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Human reproduction is complicated an important, and it's going to take a four part series for us to cover it. Today, we're kicking that off with the female reproductive system, starting with how sex hormones affect oogenesis and ovulation, continuing through how the ovarian and menstrual cycles mature and release oocytes, and create a comfy uterine environment for a fertilized egg.

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Table of Contents
Female Reproductive Anatomy 1:58
Sex Hormones Affect Oogenesis and Ovulation 3:15
Ovarian and Menstrual Cycles Mature and Release Oocytes 4:05
Uterine Environments and Fertilized Eggs 7:10

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Hank: 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.

[CrashCourse A & P intro plays]

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 post-ovulatory 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.