Hello! I am Dr Aizaz from medicovisual.com back with another visual lecture. In today's lecture
we will talk about the events that occur in first week of development after fertilization and these
includes, we will discuss what is cleavage? what is, how blastula forms and some related concepts
that are related to these topics. Along with that we will finally also discuss what is
implantation and how implantation occurs. So, let's start the lecture. So, previously we had
discussed that how the with the fusion of male and female gamete that is with the fusion of
sperm and ovum a single celled embryo that was formed was called zygote and zygote it consists of
male and female pro nucleus and it is still surrounded by zona pellucida and corona radiata.
If you want to see it in 3D so here is that in 3D. As you can see this is the corona Radiata,
this blue cells and this is translucent, this yellowish translucent zona pellucida. Right,
and here is the zygote, here is that, that membrane of zygote. Initially it was actually
the membrane of oocyte but now it is the cell membrane of zygote and inside it are the male
pro nuclei and female pro nuclei. Clear? So, this was zygote and as you know zygote consists of
23 chromosomes from male and 23 from female but only single copies were there initially. Right,
so 23 chromatids were transmitted into the ovum which was also having 23 chromatids
but these are no chromatids to form the full chromosome. They have to undergo DNA replication
and so they do undergo DNA replication that is S phase of interphase. Right, and then now they
have to divide, now they have to undergo a cycle of mitosis a cycle of cell division so that they
can be divided into two daughter cells. Right, so the chromosomes that are now formed they Arrange,
they are arranged on the equator and now mi, in during mitosis these chromatids will separate
from each other and they will go to the two daughter cells. Now, as they are still enclosed
by zona pellucida they can not grow further. Right, these cells they cannot grow further and
that is why the size of this, these two daughter cells it is half the size of that of parent cell.
I mean the size of these two daughter cells by the way these are called blastomeres the size of two,
these two daughter cells is about 50% of that of zygote. Right, and this type of cell division,
this type of, this is a specialized type of mitosis in which the size become half with each
subsequent division. This is called cleavage. Why it is called cleavage because if you look
carefully it looks like someone has cleaved the zygote with a knife, like someone has cleaved the
zygote into two parts. Right, so after cleavage these two daughter cells are formed. Right,
again they will undergo DNA replication in between before starting the new cycle
of cell division, before starting new cleavage cycle they will have to undergo
DNA replication to form the complete chromosome. Of course, they were initially chromatids only.
Now normally what happens that let's suppose now we are talking about normal mitosis not
about cleavage. So, this is, not this cell, let's suppose it has to undergo mitosis so
what will happen that during M phase, after M phase it will be divided into two daughter cells.
Initially, of course, these two daughter cells will be half the size of parent cell. Right,
and of course chromatids will be separated but initially they will
just like cleavage they will be half the size of parent cell but after they have completed mitosis
before they will start a new cycle of mitosis they will, they will rest and they will grow,
they will increase their size and this is called G1 phase of cell cycle and
first they will grow in size again because they will grow in size and then they will
replicate their DNA that is S phase of cell cycle and then further they will go, grow.
Right, so, now after G1, growth one phase followed by S phase and then growth two G2 phase
finally these daughter cells that are formed they are as big as huge as the parent cell
almost the size of the parent cell. This is, this is what happens normally during mitosis
but if we talk about cleavage again after they undergo mitosis because they are
limited by the presence of, by the enclosure of zona pellucida they cannot undergo G1 and G2 phase
so they will only undergo S phase and then they will start another cycle of mitosis or
modified mitosis which we call Cleavage. So, in cleavage two things are happening. What, what
is the difference between cleavage and mitosis? There are two differences number one there is
no growth phase G1 and G2 phase of cell cycle in cleavage number one and number two because there
is no growth phase in cell cycle during cleavage. It means the size of, size of daughter cell with
each subsequent division will become smaller and smaller that is 50 percent of the parent cell. So,
again this is the zygote and as it is cleaved into two cells and subsequent four cells and
eight cells and so on it continue to move towards the uterine cavity by the ciliary action of,
ciliary action of cilia present inside the fallopian tube or uterine tube and with each
cell division, with each cleavage the daughter cells that are formed they are called Blastomeres.
Right, so, early embryo, early embryonic cells they are termed as blastomeres and as you know
it was initially surrounded by corona radiata as well but within about two days of fertilization
at two cell stage or sometimes at four cell stage corona radiata is destroyed but it, it is still
surrounded by zona pellucida. so, it continued to move forward and it continues to undergo cleavage.
Now the, these early embryonic cells from zygote up to about eight cell stage these
early embryonic cells they have capability to form a complete organism. It is, it has come,
it has capability to form each and every cell of the embryo,
each and every cell of the embryo proper the body as well as the placenta as well. Right,
so, these cells up to about eight cell stage sometime beyond that as well they maintain that
capability. So, up to about eight cell stage these early embryonic cells they are called totipotent
cells. Why they are called totipotent cells? Because they have potential to form a totality
of an embryo, each and every cell of an embryo including the extra embryonic tissues I mean the,
the placenta, the membranes and everything. Right, so that is an important concept Here.
Now at this stage if we remove one of one of these blastomeres it can form another organism.
Right, I mean another and this is the basis of formation of identical twins
at two cell stage or four cell stage. If the embryo separates from each other it can form
two identical human beings and these are called identical twins. Why it can form? Because it has
potential to form the totality of an embryo, they have potential, they are called totipotent cells.
So, as it moves forward what happens that these cells they are squeezed inside the zona pellucida,
they are squeezed inside the limited space enclosed by zona pellucida. Right, so, as
they are squeezed they are, they are you can say pushed against each other and they are compacted
against each other in a closed space, they are compacted and it is not just that but also
some special cell to cell communication starts in between them. Right, special channels are formed,
right, desmosomes are there e cadherin and different substances are there to connect the
cell, to cause cell to cell communication. So, cells communicate closely with each other to,
to plan for the future embryo, for the future baby. Right, and this is called compaction.
And at about 16th cell stage this early embryo it starts to look like mulberry. You know mulberry
fruit. So, because of its appearance that resembles the mulberry fruit here is the 3D
simulation that I created in blender 3D. Because of this resemblance it is named as Morula. So,
early embryo at about 16 cells stage some books even consider the early embryo as morula at about
12 cell stage but other consider at about 16th cell Stage. So, this is a morula and it continues
to grow and undergo cleavage and finally it comes into the uterine cavity and as we have discussed
in the lecture of female menstrual cycle that there are special uterine glands here that secrete
uterine milk a nutritious fluid is secreted during the secretory phase of or luteal phase of
of this cycle, menstrual cycle. So, due to this fluid what happens that two things happen first
the this early embryo it gets nutrition in the, in the uterus but my question is that how it was
getting the nutrition in the uterine cavity, in the uterine tube? Yes! what do you Think?
Well! that is simple human's human embryo, human zygote do not accumulate
much yolk as in bird and small cells their egg contains sufficient yolk to feed the embryo
but still human zygote consists of enough yolk, the little amount of yolk or you,
you should call it not yolk but enough amount of nutrition in the zygote that it can survive some
early cell divisions up to morula and some further cell stages till it reaches the uterine cavity.
And as it reaches the uterine cavity its inherent yolk analogue I can call it yolk analogue as its
inherent food or yolk analogue runs out. Now it can get nutrition from these, from this uterine
milk. Right, and it gets nutrition from uterine milk. Another thing that is happening here is that
the cells in the outer layer of, of this embryo, this early embryo these cells they have special
sodium channels and with these sodium channels they pick up sodium from this uterine fluid and
take the sodium inside. As they pick up the sodium and take it inside water will follow. Right,
and as the water will follow what will happen that a special, special cavity will be formed. Right,
and now zona pellucida will disintegrate. How zona pellucida disintegrated? Actually
we are not sure how it happens but there are three factors and at least one of this factor
or maybe more than one of these factors contributes disintegration of zona pellucida
what are those factors let's discuss now? As this the, as it becomes filled with fluid there
is a physical pressure that is created inside and this physical pressure may causes the, may cause
the weakening of zona pellucida which leads to that characteristic Hatching, number one thing.
Number two is that these outer cells they create some special enzymes and there is strong evidence
of this hypothesis that it creates specific enzymes that weaken the zona pellucida and,
and finally some, some substances produced by the uterus of mother they also probably helps in
disintegration of zona pellucida. So, now finally zona pellucida disintegrates and now cleavage
stops and now normal mitosis will start. right, now as they can grow normally, normal mitosis will
start and it will grow further and now this cavity that is formed this cavity is named as blastocele
and these cells that are here, that are lying here these are now called inner cell mass
or they are called embryoblast and these cells that are in the outer layer these are called
outer cell mass or they are also called Trophoblasts. Now why there is this need of
demarcation? Actually these two cells varieties they have quite different functions
probably during compaction they might have decided that these inner cell mass or embryo blast it will
form the actual body of the upcoming human being, upcoming human being to be formed human being.
Right, so they will form the the body of baby, the whole body of baby. Right, and these outer cell
mass or this trophoblast it will arrange the, it will, it will arrange for the nutritious needs of
the baby. What I mean by this is that it will form placenta and related structure and this will form
the embryo proper the, the major parts I mean the body of, of the baby. Right, so, here is another
important concept that what is pluripotent? Now these two cells are pluripotent they are
different from totipotent they have potential to form variety of different cells of the body
but they cannot form totality of an embryo, they cannot form each and every cell of an embryo.
Well! how come these inner cell mass although they can form complete organs or everything that
is required to form the body, every cell of the body but it cannot form placenta. Similarly these
trophoblastic cells they can form each and every cell of placenta but they cannot form the cells of
embryo. So, now they are not totipotent now they are Pluripotent. So, they have become slightly
more differentiated they, their function is now slightly become more specialized. Right,
now they have lost that toti potency and they have gained the pluri potency at this stage. They have,
there are two poles that are established or two sides you can simply call sides of the embryo.
Right, what are these two sides? The this side on which inner cell mass is there, this pole, this
side is called embryonic pole. Right, and opposite to that side is the, this ab embryonic pole.
Right, now at this stage the embryo which was initially morula it is now named as blastocyst.
Right, now it is time for this blastocyst to implant into the endometrium of his or her mother.