hey there primed in this video we're going to discuss the female reproductive cycle which includes the menstrual cycle and the ovarian cycle we're going to talk through the hormones the body changes that happen and that branching point between either repeating the cycle or going into our pregnancy cycle we'll touch on some U Genesis here where we're talking about the development of the egg but really we're going to be focusing on the hormonal control and regulation of this cycle in females now there is a lck going on in the female reproductive cycle and in general these are going to be average days and average times so in real life this is going to look very different but for something like the MCAT the important thing to recognize is that we have basically the mechanism cycle which is our hormones and our ovaries and then we have what's happening in the uterus or the menstrual cycle so we have the ovarian side which is in the ovaries which are our reproductive organs and involves a lot of the hormonal controls in the ovarian cycle we have three phases the follicular phase which goes from about day 1 to day 13ish then we have the ovulation phase which happens on average at day 14 although in real life females this can happen anywhere from day 7 to day 20 but for the purposes of the MCAT we'll say it's right in the center at day 14 then after ovulation we have the ludal phase of the ovarian cycle and that word's going to make a lot of sense once we talk through what's happening after the ovom travels to the Fallopian tubes after ovulation in our uterine or menstrual cycle we also have three phases we have our menstrual phase which is usually from days one to day four or all the way up to day seven this is when menstration is happening and the uterine lining is shedding then we have what's known as the proliferative phase where we're regrowing that uterine lining this happens between days 5 and 14 finally we have the secretory phase which happens after ovulation usually around day 15 and goes to day 28 so we can see here that the secretory phase kind of overlaps with ovulation and the ludal phase the menstrual phase overlaps with the follicular phase and the folicular phase also includes some proliferative phase so there's kind of connections between the three phases on both the ovarian side and the uterine side okay so we have the two cycle definitions we have our ovarian cycle and our menstrual or us in cycle definitions but what is happening what is the cycle well the cycle is the development of our ouo site and the release of the ouo site into the follicular tubes to be available for fertilization by the sperm so in order to understand what's happening we do have to have a brief discussion of uu Genesis this is not going to be the focus of the video nor is it heavily tested on the MCAT but I do want you to kind of understand from a uh gametogenesis perspective what's going on so in humans an interesting component of the female reproductive cycle is that when we're born we're actually born with all of the primary oo sites that we're going to have in our lifespan this happens at about 18 to 22 weeks in the prenatal phase and we our ovaries develop the primary uo sites so this is our two and these are our diploid cells and they are arrested or Frozen in prophase 1 of meosis so when we're born we have a bunch of these primary Ides that are Frozen They're not progressing through meosis at this stage they're still diploid but they're ready to go and we're not going to produce any more of these cells after birth so we kind of have what we have upon birth uh this is very different than the male reproductive cycle which continues to go through spermatogenesis and produce new sperm cells throughout their lifespan so we have this Frozen set of primary o sites and then when we hit monarchy or the onset of PU with our first menstrual cycle we start something called follicular Genesis folliculogenesis which just means that we're developing folicles what are follicles this is where it gets a little confusing in terms of the nomenclature but follicles are basically our oo sites surrounded by a fluid and some other cells like granulosis cells so I'm going to show you kind of the process of developing follicles I don't want you to stress about memorizing this but I do want you to recognize in general what's happening to our oo sites in this phase so this is after monarchy as we hit puberty so you can see here this is the path of developing follicles and again don't worry about memorizing I just want to show you kind of what's going on so our primordial follicle all right which is here this guy is where we start at Birth so this is our primary oo site right in the center here surrounded by fluid and then these little purple cells are called granulosis cells then once we hit puberty we start to develop these follicles and so we're going to see that our granulosis cells change shape and start to replicate and we start to get a little bit bigger and we produce these primary and then secondary follicles this is something that happens before we get into our hormonal regulation phase so a whole bunch of these cells at once are developing into our secondary and then all the way into our antral follicles so we have these big chunks of developing follicles and it's kind of like a selection period where you have this big group of follicles and then we have this fun little selection moment where only a few are chosen to fully develop and once they're chosen they're called graffian follicles they're now going to be subject to our FSH and LH response so when our hormones start coming into our ovaries these are the follicles that are going to respond to that and be stimulated we can see here that we have our granulosis cells our big follicle with our space is now bursting here after ovulation allowing our ouo site to go into our peritoneal cavity our abdominal cavity and then travel to our Fallopian tubes to either be fertilized or to go through our menstrual cycle so a couple things to notice when we're in our primordial folicle we're in our primary oo site all right this is our 2N it's our diploid cell and it's held in a rested in prophase 1 of meosis once the follicle reaches the graffian stage which is also known as tertiary follicle then we go through we go through our metic phase and become a secondary oo site this is now a haid cell so it's just n and we develop a polar body so that division happens where we have a polar body that doesn't really have much in it and then we have our secondary oouo site this secondary oouo site is what's going to go off and develop into a zygote if it encounters a sperm or again be sheded out with the uterine lining if it is not fertilized so now we have kind of the players here so from here on out when we talk about an ovom I do want you to think about it as the secondary oooo site developed cell the haid cell prior to this there's a lot of follicles that never fully develop and produce a haid germline cell so a lot of these follicles um end up atrophying and going through apoptosis they don't get selected for to make it to the final stage and become a secondary o site that's pretty much all I want you to know if you like to learn this stuff for fun or to prepare yourself for medical school when you will need to know all these stages please feel free to do so but just know on the MCAT it's really more about knowing that we have diploid cells um as our primary primordial follicles and then we do end up as haid cells before right before ovulation right and those are the ones that have been selected for we only have a couple follicles that make it to that stage now let's get into the hormonal control and regulation which is really where a lot of these MCAT questions come up okay so you may have seen something like this in your book where it's kind of showing you everything at once it's showing you the days of this cycle on the top with the follicular ovulation here at number 14 and then ludal phase showing you the basil body temperature an interesting feature is that female body actually increases in temperature by about3 degrees Celsius um after ovulation then we have our hormone levels here and this can feel very stressful and complicated as you see all these graphs what do you need to know about it then we've got our ovarian cycle the development of the follicle and release of the follicle as an ovom right it's it's comp at and then at the bottom here we have our uterine cycle which is showing you the uterine lining the shedding proliferation and then secretory face so there's a lot here personally for me I like to just go step by step and describe what's going on so I'm going to switch my page and actually write it out for you but please feel free to reference this diagram will come back to it as we talk through each step and I'm going to do that by having our hpg our hypothalmic anterior pituitary gonadal axis handy since we'll will be talking about our hormone release okay we're going to start with day one we're going to assume that this is a regular cycle um and day one is the start of menstration this is where the uterine lining is shedding all right so we're breaking down the uterine line we have not fertilized our ovom and so we don't have a zygote and we're going to go ahead and go through menstruation so what's happening here why is menstration triggered this is because we actually see a drop in estrogen and progesterone I'm going to write it out here but then in the future I'm just going to use e and P okay estrogen and progesterone are both necessary to maintain the uterine lining when these levels drop we shed that lining so that's what's happening again here if we go back to our little phase here we're all the way at day one what we can see is that our estrogen which is our Gray Line is pretty low right our estrogen and our progesterone are pretty low here at day one we see that they've dropped off pretty heavily towards the end of the cycle so that is what's triggering the menes the shedding of the uterine lining okay so what impact is that lower estrogen and progesterone having on our hpg axis well let's take a quick reminder the hypothalamus is up in the brain here that produces G&R gatot tropin releasing hormone which then goes down to the anterior pituitary and stimulates the release of lutenizing hormone and FSH that then goes to our gonads in this case our ovaries which stimulate the release of estrogen and progesterone our steroid hormones if you need a review on this cycle and any hormone Cycles please feel free to check out my videos on the endocrine system so we now have lower estrogen and progesterone right we're decreasing that so what's been happening here and this is generally what's happening is the high levels of estrogen and progesterone have a negative feedback effect when we decrease our Inhibitors right our negative feedback what are we going to do our end result is going to be to actually increase our levels of G&R so what's happening here is that low estrogen low progesterone is releasing the inhibition the negative feedback from our hypothalamus allowing us to increase our levels of G&R which then of course will increase our levels of LH and FSH GNR and LH and FSH will always be going in the same direction it's the estrogen progesterone stuff that's going to be doing our feedback loop okay so now we're going to get into the connections that the MCAT wants you to make which is not just okay we know the pathways and the negative feedback but let's talk some cell biology here and maybe even some genetics because as you know the MCAT loves mixing and matching multiple categories so want you think about something estrogen is a steroid hormone what does that mean that means that it can freely go through membranes and most steroid hormones their mechanism of action is to bind to a cytosolic receptor so they get into the cell through passive diffusion they bind to their cytosolic receptor and then they go into the nucleus and they have their mechanism of action by regulating gene expression at the DNA level and so our estrogen receptor and our estrogen are our on and off switch for the production of G&R and LH and FSH to a lower level in the anterior pituitary so this is one of the things I want you to think about as we're going through this is I want you to visualize what's happening here when I said lower levels of estrogen are increasing our GNR that really means and you don't have to memorize the mechanism but I want you to think about it okay we have less estrogen binding to its receptor so less estrogen and receptor molecules are getting into the nucleus which means they're not inhibiting gene expression as much they're not binding to the promoter and inhibiting our gene expression of our G&R so we're able to produce more G&R so that's what we're talking about when we say something simple like there's a negative feedback mechanism right I want you to make that connection because of course our steroid hormone mechanism of action our cell diffusion our transcription translation stuff these are all testable topics on the mcap okay let's get back to our pathway so now we have our estrogen and our G&R and our LH and FSH so we can see that these guys are starting to increase and again if we go back to our little path here so we can see our FSH and LH we can kind of start it from the back here right they're starting to go up right and then they kind of platten out they go up a little bit they they stabilize at a certain higher level so we'll talk about that shortly and again again FSH and LH can be seen as a proxy for G&R levels as well so as we're going through this follicular phase what we're doing is our increased lutenizing hormone and FSH are stimulating our follicles to develop so we have our follicular development happening and that's happening all the way through Days 1 to 14 and as the follicle develops it releases estrogen and again I'm just going to write as Big E so now our estrogen levels are going back back up because our follicle as it's being stimulated by our FSH is now releasing estrogen and again it's going to be doing negative feedback as those levels are higher now it's going to bind to its receptor get into the gene promoter region and stop G&R from getting transcribed and translated so what we're seeing here is we'll see a leveling off right will decrease the G&R and I call it stabilizing LH and FSH levels and this is because we don't want to be developing too many follicles past our tertiary point right we want to only develop a couple at a time and really only ovulate one egg at a time so we want to make sure that we're not developing like 10 follicles which is what would happen if we kept increasing FSH but then and we'll actually kind of pull this here right before day 14 or right before ovulation something wild happens as estrogen increases in concentration in the body it reaches reaches this magical estrogen threshold and as far as I can tell we're still trying to figure out exactly why this happens but what happens here is once a certain amount of estrogen has been released once we have a high enough level of estrogen it turns into a positive feedback so all of a sudden that estrogen estrogen receptor that's binding the prono region and inhibiting is now acting as an activator and there's some theories that say that it's binding to a different uh portion of the DNA um it's interacting differently with the transcription factors that's all beyond the scope of the end cap what you need to know is that something changes and all of a sudden we get this positive feedback on our G&R and LH and FSH production so what is that going to do for this very short period of time it's going to massively increase G&R and cause an LH and an FSH Spike but the important one here is going to be our LH Spike all right it's going to do it for FSH to some extent as well but LH spike is really what we care about because that LH Spike causes ovulation so we can see that here on our graph all of a sudden right everything's going low but we see how our estrogen our Gray Line is going up and up and up and then all of a sudden it reaches some sort of magical threshold and boom these guys that had been kind of decreasing suddenly Spike and the LH spikes way more um the action the mechanism of action really impacts the LH release and that LH release allows for the follicle to burst it's kind of like it's like boom pressure burst and then all of a sudden we have our ovam release so we'll go back to our follicular development here for just a moment and see that this phase ovulation is happening because of this LH spikes so then what happens well now we have we're ovulated we're off goodbye ovam go off and Conquer and our Corpus ludum is formed the Corpus ludum I like the name it's kind of like ludus corpse right so d to the LH lutenizing hormone we develop this ludum corpse and this is all of the granulosis cells from our graphi and follicle and they're still there and they kind of Reform and they replicate and what these guys do that's really interesting is they start to release progest so let me go back to our writing phase now ovulation has happened and then we form the Corpus ludum from our previous follicle the oam's gone the Corpus ludum is what's left over this guy is producing progesterone so our progesterone levels are going up so again let's go back to our little figure here again progesterone has a negative feedback only only estrogen does the funky thing so progesterone goes up now we're doing negative feedback that means that our G&R FSH and LH levels will level off and this prevents multiple ovulations this prevents more follicles from developing so this is key that the Corpus ludum has to keep producing progesterone so that we don't accidentally ovulate twice in one cycle right we don't want to be doing that so now we have these G&R LH and FSH levels let's go back to our graph of our hormones so we can see what's happening there all right so this is again post ovulation see how that progesterone just goes that's all because of the Corpus ludum meanwhile because we don't have a follicle anymore our estrogen levels are kind of falling off some of the estrogen is being produced still by the Corpus ludum those granulosis cells so we still kind of have a leveling off but in general it's the progesterone that's preventing M multiple ovulation from our Corpus ludum and as we go through the Corpus Lum kind of runs out right it slows down that progesterone production and if there hasn't been a fertilization event it's going to go down enough and our estrogen and our progesterone levels will tank down enough to start that GNR LH and FSH going back up again if we stop the negative feedback by having low levels of estrogen and progesterone we're going to allow that G&R LH and FSH to come back online so that's what's happening then we restart our cycle all right we're going to summarize this and add in what happens if there is a fertilization event but before we do I'm Amanda Bram and I've been coaching premedical students on their MCAT Journey since 2019 please remember to subscribe to this channel for more videos on MCAT content test taking strategies and mental fitness tips to help you perform your best on testing and if you'd like live online lessons with me plus a whole bunch of resources and test taking strategies please feel free to check out our next available live online MCAT cohort in the link in the caption below okay ready to summarize we have three phases broken up by a 28-day cycle from Days 1 to about 14 we have our follicular phase in our ovaries or our menstrual and Pooler phase in our uterus what's happening there is we started off with really low estrogen and progesterone levels they bottomed out after the Corpus ludum stopped producing those hormones that released the negative feedback from our hypothalamus and anterior pituitary allowing G&R LH and FSH levels to start to rise as that happens the follicle that's being stimulated by FSH releases estrogen from its granulosis cells that estrogen has a negative feedback loop it prevents more DNR and LH and FSH from being produced used so that we don't produce too many developed follicles but at some magical time right before ovulation usually day 13 or 14 something crazy happens and our estrogen threshold is released which means we have enough estrogen to transform it into a positive feedback modulator so that estrogen now increases the transcription translation of GnRH LH and FSH and that results in a spike of those hormones particularly LH LH that lutenizing hormone spike is what allows that tertiary follicle to burst and release that ovom out into our abdominal cavity and into the follicular tube then we have our ludal phase right in our secretory phase in our uterus this is where our leftover cells from our follicle produce lots and lots of progesterone and that progesterone has that negative feedback keeping the G&R LH and FSH levels low so that we don't ovulate multiple times in a cycle eventually though those progesterone and estrogen levels bottom out restarting the cycle but what happens if we fertilize the ovam what happens if we form a zygo if fertilization and implantation occurs this is where sperm and egg meet they form a diploid zygote which then forms a blasticus which then embeds itself into the uterine lining and what does that bunch of cells do it produces HCG human chonic gatot tropin HCG is an analog of progesterone analoges are like synonyms right they basically act the same and have the same mechanism of action on receptors as that original hormone so a progesterone analog is going to act just like progesterone so it's going to bind to those receptors up in the hypothalamus and anterior pituitary and keep our G&R LH and FSH levels low again maintaining the pregnancy so that we don't shed that uterine lining remember it's those increasing levels of G&R LH and FSH and low levels of estrogen and progesterone that drive us to shed the uterine lining the HCG prevents that and helps maintain the pregnancy once the second trimester hits now the placenta is producing enough estrogen and progesterone on its own to maintain the pregnancy and so our hcd levels drop off now there's a lot more to this of course but on the MCAT the broad strokes and the connections to other biochemistry and biological comp concepts are really what's heavily tested so as you're moving through these processes and Pathways and Cycles I want you to think to yourself wait do I know where that estrogen receptor is do I know what's happening to increase our levels of G&R yeah gene expression right we know about that from genetics so please feel free to make those connections check out my other videos again on the endocrine system and my genetics videos to help connect those dots and as always happy studying