The ovarian cycle is the set of happenings over the course of about 28 days in the ovary that repeats itself every month-ish, okay? And there are two major stages or phases to the ovarian cycle. There's a follicular phase and a... phase and the sort of fence post between the two is ovulation.
Okay, during the follicular phase, that sort of most mature, most primed vesicular follicle is growing, growing, growing, growing, growing. Okay, and during the luteal phase, which is aptly named, okay, This is after ovulation, so that same follicle is now a corpus luteum. Still active, but not actively developing or nourishing an oocyte.
Instead, actively secreting hormones. Now, in reality, only a small fraction of women actually have a 28-day cycle, okay? Mostly because the length of the follicular.
phase varies from one woman to the next, whereas the luteal phase is very predictably 14 days long. Isn't that interesting? Let's look at the stages of follicle development. In other words, next slide.
We already know that primordial follicle will give rise to a primary follicle, which will give rise to a secondary follicle. Okay, we already know that follicles house oocytes, specifically one follicle per oocyte, or you could say it the other way around, one oocyte per follicle. In a primordial follicle...
Thin cells surround a primary oocyte. In an effort to become a primary follicle, those thin cells bulk up into cuboidal-shaped cells, and the oocyte, the primary oocyte within, enlarges. And that transition from primordial to primary follicle can take about a year. Now that we have a primary follicle, we want to transition to a secondary follicle, in which case the follicular cells surrounding the oocyte, they make copies and that surrounding epithelium becomes layered or stratified.
Once we have more than one layer of cells surrounding that oocyte, we stop saying follicular cells and we start saying granulosa cells. Okay. And now, instead of saying primary follicle, we start saying secondary follicle. Okay.
And those granulosa cells. are communicating back and forth with the OO site and developing back and forth because of cell signals, because they're connected via gap junctions. So they're very sort of co-developing.
Okay, same topic, we just need more space, so next slide. Now we want that secondary follicle to transition into a vesicular follicle. Granulosa cells and surrounding connective tissue will condense to form fecal cells. Fecal cells, okay, and the oocyte will secrete a glycoprotein membrane around itself.
that's quite thick called the zona pellucida. Zona pellucida. Okay. When space is full of clear fluid, start to kind of split open between adjacent granulosa cells, we say that's an early vesicular follicle, okay?
Whereas when all of those fluid-filled spaces merge to form one large cavity, We say that's a later or mature vesicular follicle. Okay, that antrum is going to continue to expand, expand, expand, creating that almost swollen looking follicle. Okay, and also kind of setting aside to one end that oocyte.
At full size, a vesicular follicle that's just about to rupture, just about to undergo ovulation, is about two and a half centimeters in length. That's really big. That's amazing. Next slide.
During the follicular phase of the ovarian cycle, remember there's a follicular and a luteal phase, several vesicular follicles, a handful, will be stimulated to grow. And that's achieved via increasing levels of follicle stimulating hormone that that's what stimulates those that handful to to grow okay and at around halfway through the follicular phase follicle stimulating levels decline and The most dominant follicle, the follicle that's gotten far enough thus far, the follicle that's... Most influential in terms of cell signals on the others. That gets selected. So the drop in FSH is kind of like a screening process that screens.
out the less dominant follicles and lets that most dominant follicle kind of shine through. Okay. In that most dominant follicle, the primary oocyte. will complete meiosis one to form daughters, the secondary oocyte and the first polar body.
Okay. And the granulosa cells in that vesicular follicle will then signal the oocyte to again, pause again, pause in meiosis two. specifically metaphase two. Next slide. We've seen at least once before, I want to say twice maybe already.
Okay. It's just growing more and more relevant. Okay.
And to help us out, this region. Here includes the zona pellucida, okay, and this outermost region. Here, it includes the equal cells. Okay. Here.
Are some of those smaller pools or cavities forming those merge to form one large antrum? Next slide. Another figure that we've already seen but again it just That's more context for us now. Right? Again, oocyte and follicle develop together.
And of course, this is the dominant follicle. Next slide. Okay, so now we know that follicle stimulating hormone plays a role in sort of identifying who that dominant follicle will be. Luteinizing hormone, as its levels rise, is what actually causes the wall of that vesicular follicle to rupture and propel that. that secondary oocyte out of ovary and hopefully into oviduct, but possibly just into the the sort of loosey goose abdominal pelvic cavity, okay, into the pericelial cavity.
Middle schmartz I mentioned earlier, it's the pain that some women feel on ovulation. It's like a little pinch, okay. And, uh, only one. Maybe 2% of ovulations will actually release more than one secondary oocyte. And if those multiple secondary oocytes are all fertilized, that's what yields.
fraternal multiples, okay? Whereas in the case of identical twins, it's just one secondary oocyte that's being fertilized, but then the zygote When it first divides, it's a little too overzealous. And it divides into separate, separate daughter cells, which end up being separate individuals.
Very, very interesting. And really so much more detail that we're not covering here about twins. All right, in the luteal phase of the ovarian cycle, the leftover granulosa cells and the fecal cells of the now spent vesicular follicle will kind of thicken up, enlarge, to form the corpus luteum. Okay.
Hold on, I'll be right back. This word is a little misleading. It's not something I would emphasize anyway, but I don't want you to get stuck on it. Fecal cells are fairly superficial in placement.
So here what's meant by internal? the fecal cells, those that are innermost, are going to enlarge to help form the corpus luteum. But that's splitting hairs. The corpus luteum now secretes, excuse me, progesterone and some estrogens, okay? If implantation doesn't occur, which is what pregnancy is.
Then the corpus luteum will degenerate. All right. In the last two to three days of the luteal phase.
The endometrium will start to erode, and that's, of course, if no pregnancy. And that's called the ischemic phase of the luteal phase, so it's like a subphase, okay? If pregnancy does occur, then the corpus luteum continues to produce hormones that will sustain the sort of fluffiness. flourishingness of the endometrium until the placenta is big enough to take over that role. So about three months from the first trimester.
Amazing. Next slide. Before we hit puberty, the ovaries are secreting estrogens, but only, whoops, only in...
relatively small amounts, okay, and, but high enough, high enough to feedback to the hypothalamus and say, hey, don't release. gonadotropin releasing hormone. Don't promote the release of gonadotropins from the pituitary, meaning FSH and LH, okay? But as we get closer and closer to puberty, the hypothalamus will release gonadotropin-releasing hormone in greater and greater and greater amounts, triggering, in turn, the anterior lobe of the pituitary to release follicle-stimulating hormone and luteinizing hormone.
Follicle-stimulating hormone will, as we've already established, help to certainly select a dominant follicle. every month, but we're talking about sort of the first ovarian cycle. That's why this says establishing.
Okay. That FSH is going to trigger granulosa cells to release estrogens and the luteinizing hormone is going to trigger fecal cells to produce or release, same, same androgens. Okay. And then granulosa cells will convert those androgens into even more estrogens.
And that continues until we reach menarche. Menarche is the first. menstrual cycle, the first period, okay? Whereas what's the last period called? Yeah, menopause.
Oops. Surprisingly, what we associate the symptoms or the byproducts that we associate with menopause Like night sweats, hot flashes, mood swings, nobody ever tells you, increased crotch sweat, surprise! Isn't actually menopause.
These are signs that menopause is approaching. It's perimenopause. It's the...
The window of time before menopause and perimenopause, laden with all these symptoms, can last a decade. Yay! Our... cycles don't become regular, predictable, and patternistic until about... We're three years in, three years beyond menarche.
Wow. Wow. So not only are you more susceptible to STDs when you're an adolescent, but you're also experiencing less predictable and yet fertile.
ovarian cycles right so there's also a greater chance just because of the lack of a pattern the lack of predictability of getting pregnant yowza let's look at the next slide Suppose we're well past menarche, okay, so we're regular. At the onset of the ovarian cycle, we're going to see a rise in gonadotropin-releasing hormone, which in turn stimulates the bloodstream. the release of follicle stimulating hormone and luteinizing hormone, which in turn stimulate follicles, but just some, to continue maturation. okay and also secrete sex hormones like progesterone and estrogens okay initially estrogen levels will rise just a bit Okay, follicles are also, however, releasing inhibin, which we also saw in the male reproductive system, okay? And together, these feedback to the hypothalamus to decrease.
the release of gonadotropin releasing hormone. In turn, this means less FH. So that's where that drop in FSH comes from to help screen out everything but the dominant follicle.
Okay. That dominant follicle is the follicle. gets bigger and bigger and bigger and therefore is producing more and more and more estrogens, okay, and these estrogens will briefly exert a positive or have a positive feedback effect on the hypothalamus, okay. as well as trigger a peak in luteinizing hormone release, which is really what triggers ovulation and therefore the formation of the corpus luteum. The corpus luteum then releases oodles of progesterone someone has been editing the publisher slides i wonder who and some estrogens okay rising levels of progesterone and estrogens as well as inhibit, okay?
Exert negative feedback on the hypothalamus and in turn the anterior lobe of the pituitary gland, okay? So that we see... Less of these stimulating hormones coming from the brain.
If you go to the next slide, this is a visual representation of what we just talked about. This is early and mid cycle. Whereas here's late follicular cycle and all of the luteal, I shouldn't say follicular cycle, follicular phase, and all of the luteal phase.
Okay. Early in the ovarian cycle, the hypothalamus is secreting. The adenotropin releasing hormone and in turn the anterior lobe of the pituitary gland is secreting FSH and LH. Okay, LH. signals fecal cells to produce and release androgens.
FSH signals granulosa cells to both secrete estrogens and convert these fecal androgens into estrogens. Therefore, we get an increase in estrogen level. In the meantime, granulosa cells are also secreting inhibin and together these feedback to the brain to say hey let's let's taper off for for at least a moment.
Okay tapering off allows us to sort of select a winner a dominant follicle. Okay initially the estrogen secreted by that dominant follicle exert a positive feedback on the hypothalamus so that gonadotropin-releasing hormone, well, release, picks back up. We get a surge peak in LH, and that causes ovulation.
here as well as triggering the formation from granulosa cells and at least some fecal cells of the corpus luteum corpus luteum really cranks up those sex hormone levels as well as uh inhibit and feed feeds back to the brain to say okay turn those down now I think the next slide, depending on how your brain works, might work out better for you, might make more sense. Okay, you can really see where FSH tapers off in an effort to select that dominant follicle. And you can also see that driving ovulation. We see a peak, a surge.
A big increase in these stimulating pituitary hormones, okay? And as they taper off, we're seeing corpus luteum. And once they taper off markedly, we're going to see degeneration, right?
Assuming no pregnancy, okay? It's primarily when LH levels get low that that corpus luteum knows, okay, my job is done. Let's look at the next slide. If fertilization doesn't occur, then certainly implantation can't occur, and that corpus luteum degenerates, again triggered by the fall in luteinizing hormone. This also causes a sharp decrease in estrogen and progesterone levels.
Okay, which puts an end to the negative feedback on the brain so that gonadotropin releasing hormone follicle stimulating hormone luteinizing hormone levels can pick back up and start the cycle all over again that's why it's a cycle we do it over and over and over again now it turns out that In actuality, the oocyte that's allowed to continue down the meiotic path, it actually is told to do so about 12 months, about a year before ovulation. Oh my God, what a mess I'm making. Okay, now I don't have to underline that.
It's all fine. All right, but it doesn't really mature markedly until about 14 days. It doesn't start showing that substantial.
maturation until about 14 days before ovulation. Let's look at the next slide, which starts us talking about the uterine cycle. We were talking about the ovarian cycle.
Now we're going to talk about the uterine cycle. The uterine cycle is a cycle of the uterine cycle. cycle is aptly named.
It's the set of events that's occurring in the uterus over about a month. Okay. Mostly in response to, um, the hormones that are, that are being sent out from the ovaries.
Okay. And there are three phases to the uterine cycle, the menstrual phase, the menstrual phase, and the menstrual phase. the proliferative or pre-ovulatory phase, and the secretory phase. During the menstrual phase, ovarian hormones are at their lowest level, but certainly gonadotropin-releasing. Hormone is rising, follicle stimulating hormone is rising, luteinizing hormone is rising.
The stratum functionalis of the endometrium detaches from the wall. And we emit both blood and tissue for three, four, five-ish days. By the fifth day, the next follicles in line are producing more estrogen.
They're getting big enough, okay? During the proliferative phase, we're starting to generate a new stratum functionalis because we just shed our old one. And by the end of this phase, the... The mucus that plugs the cervical os has thinned.
The cervix is sitting relatively low. It is soft to the touch. And the cervical os is relatively open. Okay?
And... The fence post between the proliferative phase and the secretory phase is ovulation. During the secretory phase, the endometrium, specifically the strontum functionalis, is really putting those final touches, those final fluffy touches. on that guest bed for implantation.
The corpus luteum is secreting lots and lots of progesterone and that progesterone is promoting the release of nutrients into the lumen of the uterus. Okay. We also see a thickened mucus plugging the cervix. Now in the next slide, which is not from our textbook, what we're looking at is the cervix of one female.
Every day for one month and I simply love this. You can really appreciate how the firmness, the texture, the position, the secretions, the wideness of the os, how all of those attributes of the cervix change over the course of a woman's cycle. So for instance, in this particular shot, look how far away the cervix is from the opening of the vagina.
Right. And maybe compare that with this. Wow, what a difference, right? Look how soft the cervix is.
Here. Wow, what a difference. Now, just so you know, some students don't realize, in order to take these these photographs, a speculum had to be inserted into the vagina. And so these smooth walls, those are actually the plastic of the speculum.
Okay. Whereas here is the true wall. Here is the true wall of the vagina.
True wall of vagina. True wall of vagina. Okay. Bumpy, bumpy rugae.
Just fascinating. Let's look at the next slide. which pairs an illustration of just the endometrium where here is stratum basalis and here stratum functionalis.
Okay, with the changes in ovarian follicles, or ovarian hormones, I should say, notice that by the end of the menstrual phase, Stratum functionalis is as slim as it's going to get. It's been almost entirely shed. Okay. Estrogen and progesterone levels are as low as they're going to get.
Notice that by the end of the proliferative phase, Progesterone levels are just starting to pick up, but estrogen levels are at their highest. And then notice that in the secretory phase, that's when we make the biggest in really thickening up that stratum functionalis layer. What's that driven by?
Progesterones. Who's secreting those progesterones? Corpus luteum.
Okay? Next slide. Really, review.
Recall that erection and engorgement, which is relevant for females too, is overseen by parasympathetic. pathway while orgasm overseen by sympathetic pathway. Okay and then you probably already know that males have a refractory period that follows orgasm.
Females do not. Next slide is just a reminder for me. So I am now on what would be slide 52, which is just a blank, a blank screen.
Okay. I want to talk to you a little bit about what our embryos look like and what they're equipped with. So I'm going to have to revisit 160 a little bit.
Probably in 160, you learned a little bit about taxonomy. So, for instance, you may know that we belong to domain Eukarya. You may know that we belong to kingdom Animalia.
You may know that we belong to phylum chordata. You may know that we belong to class. Mammalia. And chances are you also know other classes in Cordata.
For instance, reptilia, amphibia. Right? These sound familiar. Okay.
Well. First of all, don't worry about these. It's not like I'm going to ask you about them, but I wanted to give you some context.
All right. It turns out that mammals and reptiles have what's called an amniotic egg. Which not surprisingly is not an egg at all, right? We misuse the word egg so often. An amniotic egg is an embryo and for extra, meaning outside, embryonic membranes or sacs okay even though lots of other chordates have reproductive containers that include an embryo, they don't have all four of these extra embryonic membranes, okay?
So let's look at a class. Classic amniote. Okay, here's what a reptile embryo. Here's what a mammal embryo.
Fairly early. Fairly early. In other words, weeks. probably okay might look like in fact you looked like this this is the lateral view okay where yeah you had a tail okay you know these little slits And what would become your pharynx that kind of looked like gills.
You didn't actually have gills, but they look like gills. Okay. And then, let's see.
Yeah, we'll do that. An amniotic embryo. We'll have directly attached to it, directly attached to it, an extra embryonic membrane or sac that's called a yolk sac, a yolk sac. I'm not even yoking. You're welcome.
And actually, I think I'll put these, because I'm worried I'm going to run out of drawing space, so I'll put these down here. A yolk sack. Okay. A yolk sack, again, directly attached to this embryo, which is sometimes called an amniote. Okay, and that yolk sack is kind of like a packed lunch or an IV bag.
All right, now. Even though one day this amniote will have fully formed and will also use a fully formed alimentary canal, it's not yet using its alimentary canal. So does it make poop? No, it doesn't make poop, but it still does make metabolic waste.
Okay, so here's a second. bag attached directly to the embryo. It's called the alantois.
Okay, and it's receiving and storing metabolic waste. It's like a waste bin. All right, all right, immediately surrounding the embryo.
There's another membrane or sac because it's immediately surrounding the amniote. It's called the amnion. Okay, also because it's in most direct contact, it's nearest the amniote, it's going to provide shock absorption.
Kind of act like a C-lomb, but outside the body. Okay, that's three extra embryonic membranes. How many do we need total? Four. Okay, here's the fourth.
The fourth surrounds the whole shebang. It's called the chorion. And because it's most exposed, it provides surface area for gas exchange. All right.
That is what an amniotic egg is. Is it actually an egg? No, it's an embryo.
It's way past egg. Way past egg, okay? Now, all members of mammalia, all members of reptilia, will have an amniotic egg, which means they have at least this embryo surrounded by these four extra embryonic membranes, okay? But... Often when we think about reptiles, we think about things that lay eggs.
By the way, birds are reptiles. Okay, there's no guarantee that an amniotic egg will have a literal shell, okay? But when that shell is present, it would be here, okay?
And there would even be another... Hmm. A little sack of fluid just within that shell that holds albumin aka egg white.
Yes, the plasma protein. Okay. to act as cushion, right? I don't want this guy bouncing around in his shell, okay? But not all members of this group will produce shelled eggs.
Okay, or a shelled embryo, okay, including us, right? Some mammals do lay eggs. An echidna, a platypus, will lay eggs. But other mammals... mammals, thousands of species of mammals, they don't have a shelled egg, they don't lay eggs, okay?
But they still, by definition, must have an amniotic egg. So how does that translate? Let me show you. Here's our embryo. What's this?
Amion? Good. I think I chose, yeah I did. Maybe just via color, can you tell what that is?
Yeah, that's a yolk sac. What's that guy? Alantois. Good. What are those shoes?
Green? What's that? Good.
Corian. Okay. In mammals that form a placenta, like you and I, what happens is the ducts of alantois and yolk sack get tied up together in a rope what is that rope called That's an umbilical cord. Okay, and the yolk sack and valentois, proper, they're bags, they hit. enveloped in a large interface.
What is that? That's the placenta. Okay. So, do humans, do human embryos have a yolk sac?
Yeah. Do we have an alantois? Yeah. They're just sort of masked by the presence of a big blob that's called a placenta.
And the placenta is an interface between embryos, circulatory system, and mom. circulatory system all right now when um we hear maybe on tv or in a movie oh my god my water broke okay what's actually happening is as the pregnancy goes on, the closer and closer we get to full term or possibly even beyond full term, the more and more the chorion... will start to thin and degrade.
Okay. Start to thin and degrade. And so it eventually will rupture.
All right. And often if the chorion ruptures, the amnion as well will rupture. Or if we maybe are getting close to being over full term, we go to the doctor and they say, I'm going to strip your membranes.
what they're doing is hopefully very carefully separating the chorion separating the chorion from um the uterine wall okay just to just to really rigid to rupture. Okay. Some background.
So when you see in our study guide, whoops, I don't know what I just did. Um, when you see in our study guide that, that But chapter 27 is wrapped in. This is part of chapter 27. That's the part that I really wanted to cover.
Okay? Really interesting stuff. And again, don't worry about the taxonomy.
I just thought that you might need it for the context. Okay?