Learning oogenesis is a terrible thing, especially when you’re just starting out. It sort of makes you want to give up right there. Stick with it, though, this isn’t as hard as it seems. Start by learning more the concept and then worry about filling in detail.
Most people know that women are born with all the eggs they’ll ever have. That’s mostly true. Women are born with all the cells that will become ovum down the road.
Before the woman is born, the primordial germ (the cells that will become a gamete down the road) enters the gonads of a female.There, the differentiate into oogonia (an immature female reproductive cell). Once they’ve done that, the oogonia start undergoing multiplying through regular old mitotic division. No fancy stuff, yet. What’s crazy is that they end up developing approximately 1 to 2 million oogonia, even though less than 400,000 will be left by the time the woman will undergo puberty.
Anyway, still in embryological development, the oogonia start growing in size to become primary oocytes – still not an immature gamete or ovum. These still have the full number of chromosomes, too. When they have 46 like this they are called diploid. If two diploid cells were to try to merge, though, we’d end up with something not compatible with life. So, through the process of meiosis I and meiosis two, we’re going to strip them down to 23.
The other important part of this process is that it’s meiosis that mixes the genes around on the chromosomes to get variations. This is why two sons or daughters of parents aren’t genetic clones. It’s kind of a big deal for evolution.
So the primary oocytes begin the first round of meiotic division. It’s very similar to mitosis, but it’s important to know that during the chromosomes line up awfully close to each other and genes have the chance to cross over. That’s called synapsis and it happens in prophase I. The mixing part is called crossing over and it occurs at a point called the chiasmata or chiasma.
And now we stop. That’s right – when a woman is born, all of her primary oocytes are stuck in prophase I. So, they’re still not eggs, but they are on their way. And now, literally nothing happens until the woman hits puberty.
So, flash forward a bunch of years and now we’re hitting puberty. The woman is awkward, pimpled, and smelly. We’ve all been through our own frustrations with that. At this time, 5 to 12 primordial follicles are maturing monthly (primordial follicles are the oocyte and the tissues surrounding them). Those primary oocytes that were stuck in are going to finally achieve their dream of completing meiosis I shortly before they are ovulated.
As the follicle grows, the cells start to change into granulosa cells – just another type of cell to contain them. So now its the oocyte surrounded by a granulosa cells, which are epithelial, and together they are known as a primary follicle.
So far so good? Alright, so then the primary follicle is going to secrete a layer of glycoproteins (a protein mixed with a carbohydrate). This layer is called the zona pellucida.
We have to add one more layer in. Remember how the granulosa cells are epithelial? What do epithelial cells sit on? A basement membrane. That basement membrane separates them a thing called the theca folliculi. The theca folliculi has an outer layer of fibrous tissue (theca externa) and an inner layer of secretory cells (theca interna).
Now, they’re called secondary follicles. There is a surge of Lutenizing Hormone (LH) which kicks of the growth phase and causes meiosis I to complete. Now we have two daughter cells of unequal size and they are haploid! So now we’re down to the right number of chromosomes, too. One of those cells is the secondary oocyte and the other is the first polar body.
You know how the ovum is pretty big? This is part of how it gets to be that. During meiosis I, enough material is created in order to make two viable cells. But instead of doing that, the process favors one over the other, allowing it to grow in size, while the other one is not viable for life. That second one is the polar body, which degenerates.
So, then the secondary oocyte enters into meiosis II but arrests again! This time in metaphase II, and approximately 3 hours before ovulation commences. How do we know it’s 3 hours? I don’t know.
The secondary oocyte will not pick up meiosis II again unless it meets up with a sperm. That’s right. A woman’s eggs aren’t really eggs until they are already fertilized. And really, at that point meiosis II is ending with a zygote and a the second polar body – which just degenerates, too.
Please remember that we only addressed oogenesis in this article and not the menstrual cycle. That is also quite complex and deserves a whole article to itself, and will probably get one. We hope that makes understanding the process easier!