Basics of egg development
- So let's look at the female reproductive cycle. The female reproductive cycle refers to the maturation of eggs within the ovaries. The ovaries initially created these eggs during gestation. In other words, when a baby girl is in her mother's womb, the baby girl's entire egg supply will be created but will remain in an inactive state. This process of egg creation is called oogenesis. Then, once she grows up a bit and reaches puberty, her reproductive cycle will start, and one egg in that egg supply in her ovaries will mature or become activated each month, and that allows it to be fertilized by sperm. By the way, another word for egg is oocyte. After an egg matures, it's pushed out of the ovary in a process called ovulation. The other major function of the ovaries is to secrete the female sex hormones, estrogen, progesterone, and one called inhibin, and we'll talk about their functions a little bit later on. So let's first discuss how the eggs are made in the ovary in the first place. So early in uteral development, precursor germ cells, which are called oogonia, and those are homologous to spermatagonia in males, these oogonia undergo a ton of mitotic divisions to make more of themselves. And then, at about the 7th month of development, these divisions stop, and all of the ones that have been produced, which is actually about two to four million, are all she'll have for the rest of her life, and that turns out to be about one to two million per ovary. So while she's still in fetal development, all of these oogonia that have been produced, they all develop into the next stage, which is a primary oocyte. And just remember that the two oo's refers to egg, and the cyte, C-Y-T-E, refers to cell. So this just means egg cell, in case you were wondering. And let me also just mention, on a chromosomal level these oogonia, the germ cells, they're 2n, which means they have two copies of each chromosome, and the primary oocytes are the same. They're also 2n. And then these primary oocytes, they begin meiosis 1, and meiosis is what our germ cells use to reduce our chromosome copy number, and by that, I just mean the number of copies of DNA that we have. So they start this process of meiosis 1, but they don't actually finish it. They just kind of get about halfway through, and then they stop. So they're stuck as these big cells. So they're still primary oocytes, but they're said to be in meiotic arrest. So when the female who's been developing in her mom's womb, when she's born, her primary oocytes are in meiotic arrest. So the question is, do they stay like this? And the answer is, some do and some don't. Let's zoom in on this reproductive system to try to explain. So this is just a closeup of the major parts of the female reproductive system, and I've cut away parts of the uterus and the uterine tubes and the ovaries so you can see sort of the inside and the outsides of both structures. And this is our key organ here. This is the ovary. So the question was, do these primary oocytes that are stuck in meiotic arrest, do they stay like that? So the answer is that the ones that are sort of destined to be ovulated, that is, to be pushed out of the ovary right about here and then to be picked up by the fimbriae and then travel along the uterine tube here, those ones get past meiotic arrest. But most of them sort of die off while they're still stuck in that meiotic arrest phase as a primary oocyte. So I've mentioned the ones that sort of get out of the meiotic arrest phase and move on to develop into secondary oocytes that are able to then fuse with sperm, but when exactly does that happen? Well, it starts at puberty. So they actually stay in this phase as primary oocytes up here, in meiotic arrest for like 12 to 13 years, give or take, and only then do they start moving forward with development by finishing off that first part of meiosis that they started and splitting into two secondary oocytes. And actually, that's not exactly true, even though that's what we'd expect. What actually happens is one primary occyte it attempts to split into two secondary oocytes, but that's not exactly what happens. What does happen is that one of the developing daughter cells develops beautifully into a normal secondary oocyte from the primary oocyte, but it turns out that when they do complete the first part of meiosis, something really interesting happens. One of these cells receives basically all the cytoplasm. So the chromosome copy number is halved, but basically all the cytoplasm is kept in one cell. So this little guy over here that didn't get much cytoplasm, it still has a full complement of chromosomes, but it still ends up being pretty small and not really very functional. So it kind of withers away and dies, and it's called a polar body. So you end up with this really large secondary oocyte, and this is what ends up getting ovulated. And so now you might be thinking, well, meiosis is two steps, right? When does the second step happen? And that's a good question. So again, ovulation happens roughly here with the secondary oocyte coming out, and this secondary oocyte sort of just hangs out in the uterine tubes, and a sperm comes along and fertilizes the egg. So let's look at that down here. So you have your uterine tube here, and you have your egg. That's a secondary oocyte now. And then a sperm is coming along, and the sperm fuses with the egg after fertilizing it. And so the sperm fertilizes the egg and fuses with it. And so, let's just zoom in on what's happening there. So here you have your big secondary oocyte, and then you have your sperm that sort of, let's say that the nucleus of the sperm is right here. It's inside the egg already. This is the nucleus of the sperm. And here's the nucleus of the secondary oocyte. Well this is when meiosis 2 happens, so the second half of meiosis. So as this sperm nucleus is traveling toward the egg nucleus to create a joint nucleus, meiosis 2 occurs, and the oocyte reduces its chromosome copy number by creating another polar body, so a second polar body that kind of divides off the cell. So the oocyte cuts its chromosome copy number in half again, and so this little bit of DNA here that's just an extra copy of the DNA the egg already has, it divides off the cell in the form of another polar body that doesn't really have that much cytoplasm, just like the first one. So again, it leaves its nutrient-rich cytoplasm behind for the sperm and the egg. And by the way, the egg has changed its name now from a secondary oocyte to an ovum, but it won't be an ovum for long. Once the sperm nucleus fuses with the egg nucleus, then it becomes a zygote. So let me just clarify that if the egg doesn't get fertilized by a sperm that comes along, then it doesn't complete that second meiotic division that it did right here, and it just gets discharged from the body in menstruation as a secondary oocyte and not as an ovum, because the name ovum is reserved for the oocyte only once it's been fertilized. So those are the basic concepts behind what goes on with egg development.