Ovary: Secondary and Mature Follicle – Female Reproductive System

by Geoffrey Meyer, PhD

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    00:00 ago. Now, we see a very large mature follicle, a secondary follicle on the left-hand side.

    00:08 You can see the oocyte within it. And on the far right-hand side, you can see a huge follicle.

    00:15 These are taken at different magnifications. And the mature follicle or the Graafian follicle, as it’s called that is about to ovulate, is very very large. It can be up to 10 or even 20 microns in size. It’s a huge structure. And because it’s so huge, you don’t often see section through the oocyte as you do on the left-hand side. It’s just a chance that when you section some of these large follicles, that you actually see the egg as well, or the oocyte. Now on the left-hand side, the secondary or the large mature follicle, it’s gone from a secondary perhaps to the large follicular stage, you can see its accumulated fluid that’s in that space. It’s called the antrum space. And the antrum space actually pushes all the granulosa cells add into the periphery. Look at the oocyte. You can see the clear stained zona pellucida around it. The oocyte is bright pink stained in this section. The cluster of granulosa cells around the oocyte are a group of cells we term the corona radiata, the crown around the oocyte. This group of granulosa cells are going to persist even after ovulation. And the very inner layer of these corona radiata cells actually penetrates microvillus projections through the zona pellucida and contact the microvilli of the oocyte. So there is communication between the two. There's interactions and control mechanisms going on. And then notice that the oocyte and the corona radiata are attached to the other cells around the periphery of the follicle by another mass of cells.

    02:21 That’s called the cumulus oophorus. So, here’s the antrum. There’s the cumulus oophorus that I’ve referred to, and the corona radiata is shown around the oocyte. The Graafian follicle is shown here as I pointed out, but I just want to mention a little change that’s occurring just prior to ovulation. If we can imagine, we could see the oocyte inside this Graafian follicle, I want to just describe a few changes. Firstly, during the follicular phase, as I said before, around the sixth day, waves of follicles are stimulated to go through this growth phase. But only one eventually is going to eventually ovulate. The rest, as I said before, undergo degeneration or atresia. Something happens by chemically inside these follicles.

    03:26 And I just want to summarize a very important concept. Have a look at the membrana granulosa in this large secondary follicle on the left-hand side. Recall that those follicular cells, those granulosa cells are secreting estrogens under the influence of FSH. They’re secreting those estrogens because they get androgens from the theca interna under the influence of LH. Well, what happens just prior to ovulation? One of these follicles, or perhaps a couple of them, actually starts to get receptors for LH in the granulosa cells. Those granulosa cells up until now have only had receptors for FSH, which controls their production of estrogens. Suddenly, about 12 hours before ovulation, or maybe even a little bit earlier, they acquire, or some acquire the ability to have LH receptors on their cell membranes as well. And that’s extremely important for two reasons. One is one of these follicles is going to get more of these receptors than the others. And therefore, they’re in a position to respond first or best to the LH surge that we see at the time of ovulation that induces ovulation of one of these follicles. So the dominant follicle, if you like, the one that has the more numbers of LH receptors, can respond to that LH surge, and therefore, ovulate.

    05:08 But another thing also happens. Those LH receptors are also very important for the eventual destination of this Graafian follicle once it ovulates. Those LH receptors are going to be extremely important on these cells for those cells to convert to produce progesterone after ovulation.

    05:30 And therefore, we enter the progesterone or the luteal phase of the ovarian cycle. And some of that progesterone gets produced by these cells just prior to ovulation. So those two very important concepts are very important to understand with regard to responding to the LH surge ovulation, and then finally, being out to produce progesterone in the corpus luteum, which is what this follicle will develop into after ovulation. Again, you can see the theca layers here. They’re becoming more engorged with blood which assists in also the ovulatory process.

    06:14 But also, at the time of ovulation, these theca layers, the theca interna anyway, is going to penetrate into the ruptured follicle. Up until now, follicular cells do not have a direct blood supply. They receive all their nutrients by diffusion from the theca interna into all the cells of the granulosa. But after ovulation, as we’ll see this theca interna layer, the theca interna, the very vascular layer you can see here quite prominently, will invade the developing corpus luteum, and convert the corpus luteum into an endocrine organ. As I mentioned earlier, a lot of the follicles undergo atresia degeneration.

    07:02 And you see certain signs of atretic follicles in the ovary. The one on the left, you can see that the cells just don’t look as pretty as they do in a normal mature or developing follicle. There's a lot of cells that are lost and moving into the antral space.

    07:18 If you look on the right-hand side on the very right bottom corner, you see these little sharp pink structures, shiny pink structures found in the ovary. These represent the zona pellucidas that have remained after degeneration of follicles of certain sizes. And again, on the right-hand side of the right-hand section, you can see a rather elongated pink structure.

    07:45 That’s called the glassy membrane because one thing that happens during atresia is that the basement membrane separating the theca layers from the granulosa layers of the developing follicle hypertrophies. And during atresia, it gets thicker and thicker, and rather amorphous in staining, and we refer to it as being a glassy membrane. These are all evidence of follicular atresia. Now, here’s a corpus luteum. On the left-hand side, this is an

    About the Lecture

    The lecture Ovary: Secondary and Mature Follicle – Female Reproductive System by Geoffrey Meyer, PhD is from the course Reproductive Histology.

    Included Quiz Questions

    1. Granulosa cells synthesize increasing amounts of estrogen (from androgen precursors secreted by theca interna cells) in response to LH stimulation
    2. In the "dominant follicle" progesterone production commences just prior to ovulation in response to LH
    3. A select few follicles have granulosa cells that accumulate receptors for LH with about 12 hours before ovulation
    4. Granulosa cells in the mature follicle secrete increasing amounts of estrogen in response to FSH stimulation
    5. The dominant follicle has the most numbers of LH receptors and so is best sensitised to respond to the LH surge and ovulate

    Author of lecture Ovary: Secondary and Mature Follicle – Female Reproductive System

     Geoffrey Meyer, PhD

    Geoffrey Meyer, PhD

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