 menstrual cycle is the cyclical changes which occur in ovary and uterus every month in a repeatable manner. Basically this menstrual cycle is causing the development of the follicles and with the development of the follicles it also prepares the uterus for the implantation if there is fertilization of the released ovum. So basically the follicular development causing the release of ovum and the uterine changes for receiving the fertilized ovum they are both interlinked. Now the duration of this menstrual cycle may range from 21 to 40 days. However for understanding we will take an example of 28 days. So day one is the start of the cycle. What happens is that at the start of the cycle the levels of the estrogen and progesterone the female hormones is very low. So because of this there is a shedding of the uterine epithelium. So there is necrosis of the uterine epithelium and there is loss of blood via vagina. So the day that the start is taken as the start of the cycle. So from this day we will see what are the cyclical changes occurring in ovary in the uterus and the various hormones. So let's see. So here in this diagram I have just drawn a line and every two days we have made one division. So we will start from day zero when the levels of estrogen and progesterone are very low. Now we know that these estrogen and progesterone exert a negative feedback on the hormones released from hypothalamus that is GNRH and also on LH and FSH. So when these levels are high LH and FSH levels are low. But we are saying that on day one these estrogen and progesterone levels are very low. That means they will not be able to exert a negative feedback effect on LH and FSH. So what happens with the start of perboginal bleeding what we call as menstruation there is rise in the levels of LH and FSH because of loss of this negative feedback. So here we are seeing that there is an increase in the levels of LH and FSH. Now what happens that rise in the FSH leads to development of the follicles. Actually the word FSH that is follicles stimulating hormones. So it causes the development of the follicles and apart from follicle development it also leads to the proliferation of the granulosa cells. The granulosa cells are the cells which surround the ovum. So here this is the ovum and these gray colored cells these are the granulosa cells. So follicular stimulating hormone also causes the proliferation of these granulosa cells. Now it is from these granulosa cells that estrogen is released. So increase in FSH causes granulosa cell proliferation and development of the follicles with the release of estrogen. So obviously what we will see is rise in the levels of the estrogen. So you see that by day four it has risen a bit. But progesterone is not changing. Here we are talking only of estrogen. Now this estrogen and FSH what they do is they also cause increase in the expression of this FSH receptors. So on the granulosa cells in the follicles there is expression of these FSH receptors. So you see what is happening is that FSH is having an effect such that whatever FSH is present it can act more on the follicle because of the expression of the FSH receptors. Plus there is also expression of the mutinizing hormone receptors that is LH receptors. So why this is important is that that as estrogen increases by day four it starts having a negative feedback on these hormones LH and FSH. So only in the beginning that FSH levels rise and they cause the development of the follicles. But very soon by day four these FSH and LH levels start falling also because of the resumption of the negative feedback. Also this estrogen has an effect on the uterine epithelium. So it causes the proliferation of the uterine epithelium. See here from days to four there is shedding of the uterine epithelium. But after that day there is increase in the thickness of the uterine epithelium. This is known as the proliferative phase. So that is occurring due to the effect of the estrogen on the uterine epithelium. Now you see I had told earlier that FSH and estrogen are causing the expression of the FSH receptors in the follicles. So what happens that despite the negative feedback being there so despite the FSH levels being going down it is able to act on the follicles causing further follicular development and maturation. So there is the appearance of the enteral cavity and there is the proliferation of the granulosa cells which keep on releasing the estrogen causing the rise in the estrogen levels. So are you understanding the link between the what is happening in the ovaries, what is happening with the female hormones and at the level of the uterus. So FSH is causing the development of the follicle. Follicle is releasing the estrogen. Estrogen in turn is having an effect on FSH and LH and also on the uterine epithelium. Okay now what happens that by day 12 this estrogen reaches a very critical level and now you see that this critical level is important since now the effect of the estrogen on LH and FSH changes. Instead of negative feedback it becomes a positive feedback. So after a certain level of estrogen is reached it causes a positive feedback effect on LH and FSH. This leads to rise in the levels of LH and FSH because of the positive feedback. Actually FSH rises around two to three fold while LH rises around six to ten fold. So there is too much rise in LH and this rise actually starts around 24 to 48 hours before ovulation and it peaks around 16 hours before ovulation. So here we are seeing a peak in the LH and FSH levels. So you see in that in the beginning it was FSH effect we were seeing now LH is coming into the picture. So now this LH that is the luteinizing hormone what it does is modifies the grand losa cells. See still the ovulation has not occurred. So this luteinizing hormone modifies the grand losa cells causing them to release progesterone. So they are being converted into progesterone secreting cells. So till now we saw that progesterone was not changing the levels were constant only almost but as the levels of the luteinizing hormone rise progesterone start rising. So yes progesterone starts rising approximately 24 hours before ovulation and since grand losa cells are converted to progesterone secreting cells so obviously the levels of the histrogens are falling now. So that is what is happening just before ovulation approximately 24 hours before ovulation. Histrogen exerts a positive feedback causing rise in LH and FSH levels which in turn causes release of the progesterone from the grand losa cells. Okay now luteinizing hormone and progesterone are responsible for ovulation. So I am not going into the details that of how they are causing the ovulation we are discussing the cyclical changes which are occurring. So here this diagram is showing that the ovum is released and some grand losa cells are left behind in the follicle. So around day 14 ovulation occurs leaving behind some grand losa cells and there are certain thica cells also in the follicle. Then you see that because of decreasing levels of estrogen this positive feedback of estrogen on the LH is gone because I told you that there should be a critical concentration of the estrogen which will cause a positive feedback. So as the estrogen levels decrease the positive feedback is gone causing decrease in the levels of LH and FSH. So are you understanding the switch initially there is no feedback then negative feedback starts which converts to positive feedback here and then again the negative feedback starts operating causing decrease in the levels of LH and FSH. Now this LH it has one effect on the remaining cells in the follicle which is luteinization. Basically they become lipid laden cells and they secrete a lot of progesterone. So the progesterone levels rise but they also secrete some estrogen as well but you see the progesterone levels rise too much and estrogen levels also after a small dip they increase. So this continues while LH and FSH levels are low you see because the negative feedback is operating getting the point. Now just as estrogen was having an effect on the uterus that it was causing the proliferation of the uterine epithelium causing the proliferation of the uterine glands and by day 14 the thickness of the uterine epithelium is approximately 3 to 5 millimeter. Similarly this progesterone also has further effect on the uterine epithelium and this is known as secretary phase. So basically estrogen is responsible for the proliferation of the uterine epithelium which is known as the proliferated phase and progesterone is having an effect of secretion of the uterine epithelium that is the glands which are there they start secreting. Then there is little bit increase in thickness also because I said that there is estrogen also which is increasing. So here the thickness at around 14 days if the thickness was 3 to 5 millimeter here it becomes around 5 to 6 millimeter but there is increase in the tortuosity of the vessels then the glands also become tortuous they start secreting. So that is why it is known as secretary phase. Okay then what happens you see this luteinization which we have spoken that these cells the remaining cells in the follicle the secret estrogen and progesterone at around 23rd to 24th day they undergo involution. Why? Because I said that estrogen progesterone they are exerting a negative feedback on LH and FSH and the levels falls so low that they are not able to maintain the corpus luteum that is what I said that is the luteinization of the remaining follicle. So that is known as corpus luteum. So if LH levels falls as the name suggests it is luteinizing hormone LH levels falls this corpus luteum will also involute. So around 23rd to 24 days that is 4 days before menstruation it involutes and since it is the corpus luteum which is secreting estrogen and progesterone once it involutes what will happen the estrogen and progesterone levels will also go down. So that happens around 23rd to 24th day the levels start going down. So since estrogen and progesterone levels decrease you see we have said that entire effect on luteus is due to these hormones that is the estrogen and progesterone. So once they go down the necrosis of the epithelium starts see there were so many blood vessels they actually were so constricted because of the lack of the stimulus to them and there is necrosis of the epithelium this leads to the shedding of the uterine epithelium which starts here when the necrosis is complete and this leads to menstruation. So there is a lot of interlinking of various cycles. So you see that in the beginning before ovulation the changes which occur in ovary is known as follicular phase. In the follicular phase there is release of the estrogen which acts on the uterus and it causes from day four I can say up to day 14 proliferative phase. So estrogen is responsible for proliferative phase. Then in the ovary there is ovulation and the ova is released and picked up by the fallopian tube. The further changes after ovulation which occur in ovary it is known as the butyl phase as named by the hormone leutanizing hormone and its effect causing the formation of the corpus luteum. The release of the hormone especially progesterone from the corpus luteum leads to secretary phase in the uterus. Finally as the levels of the estrogen and progesterone go down due to the involution of the corpus luteum there is shedding of the uterine epithelium which is known as menstruation. So there is a whole connection between what is going on in the ovary the levels of the various hormones and the uterus. We cannot see it independently separate from each other. We have to link the various cycles with each other with the levels of the hormone. Well thanks for watching the video. If you liked it do press the like button, share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.