilized, or dies to make way for a new egg.
This should all sound familiar if you were born with female anatomy, or know anyone who
was, because it’s a big part of the well-known monthly menstrual cycle.
But the truth is, menstruation is only one part of one cycle.
The menstrual cycle is what happens in the uterus to prepare for a fertilized egg. The
other cycle, the ovarian cycle, is all about the maturation of the follicle and egg, and
it’s actually what drives the menstrual cycle.
Every day, even before birth, a bunch of follicles will begin a process of maturation, very slowly
morphing from primordial follicles into what’s known as late-tertiary follicles, which are
the ones that will support a fully developed egg.
This process takes 375 days.
But out of that bunch of follicles -- usually about 20 or so -- only one follicle will end
up supporting a single, mature egg. The rest won’t get the hormonal boost they need to
bring the egg to completion. This is what happens to the one that start maturing before
puberty, for example, so they undergo atresia, a kind of programmed self- destruction.
And because I keep mentioning puberty, which you’ve probably been through yourself, it
should come as no surprise that all of this activity is regulated by sex hormones.
Starting around puberty, the hypothalamus and pituitary set up two concurrent cycles
-- the ovarian cycle in the ovaries, which ripens eggs and secretes sex hormones, and
the menstrual, or the uterine cycle, which prepares the uterus to capture and nourish
any mature, fertilized eggs.
When puberty begins the hypothalamus starts up the ovarian cycle by secreting gonadotropin-releasing
hormone about once a month. This is a sex hormone that stimulates the anterior pituitary
to release two more hormones: follicle-stimulating hormone -- you’ll often hear it called FSH
-- and luteinizing hormone, or LH.
The follicle-stimulating hormone lives up to its name by stimulating the growth of a
follicle -- but only one: the one that happens to be furthest along in development at the time.
The FSH drives that one lucky follicle to keep growing, by triggering the follicle itself
to secrete its own estrogen hormones, which locally signal the follicle to mature even more.
That surge of follicle-secreted estrogen then ends up stimulating the pituitary to secrete
another pulse of luteinizing hormone to finish the job.
The LH gets to work on the oocyte that’s been dormant inside the follicle, and triggers
it to finally start dividing again -- getting it to complete meiosis I and move on to metaphase II.
This whole process takes about 14 days, at the end of which, the follicle -- which is
now mature -- pushes up against the ovary wall, ruptures, and, with the help of enzymes,
breaches the wall and ejects a single, now mature, oocyte.
Congratulations. You’ve just ovulated.
The damaged follicle now slows its estrogen production while morphing into a different
structure, called the corpus luteum, which eventually degenerates.
But first it releases a final hormonal swan song -- a bunch of progesterone, a little
estrogen, and some inhibin -- that together stop the release of FSH and LH.
They also prepare the uterus to receive the oocyte, which is now on its way down a fallopian
tube, where it might meet a nice young sperm.
The tubes are about 10 centimeters long, and interestingly, they aren’t actually connected
to the ovaries. This means that when the egg pushes through ovary, it has to float a short
way through the peritoneal cavity before it’s caught by a fallopian tube.
Now, only if and when an egg fuses with a sperm does it actually complete meiosis II
and officially become an ovum.
But, whether it’s fertilized or not, the egg works its way down the tube until it enters
the uterus, a hollow, thick-walled, and very stretchable muscular organ that sits anterior
to the rectum and posterosuperior to the bladder, and ends with the cervix.
And the uterine wall is composed of three layers: the perimetrium on the outside; the
bulky, smooth muscle myometrium that contracts during labor; and the inner mucosal lining,
the endometrium, which consists of a thin, deep basal layer, and an outer functional layer.
If fertilization does happen, then the new embryo snuggles into the endometrium for gestation
-- but the uterus is only receptive to implantation for a short time, about a week after ovulation.
If the egg isn’t fertilized, that outer, functional layer sloughs off.
And that’s the first phase of the uterine, or menstrual cycle -- the series of changes
that the endometrium goes through every 28 days or so, in response to changing hormone
levels, and in coordination with the ovarian cycle.
The shedding of the functional layer is triggered when the progesterone and estrogens that were
being produced by the corpus luteum start to drop, about 10 days after ovulation. This
phase lasts about 5 days.
Meanwhile, the FSH and LH released from the anterior pituitary start to rise again, stimulating
the next round of follicles, which begin to make estrogen.
This heralds the start of phase two of the menstrual cycle, the proliferative, or pre-ovulatory
phase, which typically lasts from days 6-14 of the cycle.
The rising estrogen levels in the follicles stimulate the regeneration of the endometrium,
building a cushy, well-vascularized habitat for another potential fertilized egg to call home.
And after the next egg is released, the final secretory, or postovulatory phase begins.
This is when the ruptured follicle forms in the corpus luteum. And if fertilization didn’t
happen, the corpus will stop producing progesterone, and the endometrium will start to shed its
functional layer. And it starts all over again.
BUT! If, by this time, the egg has met a nice sperm and gotten fertilized, then the pulse
of progesterone from the corpus triggers even more thickening of the functional layer of
the endometrium, and a secretion of nutrients that will tide an embryo over until it has
implanted itself in the blood-rich lining.
Which is a big if.
Like, its whole separate video “big.” So that’s where we’re going next time!
But for now, you learned all about female reproductive anatomy, how sex hormones affect
oogenesis and ovulation, and how the ovarian and menstrual cycles mature and release oocytes,
and create a comfy uterine environment for a fertilized egg.
Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon
Patrons whose monthly contributions help make Crash Course exist not only for themselves,
but for everyone, everywhere. If you like Crash Course and want to help us keep making
videos like this one, you can go to patreon.com/crashcourse
This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio.
It was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr.
Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound
designer is Michael Aranda, and the graphics team is Thought Cafe.