hi everybody and welcome back today we are going to be looking at oogenesis which is a type of gametogenesis and it's the way in which we produce egg cells it's important to note in this video i'm going to be covering the physical structures that you need for ogenesis specifically i'm going to focus on the ovary and what's happening inside the ovary i'm not going to be doing the entire menstrual cycle i will be doing a separate video on that so you can see how each of the hormones plays a role in the ovarian cycle and the uterine cycle so that's going to be in a separate video today we're just introducing you to the structures and how they form and what they become and how do we eventually make an egg cell so to do that we're going to break this into two parts and i'm going to look at the ovary as we can see in this diagram here i'm going to go through step by step what's happening inside the ovary which is the site of ogenesis and then i'm going to look at our genesis itself in how we actually produce our egg cells and how it's slightly different to how we produce a sperm cell the product at the very end is slightly different and the number is different as well so let's begin with the ovarian cycle the ovary as you can see here we've zoomed in on it we've cut it open we're looking on the inside and the ovary is the site of ogenesis and what's really nice about this very simple picture is that it captures every little important phase that we're going to go through and you can study it in this order and we're going to follow along in the arrows now you will be asked to label these kinds of diagrams and so i'm just going to familiarize yourself with the words first and then i'm going to go into the detail so first of all we have something called a primordial follicle and a primordial follicle essentially is a undifferentiated cell at this point it hasn't turned into an egg cell but it's going to turn into one we often refer to them just as follicles now these follicles will grow a little bit bigger they will become more mature as you can see it's getting a lot bigger more cytoplasm and then it becomes something called a graphene follicle which i'll explain as well now that graphene follicle is a very large follicle and inside of it you can see is our oocyte which is this little round structure in the middle and we want to get that out and as you can see the oocyte leaves the ovary and left behind is a structure called the corpus luteum the corpus luteum used to be the graphene follicle but now because it's empty we call it the corpus luteum and slowly but surely as we progress the corpus luteum shrinks and it disappears now how does the actual production of eggs work and under the influence of what hormones that is what i'm going to explain now to you now how do we get these small follicle cells which are not differentiated they don't have a job yet how do we turn that into an egg so we can also call these follicles primary follicles and they are stimulated by a hormone called fsh and this is really important fsh is a hormone that is secreted by the pituitary gland and it is sent to the ovaries and essentially like the name gives it away it stimulates the follicles to continue growing and as they grow and develop and they increase in size they get more cytoplasm they're going through a few more stages of meiosis they turn into a large structure called the graffian follicle now the graffian follicle has the oocyte or the ovum sitting on the inside of it but you'll notice that it also has this really large layer sitting on the outside and all of these components are needed in order to have a successful ovulation which then brings me to how do we get the cell or the secondary oocyte out of our ovary so in order to get our oocyte or our ovum out of the ovary we need one more hormone to play and that is luteinizing hormone or lh and it is also produced by the pituitary gland and it is secreted and it causes what we say as ovulation ovulation is essentially the bursting of the egg cell out of the ovary and into the fallopian tube and it does need to burst to some extent because the ovary and the fallopian tube don't actually touch each other they're really really close but we need to get the egg from one location to the next now the graffian follicle is the structure that's going to release our ovum on ovulation day and ovulation day occurs around day 14 somewhere right in the middle of the cycle now that the graffian follicle is empty it becomes a structure called the corpus luteum and corpus actually literally means dead but in a sense it just means empty and the corpus luteum plays a really important role and it's linked to pregnancy and so under the influence of fsh and lh you're still secreting these hormones from the patient gland in the brain it starts to shrink and the graffian follicle becomes the corpus luteum now the corpus luteum begins to secrete progesterone now progesterone is the pregnancy hormone and so let's just unpack what all of these hormones are doing because this is really important if follicle stimulating hormone is the hormone that stimulates the follicle in other words it's the hormone that makes the follicle and luteinizing hormone is the hormone that releases the egg then these two hormones are responsible for growing and maturing our egg or over now that we've made it and we've released it we need to wait to see if we get pregnant or not and so that is where progesterone comes in once the egg cell is released progesterone comes in and it prepares the uterus for implantation for possible pregnancy and the corpus luteum will stick around for at least the first three months of pregnancy ensuring that enough progesterone is secreted to make sure that there's no miscarriage and loss of baby however if no fertilization takes place then progesterone levels drop why because the corpus luteum starts to disintegrate because no fertilization happened we don't need it anymore and it slowly but surely makes less and less progesterone and this then allows us to start the whole cycle again and i'm going to go into the finer details of how progesterone fsh and lh interact with each other as hormones in a separate video now that we've produced our oven we do need to make ourselves a little bit more familiar with the ovums structure and it's actually quite a very simple cell and we have our nucleolus and our nucleus remember that the nucleus in this instance is n which means that it is haploid carries half the amount of chromosomes we have cytoplasm and there's actually a lot of cytoplasm in an egg cell there needs to be because this is where eventually a human or a another [Music] organism is going to grow and develop we have the plasma membrane as we do in other cells and then we have two unusual structures that you don't always see and first of all you have something called the zona plusida which is a layer of jelly that sits on the outside it's quite dense the sperm cells often get stuck inside of that jelly layer while they're burrowing in and it performs a assertive protective layer on the outside now this next little bit that i want to tell you is really important because it's comparing spermatogenesis to o genesis and in the beginning they are very very similar however there is a major difference to what we make at the end and for now we've learned meiosis and how meiosis can take one cell and turn it into four haploid cells but this is actually not 100 the case if you compare spermatogenesis to ogenesis now if we look at spermatogenesis it follows the standard meiotic divisions we go from a diploid 2n cell we produce two haploid cells those two cells in turn each produce their own haploid set and they then grow tails and they become mature and now we have four sperm cells which is what we normally follow in meiosis but in our genesis you'll notice there's something different happening and so yet again we start off with a diploid oocyte and it goes through the first meiotic division but what you'll notice is that yes we have half the chromosome numbers but you'll notice the difference in size between these two cells and that is because this cell is going to get all the cytoplasm and it's going to continue its journey onto becoming an ovum this smaller cell is what we call a polar body a polar body is essentially a cell that is not going to become an egg cell it doesn't have enough cytoplasm it doesn't have all the cell machinery necessary and actually what happens is it degenerates now this bigger oocyte goes on to its second meiotic division lower down here and yet again we are going to finish meiosis where we produce one small polar body and then one very very large oocyte or egg cell and so that means technically if i were to draw this out in for our ogenesis we started with one cell and we produced one large cell that we want to keep and one small polar body this little polar body will divide and make two more little polar bodies but nothing will come from there it's this largest cell that will go on to finish meiosis ii and produce one very large cell that is our ovum and then one small polar body so we still have four cells being produced in ogenesis the only difference is we only use one of them and that is this really large cell that has all the cytoplasm inside of it so let's do a terminology recap so we looked at ogenesis which says the production of ovum and it is where we go through meiosis and we take a diploid cell and we form haploid cells and essentially how it's slightly different to spermatogenesis is we take one cell and we still produce four but only one of the four is actually used to form a egg cell we then looked at the word follicle and a follicle is essentially a cell that is on its way to becoming an ovum it's almost like an underdeveloped cell now these particular follicles grow really large inside of the ovary and they become something called the graffian follicle and all of this process of ogenesis that's happening inside of the ovary is being maintained and monitored by two main hormones one of them being fsh follicle stimulating hormone this is the hormone that starts the process and luteinizing hormone it is the hormone that is responsible for ovulation or essentially putting the egg out of the ovary and into the fallopian tube now we can often refer to the egg cell as an ovum or an oocyte and once ovulation has occurred of the ovum we are left with the corpus luteum let's not forget that the graffian follicle becomes the corpus luteum and this is a shrinking structure that is slowly but surely shrinking because of fsh and lh acting upon it making sure that it's doing its job and making sure that we don't release another egg you see we don't want to release an egg while we're trying to already mature another one we've got to do one at a time and so the corpus luteum secretes a hormone called progesterone the pregnancy hormone this is the hormone that's going to maintain the pregnancy if fertilization occurs if however fertilization doesn't occur then the corpus luteum secretes less and less progesterone until it disappears and the whole cycle needs to start again we also learned about polar bodies and polar bodies are these small cells that are produced at the end of meiosis they are not going to become egg cells and when we do go through our ogenesis we often make one egg cell and three polar bodies now i hope this video was very helpful for you today please use it as a consolidation video remember i'm going to put out a menstrual cycle video specifically looking at the hormones what they do and how they act upon the body often this is the most technical part of this topic and i hope you enjoyed and i will see you soon [Music] bye