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Video transcript

- [Voiceover] We kind of looked at the first half of this graph, the follicular phase, and we kind of understood that the follicular phase is all about stimulating growth of the follicles in the ovaries to eventually cause the ovulation of an egg. This stimulation of the follicles comes from the release of various hormones, FSH, LH, estrogen, and so on. The second half, after ovulation, is called the luteal phase. It's called the luteal phase because it's about the development of this yellow structure here, called the corpus luteum. So let's talk about it. Here, after ovulation, we have an egg that's been expelled from the follicle. We therefore have the remainder of the follicle. At this point, luteinzing hormone, LH, is really high. Together, LH and FSH are going to induce the old follicle here to turn into a structure called the corpus luteum. That just means "yellow body" in Latin. It's actually a bit yellowy because of some yellow pigments it has in it. Remember the follicle was what released estrogen from its granulosa cells. Since this is not a follicle anymore, it greatly reduces the amount of estrogen it makes, and it actually begins to mass-produce progesterone. Just to clarify, though: it still does make some estrogen. It's just not really its primary product. Progesterone is. So that's why you see this dip in estrogen here, and this almost uptick in progesterone that gets produced. Let's think about this for a second. We've ovulated, so there's a chance our egg could get fertilized by a sperm. Once it gets fertilized, it's going to need a place to hang out and grow. It's going to have to implant into the lining of the uterus called the endometrium. What would be really helpful right about now is if we had a nice, vascular, hospitable uterine lining that our fertilized egg could implant into to support gestation of our new embryo. By the way, at this stage of implantation, our embryo is actually called a blastocyst. I'll just write that down over here. So in comes progesterone. Let's just look at the word for a second. "Pro," meaning "for," and "gest," referring to gestation. This last part just kind of clues you in that it's a hormone. So progesterone is a pro-gestation hormone. It's what mainly stimulates the uterine lining to prepare for implantation and gestation during this phase called the secretory phase. In this secretory phase, progesterone does a few things. It increases blood flow to the endometrium by stimulating the development of special arteries in the endometrium called spiral arteries. You can see them here in red. These spiral arteries allow the embryo to eventually have good access to nutrients from the mother's blood stream. Progesterone also increases uterine secretions from special glands in the endometrium. These secretions are important for nourishment of the embryo. Finally, progesterone actually reduces the contractility of the muscles of the uterus. Remember, the uterus has a lot of smooth muscle in its walls. The progesterone actually reduces the contractility of those muscles so that the growing embryo doesn't really get too disturbed and doesn't get expelled out of the mother's body too early by those muscles contracting. Since we really want that nice, hospitable environment for gestation, the corpus luteum produces lots and lots and lots of progesterone. By the way, there's still a reasonable amount of estrogen kicking around, and it's really both the little bit of estrogen plus the lots and lots and lots of progesterone that helps to ready the endometrium for pregnancy. The corpus luteum's hormones are doing other things, too, though. The progesterone and the little bit of estrogen produced by the corpus luteum, they're going to suppress the FSH and LH production by the anterior pituitary, by that process of negative feedback. So you can see their levels dipping pretty low here. On top of the estrogen and progesterone negatively feeding back on FSH and LH release, the corpus luteum is also producing inhibin. You can see the amount of inhibin peak here when the corpus luteum reaches around its maximum size. That inhibin actively inhibits FSH release from the anterior pituitary. Unfortunately for the corpus luteum, it kind of needs FSH and LH to survive. Since they're being suppressed by the corpus luteum's own hormone release, the corpus luteum starts to atrophy. It starts to wither away and die off. When it dies off, progesterone and estrogen levels drop. And when progesterone and estrogen begin to drop, two things happen. The first thing is that the end of the luteal phase is triggered. You can see, we've reached the end of the graph here. Basically, we've triggered the end of the luteal phase, but the start of the next follicular phase. At this point, menstruation begins to occur and the endometrial lining that has built itself up and prepared itself for implantation starts to shed, and it'll be lost through the vaginal canal in what we commonly call the menstrual period, or menses. Notice that that sort of takes place at the beginning of each reproductive cycle. Generally speaking, menstruation is a sign that pregnancy has not occurred. This period can last anywhere from two to seven days. FYI, women lose usually around 40 milliliters of blood per menstrual phase. The second thing that happens when estrogen and progesterone levels drop is that they stop exerting their negative feedback effects on FSH and LH release from the anterior pituitary. So the FSH and LH levels begin to go back up again. This increasing FSH then goes on to stimulate more follicular development in the ovaries to start the whole cycle all over again over the course of another 28 days. Now, what I told you about the corpus luteum withering away and dying is only actually true when no pregnancy is occurring. That's the case in most reproductive cycles, and that's why I covered that first. But it's important to know that things are a little different if pregnancy does occur and a fertilized egg is indeed implanted into the endometrium. We said that the corpus luteum's estrogen and progesterone release suppresses FSH and LH, and suppression of FSH and LH, in turn, cause the corpus luteum to atrophy, right? That's because the corpus luteum needs luteinizing hormone to survive. Well, when a blastocyst implants into the endometrium and gets established there, the resulting embryo that develops from the blastocyst starts to produce a special hormone called human chorionic gonadotropin, or HCG. This HCG is structurally really, really similar to luteinizing hormone, so much so that levels of HCG produced by the embryo are enough to keep the corpus luteum alive. Because remember, we said that the corpus luteum relies on LH to stay alive. So now with the corpus luteum remaining alive, it can continue to produce that estrogen and progesterone that's necessary to maintain the endometrial lining and keep it nice and supportive of the pregnancy. There's a couple implications of this. First, because the HCG production is unique to the embryo, most pregnancy tests work by checking for the presence of HCG in the blood or in the urine. Second, the corpus luteum doesn't produce progesterone for the entirety of the pregnancy. It does most of the progesterone production for about the first two to three months, and after that, a joint organ of exchange shared between the mother and the fetus, called the placenta, that starts to take over as the major producer of progesterone. Third, the corpus luteum being rescued, or kept alive, means that its continued hormone release maintains that endometrial lining. So the lining isn't lost in menstruation. It's kept in order to support the pregnancy. Just to recap: if no pregnancy occurs, then the corpus luteum withers away and the reproductive cycle just continues to repeat itself for about 28 daily. And if pregnancy does occur, the implanted embryo starts to produce HCG, which rescues the corpus luteum from atrophying and the endometrium is thus maintained. No menstruation happens because you're not losing the endometrium and the reproductive cycle is put on hold for the duration of the pregnancy.