so we have this primary oocyte that is arrested in diplotene phase of prophase one of Meiosis.
Right, and in the middle cycle and let's search i have discussed these things in the previous
lecture if you do not understand please watch that lecture, right, so in the mid of cycle
what happens that there is LH surge and this LH surge, it causes the release of this Arrest.
Right, this release of arrest Happens, right, and this primary oocyte it resumes the meiosis
one. Right, and it completes the meiosis one. Right, and as the meiosis one is completed
it will divide into two daughter cells, Clear, but this does not happen like this. It undergo,
it undergoes a special type of meiosis such that that one of the daughter cell it gets
all the cytoplasm and Organelles, Right, and the other daughter cell it gets very little
to no cytoplasm. Right, this, this oocyte that is formed from primary oocyte and now
because it has completed meiosis one it is now called secondary oocyte. This oocyte that
is, that has got all the cytoplasm, that has stolen all the cytoplasm and very little is
left for its sibling it is called secondary oocyte and the other one, the other cell is
polar body. You know it has completed meiosis one, it then starts meiosis 2 and then arrest
at metaphase 2. Right, we will discuss that later in the other lecture of oogenesis. So
this is polar body, so you know how polar body forms. let's look it in a simple diagram.
So, here we have primary oocyte when the LH surge happens it is converted into this secondary
Oocyte. Right, and the polar body that consists of very little cytoplasm. Now here that's
how polar body forms but my question is that why this selfish behaviour of oocyte at the
first place? Why there is that selfish behaviour that it has got all the cytoplasm and it has
left very little cytoplasm for this polar body? So my question is that what is the real
significance of this polar body? So, what happens that it is released, ovulation occurs
Right, and it, it comes this secondary oocyte. You know it is still secondary oocyte, it
is not a mature ovum. Right, So, now what happens here let's suppose a sperm comes,
right, and the sperm comes, and when the sperm comes, it attaches to sperm receptor on zona
pellucida and then it quick, it quickly completes the meiosis 2, it was arrested in metaphase
2. So it quickly completes the meiosis 2, Right, as it completes the meiosis 2 again
it is divided into two daughter cells and again one cell gets very little cytoplasm
and other cell gets almost all the cytoplasm so this is the second polar body that is Formed.
So, this is, this is now not secondary oocyte it is now Ootid, it is ported just likes spermatid
it is now ootid. This ootid then gradually differentiates into mature sperm, right, sorry
not sperm it is it gets differentiated into mature ovum. Right, so in this diagram again
this is the secondary oocyte as the sperm comes and attaches with zona pellucida on
its receptors on zoona pellucida it is converted into ootid and second polar body so what it
is later converted into mature ovum and second polar body. How many polar bodies are formed?
Yes, four. four polar bodies here we have two polar bodies so it is actually controversial
that how many polar bodies are formed it is said that usually two polar bodies are formed.
Right, first polar body due to, due to meiosis one here is meiosis one and the second polar
body due to meiosis Two. Right, so two polar bodies are formed and these are formed due
to unequal distribution of Cytoplasm. I will discuss the behaviour later. Right, so but
usually two are formed but sometimes this polar body it does not get disintegrated at
this, at this time and the cytoplasm, and cytoplasm and organelles may be present in
this polar body and sometimes this polar body can also undergo meiosis 2. Right, and it
can form, it can be divided into two polar bodies or two daughter cells. So, we can form
then three polar bodies. So usually how many polar bodies are Formed? Two, but three polar
bodies can also be formed not four. You know this is converted into these two and now there
is no existence of this it is converted to these two so total usually two polar bodies
sometimes three polar bodies can be formed. Now the question is that why unequal distribution
of cytoplasm is there? Can you answer this question? Yeah, you cannot I think so let
me answer this question. One reason is very simple that it is a necessary evil for the
sake of baby. You know mothers even at this stage care for their baby too much. Right,
you know the lazy sperm they bring very little cytoplasm, actually I, I think, I should not
call them lazy there is a reason for that you know sperm has to travel through that
harsh pathway of female reproductive tract so for that reason they have to keep the cytoplasm
very little I don't think it's their Fault. Right, so sperm bring very little cytoplasm
so almost all the cytoplasm towards the embryo and organelles, almost all the cytoplasm and
organelles is contributed to the embryo by ovum. Right, because sperm do not, almost
do not contribute any cytoplasm. One reason is this so that is why it is taking all the
cytoplasm. Right, another reason and let's discuss the other reason. So let's suppose
it has now completed the meiosis 2, it, it has now fused with, with this sperm and zygote
is formed. Right, now, Right, so we have zygote which is formed by fusion of male and female
nucleus and cytoplasm is, cytoplasm is mainly of female the ovum. Right, so this is zygote
it is now diploid cell, it has 23 chromosomes from ovum and 23 chromosomes from Sperm. Right,
so you know it is still enclosed by zona pellucida. Right, now it will divide by the process of
mitosis. Right, it will divide by the process of mitosis into two daughter cells and as
it is developing further it will also continue to move toward uterus where it has to implant.
Now, you see something special here that as it is dividing what happens that it, the two
daughter cells that are formed they have, they are half the size of their parent cell.
Right, the nucleus, the cytoplasm is equally divided among these daughter cells that 50%
cytoplasm is taken by this cell and 50% by this Cell. Normally, what happens that let's
suppose this is a parent cell, normally if it divides the daughter cell, that will be
formed, they will be almost of the same size as parent. Why? Because they will accumulate
more cytoplasm later on but here they cannot accumulate more cytoplasm. Why they cannot?
Reason is simple because they are still enclosed by zona pellucida, they cannot create more
cytoplasm, they cannot create more organelles. Right, so that cytoplasm, that was initially
present in the mature ovum that has to be Divided. Right, so it divides further and
again you know, that initial cytoplasm is divided into four cells and then eight cells
and then 16 cells. Right, this is now called morula. Morula means, that is it looks like
bunch of grapes so it is called Morula. Right, then what happens that it comes into uterus
and because uterus is rich in fluid, right, uterus is rich in fluid so fluid can enter
into zona pellucida. Zona pellucida is not completely Impermeable, it allows the fluid
to enter inside and as the fluid enters inside, the zona pellucida stretch, it stretches,
Right, and blastocyst is now formed as it more and more fluid enters and it keeps on
stretching this zona pellucida it sorts of ruptures that is like hatching of an egg,
right, so it ruptures and now it can continue normal mitosis now these cells can grow and
accumulate more cytoplasm just like any other cell. But before that this mitosis, that was
occurring this is a special type of mitosis in which the, the daughter cells have 50 percent
size of their parent cell this type of mitosis or this type of special cell division is called
Cleavage. Are you getting the point? Now, the reason is simple that why it is having
lots of cytoplasm at first place why it has left no stone unturned to accumulate as much
cytoplasm as possible, you know Sperm, not sperm, ovum, mature ovum is largest cell of
the female body. Right, the reason is that imagine if there was not enough cytoplasm
here if the size of this cell was small and if cell, cell was not having enough cytoplasm
what would happen that maybe it can undergo two or three divisions after that no cytoplasm
would have left and now after that it could not have completed further cell division so
to allow this cleavage process to proceed further it needs a lots of cytoplasm at first
place. This is the one reason, and that is why it steals the cytoplasm of its Sibling.
What is its sibling? Polar body. So, two reason you have learned, one reason is that cytoplasm,
sperm brings very little Cytoplasm, so all the cytoplasm and organelles are contributed
by ovum, the second reason that why it needs Cytoplasm, because it has to undergo cleavage,
it has to undergo repeated cycles of cleavage till it reaches the uterus where zona pellucida
ruptures, and then it can stop undergoing cleavage and continue normal mitosis. So,
two reasons, third very important Reason, third reason is that let's suppose if equal
cytoplasm was Distributed, right, then how many oocyte we would have had at this Point?
One, two, three, four not four this will also have divided so one, Two, Three, four, yes,
four you are right. I was Wrong. So, total we have one, two, three, four so total we
would have how many Ovums? Four of them, but nature does not work like that. You know usually
a human female, usually she produces single best ovum, I think females are Perfectionist,
they produce a single best ovum each month and hope that it gets fertilized. Right, opposite
to that male creates millions of sperm that are released with each ejaculate and not all
of the sperm are perfect, about 10 percent of sperms are created faulty but fortunately
these faulty sperm they cannot move so they remain behind and the perfect sperm is able
to bind with the perfect ovum. Right, so right now, if you are sitting here listening to,
watching this video Congratulations! Why? Because you are produced from the perfect
most of ovum that fertilize, that was fertilized by the perfect most, perfect sperm. So, nature
tries to create the best Thing. Thank you so much for watching this video. If you like
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