maybe you’ve noticed that every time wetalk about a new system, we highlight its importance by saying how you’d die withoutit. like, without your muscular and skeletal systemsyou’d collapse into an inert bag of goo. or how if we magically removed your respiratoryor circulatory system, you’d die in a couple of minutes ‘cause your cells wouldn’thave oxygen.
Free Continuing Education For Nurses, that’s because most of our bodies’ systemsare just trying to keep us alive, minute to minute. but one of those systems doesn’t really care ifyou 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 aboutinvesting 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, internalsex 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, externalgenitalia, and particular brain parts that help the gonads and gametes do what they need to do, whichbasically is mate, combine alleles, and make babies. now, all animals have their own particularand fascinating anatomical methods for getting their gametes together, and we could do awhole course just on that, and never run out
of material, and let me tell you, i wouldlike to do that. but while we may seem kinda tame comparedto animals that turn bright red, bite off penises, or starve themselves for a chanceto breed, our systems are still plenty complex. in fact, it’s gonna take most of the monthto 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 talkingabout spreading our alleles around. but when
we visualize sex, what most of what we pictureare our anatomies. who has what. and what goes where. in an anatomical female, that involves thevulva, which includes the mons pubis over the pubic bone, and labia majora and labiaminora — the elongated skin folds that surround the vestibule, which contains both the urethraland vaginal openings. beyond that is the vagina, which i’m sureyou 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 focuson the bathing-suit parts, those are only the genitalia -- the external sexual organs.
and they’re really just a means of gettinggametes together. reproductively speaking, they’re the least important parts of thesystem. the ground control of the female reproductiveanatomy -- the place where the orders are given -- are of course the ovaries. their main job is to produce and release female gametes and sexhormones like estrogen and progesterone. you’ll remember from biology that gametesare haploid cells, meaning that they only have one set of chromosomes, and are formedby meiosis. when a sperm fuses with an egg, they makea diploid cell, which has all the genetic instructions required to make a baby. andpretty much everything about how our reproductive
systems work is designed to make that happen. each ovary lives inside a fibrous sac thatconsists of a layer of connective tissue called the tunica albuginea, and another layer ofcuboidal epithelial cells called the germinal epithelium, which is actually part of theperitoneum that lines the abdominal cavity. the ovary itself contains a cortex that housesdeveloping eggs, and a medulla that contains most of the ovary’s blood vessels and nerves. but the business of passing on alleles andsaving humanity really begins in the basic reproductive units in the cortex -- the ovarianfollicles. these are tiny-sac-like structures that eachhold a single primary oocyte — a sort of
incomplete proto-egg — along with a bunchof supporting follicle cells around it. females are born with essentially all of theseearly versions of eggs in all of the primordial follicles they will ever have -- around 1million 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, sometimesforever. 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 itdoes for some other animals. like, if you’re a salmon or a mayfly, then all of your eggswill mature at once, and then you’ll mate,
reproduce, and usually die, in quick succession. i mean, people talk about living fast anddying 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 makeway for a new egg. this should all sound familiar if you wereborn with female anatomy, or know anyone who was, because it’s a big part of the well-knownmonthly menstrual cycle. but the truth is, menstruation is only onepart of one cycle. the menstrual cycle is what happens in theuterus to prepare for a fertilized egg. the other cycle, the ovarian cycle, is all aboutthe maturation of the follicle and egg, and
it’s actually what drives the menstrualcycle. every day, even before birth, a bunch of follicleswill begin a process of maturation, very slowly morphing from primordial follicles into what’sknown as late-tertiary follicles, which are the ones that will support a fully developedegg. this process takes 375 days. but out of that bunch of follicles -- usuallyabout 20 or so -- only one follicle will end up supporting a single, mature egg. the restwon’t get the hormonal boost they need to bring the egg to completion. this is whathappens 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, whichyou’ve probably been through yourself, it should come as no surprise that all of thisactivity is regulated by sex hormones. starting around puberty, the hypothalamusand pituitary set up two concurrent cycles -- the ovarian cycle in the ovaries, whichripens eggs and secretes sex hormones, and the menstrual, or the uterine cycle, whichprepares the uterus to capture and nourish any mature, fertilized eggs. when puberty begins the hypothalamus starts up theovarian cycle by secreting gonadotropin-releasing hormone about once a month. this is a sexhormone that stimulates the anterior pituitary to release two more hormones: follicle-stimulatinghormone -- you’ll often hear it called fsh
-- and luteinizing hormone, or lh. the follicle-stimulating hormone lives upto its name by stimulating the growth of a follicle -- but only one: the one that happensto be furthest along in development at the time. the fsh drives that one lucky follicle tokeep growing, by triggering the follicle itself to secrete its own estrogen hormones, whichlocally signal the follicle to mature even more. that surge of follicle-secreted estrogen thenends up stimulating the pituitary to secrete another pulse of luteinizing hormone to finishthe job. the lh gets to work on the oocyte that’sbeen dormant inside the follicle, and triggers it to finally start dividing again -- getting it tocomplete meiosis i and move on to metaphase ii.
this whole process takes about 14 days, atthe end of which, the follicle -- which is now mature -- pushes up against the ovarywall, ruptures, and, with the help of enzymes, breaches the wall and ejects a single, nowmature, oocyte. congratulations. you’ve just ovulated. the damaged follicle now slows its estrogenproduction while morphing into a different structure, called the corpus luteum, whicheventually degenerates. but first it releases a final hormonal swansong -- a bunch of progesterone, a little estrogen, and some inhibin -- that togetherstop the release of fsh and lh. they also prepare the uterus to receive theoocyte, 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, andinterestingly, they aren’t actually connected to the ovaries. this means that when the eggpushes through ovary, it has to float a short way through the peritoneal cavity before it’scaught by a fallopian tube. now, only if and when an egg fuses with asperm does it actually complete meiosis ii and officially become an ovum. but, whether it’s fertilized or not, theegg works its way down the tube until it enters the uterus, a hollow, thick-walled, and verystretchable 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 threelayers: the perimetrium on the outside; the bulky, smooth muscle myometrium that contractsduring 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 newembryo snuggles into the endometrium for gestation -- but the uterus is only receptive to implantationfor 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 28days or so, in response to changing hormone levels, and in coordination with the ovariancycle.
the shedding of the functional layer is triggeredwhen the progesterone and estrogens that were being produced by the corpus luteum startto drop, about 10 days after ovulation. this phase lasts about 5 days. meanwhile, the fsh and lh released from theanterior pituitary start to rise again, stimulating the next round of follicles, which begin tomake estrogen. this heralds the start of phase two of themenstrual cycle, the proliferative, or pre-ovulatory phase, which typically lasts from days 6-14of the cycle. the rising estrogen levels in the folliclesstimulate the regeneration of the endometrium, building a cushy, well-vascularized habitatfor another potential fertilized egg to call home.
and after the next egg is released, the finalsecretory, or postovulatory phase begins. this is when the ruptured follicle forms inthe 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 nicesperm and gotten fertilized, then the pulse of progesterone from the corpus triggers evenmore thickening of the functional layer of the endometrium, and a secretion of nutrientsthat 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.â€so that’s where we’re going next time! but for now, you learned all about femalereproductive anatomy, how sex hormones affect oogenesis and ovulation, and how the ovarianand menstrual cycles mature and release oocytes, and create a comfy uterine environment fora fertilized egg. thank you to our headmaster of learning, linneaboyev, and thank you to all of our patreon patrons whose monthly contributions help makecrash course exist not only for themselves, but for everyone, everywhere. if you likecrash 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 inthe doctor cheryl c. kinney crash course studio.
it was written by kathleen yale, edited byblake de pastino, and our consultant is dr. brandon jackson. it was directed by nicholasjenkins, edited by nicole sweeney, our sound designer is michael aranda, and the graphicsteam is thought cafe.
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