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  • 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 minorathe 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-eggalong 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 videobig.” 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.

Maybe you've noticed that every time we talk about a new system, we highlight its

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Reproductive System, Part 1 - Female Reproductive System: Crash Course A&P #40

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    JillHuang posted on 2021/04/12
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