If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:45

Video transcript

we're going to talk about the ovarian cycle the ovaries are two structures and a female's reproductive system that produce her eggs each month her eggs go through a maturation process called the ovarian cycle and that cycle creates a secondary oocyte that can be then fertilized by a sperm to result in a pregnancy the ovarian cycle is also responsible for what we commonly know as the menstrual cycle basically the primary Oh sites that are destined to be ovulated will develop in the ovaries complete meiosis one just before ovulation and then they'll be ejected out of the ovary as a secondary oocyte to be picked up by the fimbria and swept into the uterine tube two to hope for fertilization so let's start from the beginning inside the ovaries eggs develop in structures called follicles these purple circles here and they start off as primordial follicles and so what a follicle is I'll just blow that up for you it's one primary oocyte so an egg cell surrounded by a layer of cells called granulosa cells and the granulosa cells develop and become more numerous as the follicle matures now the granulosa cells also secrete a few hormones estrogen a little progesterone and some inhibit and we'll talk about the functions of those a little bit later on so let's put a timeline on this now the ovarian cycle lasts 28 days this is day zero here at the primordial follicle or going counterclockwise all the way over here this is day 13 here where the secondary oocyte gets ejected or ovulated that's day 14 and then the rest of the time spent getting back to the primordial follicle stage are days 15 through 28 so now you have an idea of about how long this all takes so you remember when we said that the granulosa cells produce hormones well as the follicles develop over the first 13 days and you can see the changes between the one here and the one here it's got a lot more purple cells around here those are granulosa cells so the the number of granulosa cells goes up and since they produce hormones what do you think happens to the [ __ ] own levels in the blood they go up so that's sort of just a general point so keep that in mind but first we'll jump back to these we know these are primordial follicles here the next stage of development are these guys here and these are called primary follicles and in the primary follicles the layers of granulosa cells and the O site the egg start to be separated by this other layer that starts to form between them that's called the zona pellucida and I'll draw here in light blue and even though the egg I've drawn in blue there's still a layer of zona pellucida even though the egg is originally drawn in blue because I want to draw the egg in blue there's still a layer of zona pellucida around it now even though the zona pellucida is there separating the granulosa cells from the the actual egg the granulosa cells can still nourish the egg through gap junctions that go through the zona pellucida and into the egg gap junctions are just little passageways from one cell to another cell where they can exchange nutrients or other signals and actually through those gap junctions the granulosa cells send through little chemicals that keep those primary Oh sites stuck at that meiosis one stage because you remember at this point all of these primary Oh sites are stuck in my otic arrest they're not dividing and reducing their chromosome copy number so as we develop from our primordial to our primary to our next follicle here called our pre antral follicle and you'll see why it's called that in a minute the granulosa cells are actually starting to divide and become a lot greater in number you can you can see that there's a pretty big difference in granulosa cell from our primary follicles to our pre antral follicle here and remember the granulosa cells are in shaded in in purple here so while the granulosa cells are proliferating this wall on the outside of the follicle called the theca starts to form the Cassell's have receptors for luteinizing hormone from the anterior pituitary and when luteinizing hormone or LH binds these thicker cells they produce a hormone called andrew steamed ion and when the the-- cos get andrew steamed i own they give it to the granulosa cells who then convert it to estrogen and release it into the blood so the blood estrogen levels start to go up at this point and so that's what these red and blue bits running down the middle of the ovary are blood vessels arteries and veins and if they look a little bit weird to you or unusual that's just because they're cut in cross-section as well now you might be wondering what an antral refers to like what you see in the pre antral follicle and this early antral follicle here it actually refers to the antrum which will be formed in the next step this space here is called an Antrim and the Antrim is just basically fluid that's being produced by the granulosa cells and it's that Antrim and the fluid in the Antrim that pushes against the edges of the follicle and causes it to expand now just so you're aware during this ovarian cycle multiple follicles are actually forming it's not just this one pre antral follicle and then this one early antral follicle and this one mature follicle you're getting a lot of these happening at one time but only one of the biggest ones is the one that eventually gets ovulated because you only ovulate one egg every 28 days and that one that gets ovulated is called the dominant follicle so let's just say that what we're seeing here is an example of the dominant follicles development because the rest of the ones that were developing along this pathway sort of degenerate and die off in a process called atresia I'll write that at the bottom here and the tree just means to degenerate so another note in the ones that undergo at Riga both the follicle and the eggs they contain die off and that means that a woman loses anywhere between 15 to 25 eggs per menstrual cycle to atresia while only one gets ovulated so you can kind of imagine how you go from two to four million eggs when you were born to having zero after about thirty five ish years of ovulation it's not just that one egg you lose via ovulation you lose quite a few so anyway back to the development of the dominant follicle it enlarges mostly due to the expanding antrum as I mentioned earlier and granulosa cells actually start to form this bit of a mound here that protrudes into the middle of the entra this mound of granulosa cells is called the cumulus oo offer us as part of the development the dominant follicle the cumulus ORS and the egg sort of separate together from the wall of the follicle and float around in the middle of the antrum like a little island and the follicle increases in size so the actual follicle is increasing in size as it gets filled with more and more fluid from the granulosa cells and the granulosa cells are just producing fluid as a byproduct of their metabolism and creation of hormones eventually this dominant follicle which at this point is called the mature follicle it starts to balloon out the side of the ovary kind of like this just starts to push out against the edge of the ovary and then because the edge of the ovary and the wall of the of the mature follicle are in such close proximity enzymes within the follicle break down that common wall between them and the egg pops out onto the surface of the ovary because now this wall is broken down and by the way an enzyme is a protein that carries out a specific task the task here is to break down the wall between the mature follicle and the ovary and that happens on day 14 so it takes day 0 to 13 of build up to get to this event when this happens some women feel a little bit of pelvic pain and actually sometimes by chance two or more follicles reach maturity and they all pop up and that's how you get twins or triplets or quadruplets or octuplets when they all pop out and get fertilized by different sperm each because they're all subsequently swept up into the uterine tubes where sperm can fertilize them so now you have the egg out here but what about the old follicle it was in the follicle actually collapses little and transforms into a structure called the corpus luteum and in this transformation the granulosa cells get a lot bigger and start to produce more estrogen progesterone and that other hormone inhibin that we mentioned before just briefly inhibin lowers the amount of FSH follicle stimulating hormone that comes from the anterior pituitary and it does that because follicle stimulating hormone actually propagates this whole process of follicle maturation as you can imagine from the name so if you didn't know this before these are the exact fall chuckles that follicle-stimulating hormone refers to at least in a female anyway if the egg doesn't get fertilized then the corpus luteum reaches a maximum size in about ten days so that's about day 25 which it's probably sitting at in this diagram and then it degenerates by apoptosis that's a process that cells use to sort of self-destruct and die off and here I'm abbreviating corpus luteum is CEO just so you know what I mean but if the egg is fertilized ie it travels into the uterine tubes and gets fertilized by sperm then the corpus luteum persists I mean it keeps living because we want it to keep producing estrogen and progesterone that's because estrogen and progesterone prepare the inner lining of the uterus that's called the endometrium for implantation which would be really handy since we have a fertilized egg now that needs to develop and that's where it does it by implanting in the endometrium of the uterus so just a final note ovulation doesn't happen forever at about age 50 to 51 females undergo something called menopause first menstrual cycles become less and less regular in other words they don't happen every 28 days like they do when you're under the age of 50 and then ultimately they stop happening entirely and that cessation of ovulation is called menopause the main cause of menopause is sometimes referred to as ovarian failure basically the ovaries lose the ability to respond to signalling hormones from the brain called gonadotropins and we know these as LH and FSH and this happens because most or all of the follicles and eggs have already gone through that process that we talked about called atresia in other words they've degenerated