so in the previous lecture we had discussed
that a female baby, in the, before the female
baby is born in her ovary she has already
created lots of primary oocytes and these
primary oocytes, they are arrested in meiosis
one in the Diplotene phase of prophase one
of Meiosis. So, these poor primary oocytes
they were arrested in the Diplotene phase
of meiosis one and they are surrounded by
the layer of, a layer of these flattened cells
and this layer of flattened cells, that these
Cells, they are called what are they called?
Follicular cells, and this structure as a
whole this layer along with these primary
oocytes this complete structure is called
primordial follicle. So The, this unborn,
this fetus, this female fetus she had formed
a lots of about four lakhs, I think more than
that lots of I’m not good at remembering
numbers by the way so she formed lots of these
primordial Follicles. Right, and they were
arrested in meiosis One. Right, and as she
is born and they remain arrested before until
the puberty. Right, and as she is born and
then before the puberty starts a lot of these
primordial follicles they die out waiting
to be resumed. Right, they undergo atresia.
Yes, you have a question? what is atresia?
Atresia, it is just a fancy word of apoptosis
of these cells right apoptosis of these follicles
apoptosis means programmed cell death so it
is nothing to be worried about it is just
apoptosis. Right, so this unborn baby, this
fetus she has now become a beautiful teenage
Girl. Right, and puberty hasn't started yet
puberty is just gonna start now and here is
again the schematic diagram of her ovary and
she is having about 40,000 primordial follicles
at this stage with primary oocyte arrested
in meiosis one. All other primordial follicles
they have died now. So we will discuss about
this one primordial follicle right now and
this primordial follicle it can now grow in
size, its follicles it can grow in size it
can increase in size so these squamous or
flattened cells they have now become low columnar
or cuboidal, right, they are still follicular
cells but they have become low columnar to
cuboidal. Right, now this primordial follicle
is no more a primordial follicle now it is
called primary follicle and because it is
still surrounded by a single layer of these
follicle cells it is called unilamellar primary
follicle. Uni means single lamellar mean Layer,
Unilamellar primary follicle, primordial follicle
is also Unilamellar but because it is obvious
so we don't call it Unilamellar. Right, so
here there are two processes that are going
on side by side there is the growth of this
oocyte, growth and development of this oocyte
that is called oogenesis and along with that
there is growth and development of these follicular
cells that is called Folliculogenesis. Although
this is the topic of oogenesis but we cannot
discuss the oogenesis without discussing Folliculogenesis,
right, some books discuss folliculagenesis
with the female reproductive cycle but I am
discussing it here with this because it is
very important and crucial to understand these
process side by side. Right, so this is primary
follicle that is Unilamellar with the primary
oocyte and right now it does not have, it
cannot recognize its mate the, sperm it cannot
recognize and bind the sperm so this oocyte
as well as these follicular cells they secrete
between them a glycoprotein coat. Right, and
this glycoprotein coat it harbours special
receptors, special species, this species specific
sperm receptors right and this glycoprotein
coat it is translucent and we call it zona
pellucida, pellucida mean transparent. So,
this is zona pellucida. Right, now talking
about follicular cells these follicular cells
although they have grown in size but they
cannot divide, they cannot Replicate, they
can not proliferate without the orders from
the higher Centres. So, from the hypothalamus
of this teenage girl gnrh is released. what
is gnrh, gonadotropin releasing hormone. So,
gonadotropin releasing factor or gonadotropin
releasing hormone is released from her hypothalamus
and it passes to the interior pituitary and
here it will stimulate certain cells to cause
production and release of two substances to
hormones FSH and LH. FSH is follicle stimulating
hormone and LH is luteinizing hormone. so,
these FSH and LH, mainly FSH acts upon these
follicles and it orders these follicles to
divide, to replicate, Right, to proliferate.
So, because of this the real name of this
FSH hormone is follicle stimulating hormone
so it stimulates the follicles so it is called
follicle stimulating hormone so, under the
influence of FSH mainly and to some extent
LH what happens that they replicate, follicular
cells replicate and now it becomes multilamellar.
Remember, 15 to 20 primary follicles start
growing under the influence of FSH, and FSH
and LH each month. Right, but at the end of
the month only one remains that is the dominant
Follicle we will discuss the details in next
lecture, the lecture of female reproductive
cycles but you just have to remember this
important thing. Right, so primary follicle
or Multilamellar primary follicle you know
there are certain connective tissue cells
here in the ovary, interstitial cells or connective
tissue cells, these connective tissue cells
they surround This. what they do they surround
this follicle and they form two layers. They
form a casing. Right, they form a casing or
covering of this follicle and this covering
is called Theca this word theca it simply
means casing or covering so this theca consists
of two layers, the inner cellular layer that
is rich in this interstitial cells or they
are now called theca cells This is, this inner
layer is called theca Interna, so this is
theca interna. Right, it is more cellular
and this outer one it is less cellular and
more vascular it is Theca externa. So, theca
interna and theca externa these two layers
are formed. Right, now Clear? So, you know
in in males these theca cells, not theca cells,
in males these interstitial cells what they
are called? Yes, Leydig, Leydig cells in male
they are called Leydig cells here they are
not Leydig cells but they are theca cells
so you can say in a sense that Theca cells
are female counterpart of leydig cells and
because of their inherent nature what they
do in males by the way? They produce testosterone.
yes, in males they produce testosterone but
here they are also producing Testosterone.
You know nature cannot be changed, by nature
they are like those Leydig cells and they
Synthesize, they synthesize testosterone,
and they synthesize testosterone on the order
of what thing? LH, in males they are synthesizing
testosterone on on the orders of LH. L for
leydig cell L for LH, luteinizing hormon.
So, here also these, not Leydig cells they
are not called Leydig cells here they are
called theca cells here. Now they are synthesizing
under the order of LH, they are synthesizing
male androgen, right, testosterone and some
other androgens. Right, so, testosterone is
being synthesized by them but fortunately
they are present in close proximity to these
follicular Cells. So, these follicular cells
they pick up this testosterone and convert
it into estrogen, these follicular cells are
real girly cells they love Perfumes, the smell,
the Scent, like things and one of that is
Aromatase. what I mean by this is that these,
these follicular cells they contain a special
hormone or not hormone special enzyme called
aromatase, aromatase enzymes convert testosterone
into estrogen. Right, what causes this production
of aromatase enzyme in these follicular cells?
Yes, it's FSH. FSH causes this production
of aromatase and this aromatase converts the
testosterone that is synthesized by theca
cells that is picked up by these follicular
cells and it is converted into Estrogen. So,
what if I ask you that what's which cells
Synthesize Estrogen? Yes? what will you say
theca cells or follicular cells? Theca cells
produce testosterone which undergo processing
to produce and then they form estrogen. Yes,
that is processed by theca cells Theca cells
produce testosterone that is picked up and
fortunately that is picked up and processed
by these follicular cells to form estrogen
and that is the reason that these cells they
are present in close proximity to these follicular
cells. Right, so estrogen has many important
functions. It is female, it is major female
sex Hormone. Right, then what is progesterone?
A good Question. Progesterone is also female
sex hormone but I like to call it, rather
scientists like to call it pregnancy hormone.
P for progesterone P for pregnancy. Although,
it is also produced in non-pregnant females
but its major function is pregnancy support.
We will discuss Later, that progesterone,
it is very important for for pregnancy. It
maintains the pregnancy. Right, we will discuss
these things later. So, estrogen it imparts
primary and secondary sexual characteristics
to the female. So, here it from, here it diffuses
into blood to impart those characteristics
but it has some local functions here that
I will discuss just Now. But, here another
point that these follicular cells because
they are converting testosterone into estrogen
they are now full of granules of estrogen.
Right, these follicular cells they are now
full of estrogen granules that is why they
are at this stage they are now called granulosa
cells. Granulosa comes from the term granules
so follicular cells this term follicular cell
as well as granulosa cell it is usually used
interchangeably you can call them follicular
cell or you can call them granulosa cell but
I think at this stage it is more appropriate
at this stage and beyond it is more appropriate
to call these cells granulosa cells rather
than follicular cells at this stage you may
call it follicular Cells. Right, so granulosa
cells are formed and due to estrogen LH receptors
are also produced on these follicular or granulosa
cells. Right, so estrogen it causes the priming
of LH receptors on what is this granulosa
cells you know LH receptors they are usually,
before this time they were usually present
mainly on theca cells. Right, theca cells
for harbouring LH receptors but now these
due to the action of estrogen now these granulosa
cells they also start to harbour LH receptors.
We will discuss its significance Later, right,
and under the influence of FSH and to and
to some extent by the LH they grow further
now they have acquired many layers maybe up
to 8 to 10 layers. Right, they are now they
are still primary follicle and they are still
multilamellar along with that let's not forget
that primary oocyte that is still arrested
but this primary oocyte it grows in size,
it enlarges, it accumulates lots of cytoplasm
as well as lots of organelles and some nutrition
as well for the embryo for the baby. Right,
and due to actions of estrogen then lots of,
almost all these, almost all these granulosa
cells they, they have acquired, they are now
equipped with LH receptors. Right, now they
can also listen to orders of LH, previously
only theca cells can listen to them now these
cells can also listen to orders of LH. Clear
up till now? Now, along with that special
fluid is secreted by this follicular cells
we will discuss the details of fluid in next
lecture, right, and due to secretion of this
fluid small pockets of fluid form in between
them, right, and then these pocket at this
stage when these pockets of fluid are there
at this stage now this follicle is called
secondary follicle and this oocyte is secondary
oocyte? No, no it is still primary oocyte.
So, remember this is a secondary follicle
harbouring inside it primary oocyte. Right,
and these pockets they then coalesce to form
a single large cavity and the single large
cavity filled with follicular fluid it is
called antrum and it is still secondary follicle.
Now you remember these LH receptors, granulosa
cells they are happy because they can listen
to orders of LH now. At this stage, it is
now the mid of cycle, at this stage in the
mid of cycle there is sudden rise in level
of LH hormone. We will discuss why that happens
later, But, there is sudden rise in LH level
in mid of Cycle, during the mid of cycle and
this is called LH surge. So, but before that
let me tell you something that these cells
these follicular or granulosa cells that just
surround these, this follicle they are called
cumulus oophorus. What they are called cumulus
oophorus, cumulus means Cloud, oophorus mean
that they are surrounding this Oocyte. so,
it looks like a cloud that is surrounding
this ovum, developing ovum or oocyte, so it
is cumulus oophorus. So, I told you that there
is and this oocyte that is still arrested,
Right, now it is still arrested in meiosis
one, Diplotene stage and when there is LH
surge this oocyte is now released from the
Jail, it is now released from the arrest.
Right, it is now allowed to complete meiosis
one you know which cells caused this arrest
at the first place? These were these follicular
cells you didn't remember that these follicular
cells they were releasing oocyte maturation
inhibitor factors that causes this arrest,
that caused this arrest. Now what I think
of this whole this story that this LH it is
like lieutenant journal, it is like army chief
and these are little policemen, these are
little policemen that have arrested this oocyte
and now because they have this LH receptors
or special ears and now these policemen they
can listen to order of their army chief, they
can listen to order of this LH, lieutenant
journal. Right, they can listen to it as orders
and on the order of these they will, on the
orders of LH they will release the arrest.
Right, and now but why they why, why the LH
orders them to release the arrest because
now the follicles, follicle has grown to its
fullest as well as at this stage oocyte has
also grown to its fullest. Now it is no more
a kid, now it can complete the meiosis. Right,
now it is allowed to complete the meiosis
because it has now fully grown. You know I
told you that about 15 to 20 follicles start
growing but all those follicles that initially
started growing not all of them are fully
grown, not all of them started producing enough
estrogen to cause formation of enough LH receptors.
So, those cannot listen to order of LH and
thus, those follicles can not, those follicles
cannot complete, cannot resume Meiosis. Right,
so those follicles that cannot resume meiosis
they again die out by the process of follicle,
Follicular Atresia, and this dominant follicle
that is now fully grown that is independent
of FSH, now it will complete meiosis one.
Clear up till now? And what happened here
that it has completed meiosis one and it is
divided into Two. Right, but meiosis in oocyte
does not happen like this. Are you getting
the point? What happens actually? Here is
that one of the follicle, sorry one of the
daughter cells, after it has completed meiosis
one, one of the daughter cells it has all
the cytoplasm while the other daughter cell
it has very little cytoplasm. So there this
is the selfish behaviour of this developing
oocyte, that it has, it has stolen even the
cytoplasm and organelles of its sibling. Right,
now this primary oocyte is not primary oocyte
because it has completed meiosis one now it
is secondary oocyte. Now it is secondary oocyte
and this secondary oocyte, it quickly start
meiosis 2 as well but it is arrested in metaphase
2 and this little, this little cell with very
little Cytoplasm, almost no cytoplasm it is
called polar body. Now the question arises
here again that why this selfish behaviour
dear ovum, dear developing ovum? The answer
to this question is there are two reasons
for this. One reason is that it has to accumulate
all the cytoplasm for the sake of baby. It’s
a necessary evil here. You know that sperm,
that lazy sperm it is not bringing any cytoplasm,
so almost all the cytoplasm and organelles
they have to be contributed by Ovum. Right,
sperm do not contribute to Cytoplasm. Right,
it contributes to nucleus but not to cytoplasm
and mitochondria and other organelles as well.
So, reason is, so, because of this cytoplasm
is mainly contributed by Ovum, right, but
the question is why it accumulates lots of
cytoplasm at this stage I will answer this
question in the lecture of polar body. So,
you have to watch that lecture. Right, we
have separate lecture on that. So, secondary
oocyte, it has taken up, it has accumulated
all the cytoplasm and Organelles, completed
the meiosis one and started meiosis 2 but
it is now arrested in metaphase 2 along with
that a polar body is formed. Now, this fully
grown dominant follicle, now it is ready to
be released in uterine tube, in fallopian
tube, in the hope that it will get Fertilized.
Right, now this pre ovulatory, this pre ovulatory
follicle, right, this pre ovulatory follicle
at this stage or dominant follicle it is called
Graafian follicle or some people like to call
it tertiary follicle. There is nothing difficult
to understand here. It is a type of follicle,
secondary Follicle, that is now fully grown
that has completed meiosis 1 and started meiosis
2 and it is now ready to be released in uterine
tube by the process of Ovulation. Now we will,
we will not discuss how ovulation occurs we
will discuss that in later lecture but here
as it grows a further in size, here an elevation,
that is called Stigma, is formed here and
here this part of ovarian wall it ruptures
and it is Released. Right, so here it is released
into uterine tube this secondary oocyte, it
is released into uterine tube and you can
see it is surrounded by, here we have a polar
body and it is surrounded by what is this
zona pellucida and granulosa cells are also
there. Right, these granulosa cells they form
a special pattern here that they look like
a crown, so this, this is called corona Radiata.
Some people think that this is cumulus Oophorus,
my friends this is not cumulus Oophorous.
These cells, most of these cells they were
cumulus, they were forming that cumulus oophorus,
but now when they are released into uterine
tube now it is no more called cumulus oophorus,
now they are called corona radiata. Well,
in a true sense they, they were, those cells,
they were, those cells that are forming cumulus
oophorus, but right now they are called corona
radiata. Right, so this secondary oocyte.
Here it is due to the ciliary action of this
uterine tube or fallopian tube it comes into
Ampulla, ampulla of this fallopian tube. And
here they keep on waiting for the sperm to
come and as soon as the sperm come and attaches
with this sperm receptor on zona pellucida
it quickly completes its meiosis one, meiosis
two, sorry it quickly completes meiosis two
and forms the second polar body and then it
matures to form complete mature ovum and then
of course, fertilization will occur and it
will form zygote that we will discuss in lecture
of Fertilization. But what happens if sperm
never come then this poor secondary oocyte,
it is still Secondary oocyte, it keeps on
waiting for the sperm in the hope that maybe
the sperm will come but sadly when the sperm
do not come it dies out, maybe of depression
or I don't know what, but it dies out. Actually,
macrophages eat, eat them. Right, and this
sad story ends here and this remaining ruptured
follicle what it will do? It will accumulate
lots of fat globules inside it, cholesterol
inside it and it will start secreting Progesterone.
It will still secrete some amount of estrogen
but mainly at this stage after ovulation it
will mainly secrete, what is that, progesterone.
Right, and now it is called corpus luteum.
You know why LH is called luteinizing hormone?
LH causes ovulation, it should be called I
think, it should be called ovulation hormone.
But, but because it is called ovulation, this
ruptured follicle it is converted into this
yellow structure, that is called corpus luteum.
So, along with ovulation it is also causing
formation of corpus luteum it is converting
the ruptured follicle into corpus luteum.
If we can say it is Luteinizing, it is luteinizing
ruptured follicles, so that's why it is called
luteinizing hormone. Right, so this was about
oogenesis after puberty. And here the last
point is that if Oogenesis, if sperm do not
come and this, this secondary oocyte, it dies
out what will happen? That, there will be
shedding of uterine wall. We will discuss
in next lecture, but then another cycle will
start and again 15 to 20 new primary follicles
will start growing and this cycle will continue
and continue up till menopause. Right, let's
review all these things. So, this is, this
young, teenage girl which is, which is just
going to start puberty and her ovary consists
of lots of primordial follicles and primordial
follicles they consist of this oocyte, primary
oocyte surrounded by Primordial, surrounded
by this flattened layer of follicular cell.
So, follicular cell grows and they form primary
follicle along with a glycoprotein coat that
is formed zona pellucida that is formed. Right,
now it cannot replicate or proliferate, it
needs order from FSH and LH, and FSH and LH
they are released by the orders of gnrh gonadotropin
releasing hormones. So, these gonadotrophes,
FSH and LH they cause the growth of these
proliferation of these cells follicular cells
and it has, it has now become multi-layered
so it is primary follicle that is Multilamellar.
Right, then what happens that interstitial
cells surround it to form two layers or coverings
that is theca interna, that is more cellular
and theca Externa, that is more vascular and
less cellular. Cells under the order of LH,
synthesize testosterone which is picked up
by these follicular cells to form estrogen,
Now, because, they it is producing estrogen
so estrogen granules form in these follicular
cells and now these follicular cells at this
stage with these granules they are called
granulosa cells. estrogen has different functions
one of the function is that it causes priming
of LH receptors on these granulosa cells.
Now these granulosa cells can also listen
to order of lH and further growth and then
further growth and this primary follicle has
now lots of layers of what is this granulosa
cells then these follicular cells they secrete
pockets of follicular fluid and they coalesce
to form Antrum. And, now this primary follicle
is converted into secondary follicle. Right,
now there will be in the mid of cycle there
will be LH surge. Right, and due to LH surge,
what will happen that follicular cells will
or granulosa cell will listen to orders of
this LH hormone to release the arrest and
there will be resumption of meiosis One. Right,
they will complete the meiosis one and it,
it will now form secondary oocyte that is
now arrested in metaphase two. Right, and
along with that polar body will be formed
and now it is fully mature follicle. Now,
it is called Graafian follicle or some people
like to call it tertiary Follicle. Now, it
will be released into uterine tube by the
process of ovulation and now it is waiting
for the sperm to come and if the sperm comes
it will fertilize it and before that, before
fertilization it will complete the meiosis
2. Right, otherwise it may die out and it
is converted into corpus Luteum. Right, corpus
luteum secrets Progesterone and it maintains
the uterine wall. we will discuss in next
lecture. clear up till now? Thank you so much
for watching this video if you like this video
please press thumbs up and please subscribe
to my channel please give your precious comments
regarding this video and please support me
on Patreon. Thank you so much.
