Menstrual Cycle for IGCSE



hello is Tom here and today we're thinking about everyone's favorite IGCSE topic of the menstrual cycle it's a complicated topic so we are gonna look at it very carefully and we're gonna think in particular about how it's controlled by hormones what those hormones are which ones we're using and why that timing is so important the mental cycle is controlled by two endocrine glands and the two we're interested in are the pituitary gland over here dude loo loo loo and down here the ovaries now the ovaries contain the OVA or its latin for eggs and it'll be the follicles which contain the over that we'll be thinking about the pituitary gland produces two hormones in particular that we're interested in today that is f SH or forgive my handwriting which is follicle stimulating hormone and L h which is luteinizing hormone luteinizing because it means turns yellow and we'll think about the significance of that a little bit later follicle stimulating hormone does exactly what it says it stimulates a follicle to develop those follicles are down here in the ovaries and they contain the over the ovaries themselves put out two hormones they are estrogen and progesterone in particular progesterone has a feedback effects on to the pituitary gland so it kind of all goes round in a circuit high levels of progesterone will inhibit the secretion of FSH and LH so there's a feedback loop there and all this is to control when this follicle here this is a follicle inside an ovary when this follicle here ruptures kind of bursts open to release the ovum this being over here so this is ovulation and we want this follicle to rupture just at the right eye so that an ovum is released when the lining of the uterus here is it just the right thickness so that if an ovum is fertilized when it gets to the uterus the lining of the uterus called the endometrium is thick enough to receive it and has got a good enough blood supply to supply that developing embryo with all the oxygen and nutrients and so on that it needs then later on in the menstrual cycle that endometrium will become even thicker but if there is no pregnancy that lining will be lost not that way but down here through there in the menses phase as we go back through that back to the start of the menstrual cycle during menstruation so it's all about timing this is an image to show us the changing thickness of the uterus lining the intimate rim day one of the menstrual cycle is defined as the first day of menstruation the first day of the shedding of that uterus lining and you can see here it being broken down and ultimately that will be shed out through the vagina then from about well date it varies doesn't it but from de Fort of day seven maybe will change into the proliferative phase and then the userís line will start to get thicker and thicker up till day fourteen when ovulation will occur all these times are approximate and they vary but then that uterus lining gets thicker until around here at which point now we're concerned with two hormones here Eastridge in' and progesterone estrogen as the american spelling east region a high level of estrogen causes the thickening of this uterus lining so you'll see that when East region Peaks around here is its first peak you'll see that the uterus lining starts to get thicker go from this thickness here to that thickness up there then after ovulation you will see that East region starts to fall again there's still a bit of it around so we still get some thickening but in particular progesterone levels then start to rise and it's this high level of progesterone there which maintains the uterus lining so estrogen thickens progesterone maintains all this is to ensure that ovulation occurs at the right stage for the lining of the uterus the endometrium houses accomplished this here is a representation of the follicle and inside the follicle there is this blue structure here which is the ovum and in the presence of FSH from the pituitary gland this follicle will be stimulated to develop you can see getting larger and larger here actually in here this is a fluid-filled cavity in there and this follicle as it develops will be secreting east region therefore you can see that as this follicle develops the levels of estrogen rise at ovulation this remnant of the follicle here as the ovum is released becomes what is called the yellow body or the corpus luteum the corpus luteum then kind of takes over on secreting duties and it secretes progesterone therefore you get this surge in progesterone after ovulation because it's the corpus luteum which is doing the job now this progesterone has two jobs one yes to maintain the uterus lining but the other is it will inhibit FSH you can see here FSH levels they surge just before ovulation and then they plummet that's because progesterone now will be pushing those levels down there's a negative feedback loop on it so as progesterone Rises FSH and indeed LH Goes Down but then as progesterone goes down towards the end of the phase well you can see the FSH levels start to go back up again progesterone Falls here because the corpus luteum itself degenerates and as it degenerates it stops releasing so much progesterone until progesterone levels are very low at the end of the menstrual cycle well that takes us back to the beginning again with progesterone low and Eastern pretty low but FSH rising and stimulating Eastridge into rise because of course now the follicle is developing again and you get estrogen released from the follicle and so it goes around just one hormone we haven't talked about that LH there LH s job is to surge just before ovulation well in fact it surges and then you get ovulation about 36 hours later it stimulates ovulation okay that's all very well so what happens after that well if there's no pregnancy then you just go around that cycle again and again but if there is a pregnancy here we have an oocyte being fertilized by a sperm so we end up with at the original over nucleus in there and the sperm nucleus and then they fuse together here we've got a fusion of those two nuclei each of these care at half the genetic material one from half from father one half from mother and they fuse to give a full complement of genetic material it divides we've got mitosis going on and we get lots of genetically identical cells going on now this then ends up at day eight or nine with a blastocyst which if you like is an early stage embryo and that is going to have this nice thickened lining of the uterus to implant into so it's gonna have a blood supply and so on Bard's have a look what's going on in here this is the development of an individual follicle becoming larger and then bursting to release the ovum and then becoming the yellow body or corpus luteum as it goes on now of course this is going to be kicking out progesterone as we said and that is going to be maintaining the lining of the uterus but this won't last forever under normal circumstances so surely then once this disintegrates we're going to lose our source of progesterone and the line is going to be lost and hey our blastocyst which is implanted down here that's going to be lost along with the lining so what's the solution well there are a couple of solutions number one this blastocyst is a little bit clever because what it can do it can release a hormone which you don't use no juicy called HCG an HCG just delays the breakdown of this corpus luteum here so it maintains for a little bit longer and it carries on releasing progesterone maintaining the uterus lining also well that won't last forever so also what happens is we have a placenta grow so ultimately this structure here turns into the placenta here and the placenta itself is an organ of the fetus it's not an organ of the mother it's an organ of the fetus and it produces progesterone so you can see in whose interest it is to produce progesterone and maintaining that uterus lining it is primarily in the interest of the fetus now of course the mother will have lovely feelings towards its fetus and so baby won't I don't know who knows what a zebra feels about the foal developing within her but this fetus has 100% motivation to keep itself there for as long as possible and to keep that supply of nutrients and oxygen strong so it releases the progesterone to maintain that uterus lining from its own placenta here is a summary table for us I think this will be really useful to get under your notes and it's a good one to memorize maybe turn it into a card sort or flip cards things like that well thanks very much and I'm gonna leave you with a picture which is really complicated giving you all the feedback loops here we go

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