Hello, Dr Aizaz here back with another visual lecture on the topic of Oogenesis. This is
a bit complicated topic and I will make it very simple for you but before we discuss
oogenesis let me tell you something regarding the previous lecture in the last lecture when
we were discussing spermatogenesis I asked a question from you guys and the question
was that you know in seminiferous tubules there are blood there is blood sertoli barrier
or blood testes barrier, Right, which prevents the developing sperm from the hazards of,
protects it from the hazards of immune system. Right, but when the sperm comes into epididymis
what protects the sperm here in Epididymis? Well, the answer is simple that here we have
another barrier and that is blood epididymis barrier. Right, in epididymis also the epithelial
cells are closely packed together but that is not a perfect barrier like a blood sertoli
barrier. Also here these epithelial cells, they secrete special factors that locally
suppress the immune system inside the Epididymis. I will, I won't be discussing the details
and details if you want to know the details here is an article that discuss it beautifully.
Right, I will share the link in description. Now let's talk about Oogenesis. [Music] Right,
you know we are all formed from Zygote, a single cell zygote single diploid cell zygote
and zygote is formed by process of fertilization by the process of fusion of sperm and ovum.
Right, sperm you know also contains 23 chromosomes and ovum also contains 23 chromosomes and
they Combine, they fuse to form zygote that has 46 chromosomes and each of our cell in
our body it contains 46 chromosomes. Right, so you know sperm and one because they contains
half, they contain half the number of chromosomes that is 23 chromosomes they have to undergo
reduction division and reduction division is meiosis. Right, and the process of formation
of sperm are ovum, these are called gametes, right, so process of formation of gametes
that is sperm or ovum it is called Gametogenesis. It is a generalized term and if we talk about
formation of male gametes that is sperm it is called spermatogenesis and if we talk about
formation of female gametes or ovum that is called oogenesis. So now we are discussing
oogenesis. So what is oogenesis? oogenesis means formation of female gametes or formation
of ovum. Right, as we have previously discussed that the process of spermatogenesis it starts
with the start of Puberty, right, but females are different. Even before a female is born
she is worried about creating her future Baby, about preparing for the future baby. Of course,
she cannot create the baby at this stage I mean she is worried about preparing for the
future Babies. so the process of oogenesis, it starts before a female is born. Right,
it starts before a female is born. In today's lecture we will discuss what happens in the
ovary of a female in the developing ovary of a female before she is born. Right, so
here is a schematic diagram of ovary of an unborn female baby and as we have discussed
with regards to the spermatogenesis, their primordial germ cells these are very, these
are quite undifferentiated cells. They migrate from the primitive streak all the way through
yolk sac ultimately reaching either the testes or the ovary depending upon the gender of
Baby, developing baby. Right, so here because we are discussing the female baby and here
is the ovary in which we have primordial germ cells and it is not a single primordial gem
cell we have lots of primordial Germ cells, rather millions of primordial germ cells and
they continue to divide, they continue to undergo mitosis and they increase their population
then they become differentiated into Oogonia. Right, actually primordial germ cells they
can Form, they are totally undifferentiated cell they can either form Oogonia or they
can form spermatogonia. Right, and in this environment, in this environment of ovary
they undergo a differentiation such that they form Oogonia. Of course, they do not form
Spermatogonia. In males they form spermatogonia here they form Oogonia. Right, of course,
the ultimate motive is to undergo meiosis. Right, but primordial germ cells cannot directly
undergo meiosis. Neither these oogonia can directly undergo meiosis. They do not have
Equipment, they do not have enzymes and other materials to undergo Meiosis. Right, they
are converted into a cell that is specialized for meiosis. Right, that is well equipped
for the Things, for the machinery, for the enzymes that are required for meiosis and
that cell in females is called primary oocyte, in males it is primary Spermatocyte. Here
it is primary oocyte and this primary oocyte is now ready to undergo Meiosis. Well, it
is not this cartoon so let's replace it with a cell, so it is primary oocyte. so oogonia
it divides and through a multi-step process it become more specialized it grows in size
and it has now become primary oocyte. And now it can undergo Meiosis. Right, but you
must be knowing that with the process of meiosis there is genetic recombination that occurs
due to crossing over, what happens that the cell that will be formed it will contain the
nucleotide sequence that will be different from all of, all other cells of the body.
Right, because this nucleotide sequence will be different our immune system it may consider
this newly formed cell as a foreign cell. Right, it may consider it as an enemy so it
may start attacking that cell and to prevent it from the hazards of this immune system
it is surrounded by a layer of flattened cell, these this layer of flattened cell it forms
from the surface epithelium of developing ovary, the cells from surface epithelium of
developing ovary, it surrounds these, this primary oocyte to form a protective layer
and now this guarded primary oocyte that is guarded and well protected by this layer of
cell it is now called primordial follicle. And This, these cells are called follicular
cells so these are follicular cell and this is primary oocyte and this structure as a
whole is called primordial follicle. Are you getting the point? That we have, we still
have primary oocyte but this structure as a whole is called primordial follicle And
now it is very happy that now it is protected from the hazards of immune system as well
as it is very ready to undergo Meiosis, so it will start meiosis one and what is the
first phase of meiosis? Prophase, so it will start with prophase and now I�m not gonna
go into details of Meiosis but let's briefly discuss what happens that we have 23 maternal
chromosomes and 23 paternal chromosomes total we have 46 chromosomes and these are homologous
pairs. Right, we have two, two lectures on the topic of meiosis one is generally in one
lecture, we generally talk about meiosis and in the other lecture I have discussed prophase
one in detail you can watch that lecture as well. Right, so here we have these homologous
pairs of chromosome and I am showing only one here so this is prophase one first is
the Leptotene stage and then they come to close together lying on top of each other
and this pairing, this lying on top of each other that is zygotene phase and then synaptonemal
complex forms and then they cross over, they exchange their part of genetic material in
the Pachytene phase. Right, crossing over and then after doing this nasty business they
separate from each other. Right, and after doing this nasty business they are arrested
here. Right, what I mean by being arrested, I simply mean that this process stops here.
Right, it does not go further. Right, so what they say is that this primary oocyte it is
arrested in Diplotene phase of prophase one of meiosis. At Least, remember that it is
arrested in prophase one of Meiosis. Right, and this arrest at Diplotene phase is fancily
called as Dictyotene phase. Don�t get confused of this word Dictyotene, it just means that
there is resting diplotine phase. Right, so in this dictyotene or resting diplotine phase
The DNA repair is done. Right, and it remains there till Puberty. It does not go further
from here. Right, and to protect this nucleus for so long, to protect this nucleus from
DNA damage it is swollen it becomes engulfed with water, it becomes filled with water.
And this large swollen nucleus it is called germinal vesicle or germinal vesicle. I don't
know how this swollen thing protects the DNA damage. but it is believed, scientists believe
that because of this the DNA somehow remain Protected, maybe from oxidative stress. So,
let's recap what happens? And all this thing is happening before the female is born so
primordial germ cells they are differentiated into oogonia and oogonia are further differentiated
into what is this primary spermatocyte, primary Oocyte, not spermatocyte, so primary oocyte
and this primary oocyte is surrounded by what is this follicular cells and this structure
as a whole is called primordial follicle. Follicular cells they provide nutritive factors
to this and protects it, protects the oocyte from the hazards of immune system. Right,
it has some other functions as well we will not discuss it into detail. Right, then what
happens that it starts the prophase one and it remains arrested in the Diplotene phase
and the nucleus is swollen and that is called germinal vesicle. Clear up till now? Here
I have a question that why a female baby do not form a complete, not a complete, lots
of complete mature ovum at this Stage? I mean she can form lots of mature ova at this stage
before she is born and After as the puberty arrives she can release an ovum one by one
while this does not Happen? The reason is that an ovum is one of the finest cells that
a female Produces, it is the largest cell of the body and it contains lots of cytoplasm
and lots of organelles why so? Because you can see that a sperm it contains very little
cytoplasm so almost all of the cytoplasm and all, all the organelles these are contributed
to the embryo by ovum, by the female. Right, the sperm it only provides nucleus but female
ovum it not only provides nucleus to the baby but also cytoplasm and other organelles especially
Mitochondria. Right, so it first of all it needs to accumulate lots of cytoplasm and
organelles so that is why its size is much larger and that is not the only reason because
it's that its size is larger I will discuss in next lecture that why ovum is much bigger
as compared to Sperm. Right, so number one reason because ovum is very large in size
I don't think our developing ovary of a baby can accumulate a lot of mature ova I don't
think there could be enough space to accommodate that number one, secondly the formation of
mature ovum it is a difficult business it needs a complex interplay of different hormonal
and local factors which can only be provided by a mature person and that can only be provided
as the puberty rises. Right, so this was the developmental phases of Oogenesis before birth.
Right, now I have a question that how this primary oocyte, how it remains arrested in
meiosis why it does not resume the meiosis. Well, here you know we have these follicular
cells and they release certain factors, certain substances that prevent these oocyte from
resuming, the maturation inhibitor, so follicular cells produce oocyte maturation inhibitor
that keep these oocyte arrested in meiosis one these are like a policeman that keep them
arrested in Meiosis. Right, now all this business of preparing for the future baby beforehand
it reminds me of our subcontinent mothers that when a guest has to come they prepare
the food beforehand and make it half done they do not completely cook the food they
make it half done and then they preserve the food. Right, sometimes they preserve it by
adding a preservative or by submerging if they want to preserve it longer they submerge
it into a lots of salt water and that is what happens with the nucleus that is germinal
vesicle is Formed. Right, what happens that it is swollen, it is enlarged it is engorged,
it becomes larger in size to protect it from Spoiling, protecting it from being damaged.
Right, and of course when the guest Arrives, these mothers, they can continue that they
can resume the cooking process and then they can finally prepare the food in a very short
duration. A lot of these primordial follicles, they die before the puberty arrives, arrives
and those that are left behind they form the mature ovum but they do not start forming
the mature ovum all at once, they form it in cycles and we will discuss that process
in next lecture. Thank you so much for watching this Video, please subscribe to my channel
please give the thumbs up and please support me on patreon. Also please give your precious
comments regarding this video. Yes, you have a question? Yes, it means that once I was
a part of my maternal Grandmother? Yes, that is a good point, you can say that once you
was, you were a part of your maternal grandmother. How come? You let's suppose that this is the
ovary of your mother when she was not born. Right, this is ovary of your mother when she
was not born and when she was in her mother's womb, right, of course, her mother is your
maternal grandmother and let's suppose this is that lucky Oocyte, primary oocyte that
later form you. Right, so this was ovum precursor it was once inside your maternal grandmother.