Biology: Oogenesis: Meiosis in Females

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Cell Reproduction
Understanding Meiosis
Oogenesis: Meiosis in Females

We have to take a look at the organ systems that generate meiotic processes, as you know, and I want to take a look
at the female system.
Now, remember what we are talking about here, we are talking about meiosis. We’re talking about; again remember
what happens. The reality of meiosis is this. We start out with a somatic cell, that somatic cell is 2N. After homologs
line up, it goes through a split. This is reality, this isn’t just on paper – and eventually becomes a gamete.
Now, we know that the gametes are generated in organs called gonads. And, the idea being that, if you are going to
take a somatic cell and place it in a situation where fertilization is going to occur, whether it’s a pond or a fallopian
tube, and anything in between, the point is, you have to get that cell ready.
And so, structure follows function, and it does the same thing in human beings, or cats, or dogs, or fish, or whatever.
Talk about structure, here is one now. This is a schematic of the human ovary, and I want to correspond what’s going
on in here to what’s happening in meiosis. So we are going to go back and forth, back and forth, to see exactly what
happens here.
All right, let’s take a look. First of all, that somatic cell, if you will, that cell that starts out as 2N, is going to be right
there all right, and we are going to call that the primary oocyte, you never know how to pronounce this thing, it’s got a
little, whatever that thing is called there, those of you who speak German know what that is called, it will come to me.
And so, so we are going to call that a primary oocyte. And that oocyte is 2N.
Now it’s got to go through meiosis, and so let’s see what’s going to happen here. Now, the oocyte becomes an
oogoniam and that’s going to be in one of these middle stages right in here. So, we’re going to go from the oocyte, I
said that backwards, we’re going from an oocyte, which is a primary oocyte, to something called a secondary oocyte.
The oogoniam, that I mentioned, is come in, actually, up here, at an earlier stage, before meiosis starts. So I said it,
now I got to tell you about it, so you don’t think that I just made it up. So let’s see what’s going to happen. We will
come back to our ovary in a second.
Oogoniam – generic cell. Primary oocyte, take a look, now once again this is inhuman, how do I know it’s not human?
Four chromosomes. All right, look what’s happening, the primary oocyte, right here, has its four chromosomes, and
what are we in? We are in meiosis one, aren’t we? Okay, and the meiosis one is going to be where you get to that
point, where you are going to go through prophase one, metaphase one, anaphase one, telophase one and you are
eventually going to end in what is called a secondary oocyte. Now, one fascinating thing that happens in meiosis one,
and I am not going to tell you why this happens, you’re going to tell me. Or I’m going to ask you, and then I will
probably tell you.
Here’s the thing, here is my ogoniam, to a primary oocyte, and then we are going to split. But now I’m going to show
you one major difference between oogenesis and spermatogenesis, sperm forming. Okay, here we have what are
going to be called the secondary oocyte, in other words, what did we just do? We just went through meiosis one.
Okay, we have our secondary oocytes. Our secondary oocytes are starting to mature, but look, one of them is teeny
tiny, and one of them is large. This is called a polar body, and it is completely non-functional, it degenerates. So now
I hope I have planted a seed. You are saying to your self, “Why”? Why, does this thing, now remember what does
this has? It still has the chromosomes that are doubled, but it has half the number of chromosomes, but they are still
doubled. And so, in essence, that gets wasted. So I want you to ask yourself why? We’ll come back to that later. So
far, so good?
Well guess what, all of you who are females and are watching this, that’s where you stopped. You see in a human
female, what happens is, when she goes through meiosis one, and she gets to the secondary oocyte state, she stops,
and meiosis two never happens, until fertilization. So, in other words, whereas in spermatogenesis, as you know, the
male is constantly making sperm his entire fertile life, the female is really only, all of her eggs, which are made at birth
– let’s go through the whole development thing. Female is born with ovaries, her ovaries, all the cells, as they are
developing, are literally waiting to go through meiosis, once a month, once roughly every 28 days, she will put a cell
through meiosis one. That cell will pop out during the menstrual cycle and it will go into the fallopian tubes. If it does
not get fertilized, meiosis is never completed, but if it does get fertilized, now we continue moving one.
So, before we move on, let’s see where we are in the ovary. So, here is my primary oocyte, we’ll put it like that –
primary oocyte. Just to give you an idea of some of the structures involved here, this is the follicle, and the follicle –
you guys you know, if you at that point in the course where you need to know about the menstrual cycle, you may
want to jump to there after this lecture, but what you are going to do is this follicle is going swell and swell and swell,
and out comes the secondary oocyte. So, that’s that situation where they’re still doubled. Then, just as a little aside,
the follicle turns into this thing called the corpus luteum, which those of you familiar with the menstrual cycle know all
about. That’s another story. All right, but anyway, here we go.
So, there’s the secondary oocyte, let’s take a look at it. Where’s the polar body? It’s gone, forget it; it’s little. All right,
but look, what’s going to happen next? Let’s say fertilization occurs. If fertilization occurs, an interesting thing
happens. Meiosis gets finished and we get the structure called an ovum, Latin for egg, but guess what? In that
second division you get still another polar body.
So, once again, let’s see what happens. All right, so we’ll go through the 2N thing. Primary oocyte – we are going to
go through our first meiotic division, polar body. Polar body actually, they probably, before they degrade will in turn
split, but that’s not important, because that polar body is history, it is of no use. But now, this thing here, with its
double chromosomes, will now, if fertilization occurs – that is a sperm, if fertilization occurs, what’s going to happen
next, is we’re going to get that and that. This will be N. If fertilization doesn’t occur, this doesn’t happen.
I want to ask you a question. Why? Why does a female pop off these polar bodies as useless bits of genetic
information and re-absorb them when, indeed, she may in fact, I mean if we’re talking about natural selection, wouldn’t
it be selected to have more eggs, and that might propagate your genes even better? Because if you pop a bunch of
eggs out, well you are much more likely to be fertilized right? Trade off time. And there are lots of trade offs in
evolution.
I have a question for you. What has to happen to this zygote, this fertilized egg? Now, there’s a zygote, and three
polar bodies, or excuse me an ovum and three polar bodies. Here’s, perhaps, what four equal eggs would look like.
Which one is more likely to contain enough material to nourish a zygote? That’s your answer. See what that egg has
done, is by preserving cytoplasm, it has allowed the zygote nourishment for the first stages of development, and as
you know in humans, that eventually goes away, because we hook up with an umbilical cord and a placenta, but until
that time, until that zygote has a source of nutrients or that embryo has a source of nutrients, a cytoplasm of the egg.
And so you see, oogenesis is very different than spermatogenesis, but sperm don’t care. Eggs, sadly being the
mother, have to nourish their child much longer than the genetic carrier of the sperm.