卵子发育

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 and 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 a site 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 inhibit 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 utero development precursor germ cells which are called ogo Nia and those are homologous to spermatogonia and males these o Gonia undergo a ton of mitotic divisions to make more of themselves and then at about the seventh month of development these divisions stop and all 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 oh go Nia that have been produced they all develop into the next stage which is a primary oocyte and just remember that the two O's refers to egg and the site c y te 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 oh go Nia the germ cells there are 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 chrome some copy number and by that I just mean the number of copies of DNA that we have so they start this process of meiosis one 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 there's still primary Oh sites 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 no sites 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 close-up of the major parts of the female reproductive system and I've cut away parts of the uterus and the uterine tubes and the ovary 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 Oh sites that are stuck in my attic arrest do they stay like that so the answer is that the ones that are sort of destined to be ovulated that has to be pushed out of the ovary right about here and then to be picked up by the femme Briain and then travel along the uterine tube here those ones get past my otic arrest but most of them sort of die off while they're still stuck in that meiotic arrest phase is 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 phases 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 Oh sites and actually that's not exactly true even though that's what we'd expect what actually happens is one primary oocyte 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 Oh sis 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 with that didn't get all 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 and dies and that's called a polar body so you end up with this really large secondary oocyte and this is what ends up getting obviated 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 the secondary oocyte sort of just hangs out in the uterine tubes and a sperm comes along and fertilizes the egg so let's let's look at that down here 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 sperm fuses with the egg after fertilizing it and so the sperm fertilizes the egg and fuses with it and so that's 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's sort of let's say that let's say that the nucleus of the sperm is right here it's inside the egg all right this is a nucleus of the sperm and here's a nucleus of the secondary oocyte well this is when meiosis 2 happens so the second half of meiosis so so as a sperm nucleus is traveling toward the egg nucleus to create a joint nucleus meiosis 2 occurs and the O site reduces its chromosome copy number by creating another polar body so a second polar body that kind of divides off the cell so the O site cuts its chromosome copy number in half again and 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 in the and by the way the the egg has changed its name now from a secondary oocyte to an ovum but it won't be an ovum Vermont 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 O site only once it's been fertilized so those are the basic concepts behind what goes on with egg development