This content details the crucial process of gastrulation during the third week of human embryonic development, where the bilaminar disc transforms into a trilaminar disc, establishing the three primary germ layers: ectoderm, mesoderm, and endoderm.
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right today we are going to talk about
during the third week right already in
previous videos we have watched that how
a fertilized ovum
fertilized oven convert into
cell stage four cell stage then how it
converts into
marula and blastocyst and then how the
Blastoise system plants into endometrium
of the uterus then we have discussed
then uh eventually how blastosis
inner Mass convert into biliminary disk
right by laminar disc so we have studied
the human development within the uterus
up to the by laminar disc today we will
discuss how a bioluminar disc during the
third week of the development
convert into Tri laminar disc let's go
back to the biliminal disk and then from
there we'll take up the lecture right
when we talk about the bilaminar disc
you must be remembering that why laminar
disc was having two layer of the cells
one layer
these are
this is epi blast
blast
this layer of the cell this epiblast and
under the epiblast there is another
layer of cells and this layer of cells
is yes I will make some unusual color
let's suppose this color
and this layer of cells is
is hypo
hypo blast
blast
this is hypo
hypo blast
blast
right this epiblast there is hyperblast
and both of them put together they make
bioluminar disc what they are making
bilaminar disc
that here there is
now epiblast and hyperblast right happy
happy blast
blast hypo
hypo blast
blast
right both of them are at biliminal disc
and then there are yeah there's a cavity
above the epiblast
yes this is a mini Arctic cavity and
below the
what is this
this is hyperblast
this is epiblast
right and this cavity is the minotic
job Seth
Seth
this is definitive joke sag and around
all that thing
if you remember
yes there was
extra embryonic misoder
extra embryonic image of
and this was connecting stock
this was the inner layer of the
extramuronic mesoderm and here is the
outer layer of extrembrionic
extrembrionic
now
right now in the lecture we will
concentrate the development of only
of the bioluminar Dust I will not talk
about the outer layers I will only talk
about epiblast and hyperblast which is
basically two cell layer and called by
laminar disc how it convert in the
beginning of the third week of
gestational development into trilominal disc
so
biliminar disc and see what really happens
the process by which
by laminar
embryonic disc
convert into
embryonic
embryonic disc this process is called
yes what is the name of this process
the process
by which the biliminar embryonic disc
convert into trilominar embryonic disc
this process is called
gastrulation this is called gastro
gastro lation
right
and that embryo which is having three
layers that is called gastrola
gas ruler right
now
we have to see what is gastrolation what
is the process by which the bileminar
disc convert into trilominar disc just
so this process is during the second week
and trilominate disc is formed during
is that right so bilateral disc is
formed during second week of the
embryonic development and trilominar
disc is formed during third week now
let's start with the process
what really happens the first thing
that there is a point let's suppose
as you know that here is a miniature
cavity and here's definitive jokes at
you are standing at the top here
and you're looking at this desk
right in the beginning
let's discuss when it is newly formed it
is very flat disc right
right
then what are the things which you will
observe down
area of thickness
and this area is called pre-chordal
plate what is it called yes
pre hurdle
hurdle plate
plate
actually this is an area
where Apple blast and hyperblast are
very sticky they are very sticky with
each other right they are sticking
strongly holding stored strongly with
each other the upper layer and the lower
layer in this area they are very sticky
with each other is that right now
now
as you are looking here let's go in the
back and have a good view of this disc
by limited disc from the back okay you
you're looking here what you will find here
here
you will find something interesting
that cells in the center will start proliferating
proliferating
in the central
axis the cells will start proliferating
proliferating
and as sellers
starting proliferating there is a longitudinal
longitudinal
there is a longitudinal Street
developments treatment
a line
you will start seeing that on the
amniotic side of the embryonic disc
when you look at the epiblast
cells in the central axis will start
proliferating and you will find they are
making a longitudinal streak
and then you will find that more
more
when you move from caudal end to the yes
yes
where the head will be from Colorado
land where there will be tail but
luckily because humans don't have tail
you can say lower back right actually
cephalic and is where the pro chordal
plate is present right here is your
central nervous system is going to
develop and around this mouth will
develop an all head and neck structures
will develop so naturally this is
felican so here should be called a land
and you find that
when you're looking on this disc on the
epiblast cells start proliferating and
they make a longitudinal line of streak
which is moving
forward from chordal end to the
safelican right and this cellular proliferation
proliferation
is are making this longitudinal line and
this line is called yes what's the name
of this line who will tell me primitive
primitive yes trick
primitive straight
and this interior part of it
right which is more wide this is called yes
yes
this is called primitive primitive
primitive node
node
what is it called
primitive node so what you really looked
on the surface of biliminar disc from
the neotic cavities you can see that
epivlastic area develop a swelling of
proliferation of the cells making a
primitive streak and primitive node
then you find
that cells from the center of the
Primitive stream
right they are going downward
downward
right let me tell you
that I make a section here I make a
cross section here and now that cross
section I'm going to show you here
that let's suppose
this is the
what is this epi blast
right and what is here yes please hypoblast
hypoblast right
and I've made a section here and you find
find
that this was your streak
streak
moving and here was your
what is this primitive node and here is your
your
Pro cardinal
play it right I've made a section here
right and what really happens in this
region where there is Street
what is happening the cells in this
region Melt Away right
right
and cells in the center also
melt away they disappear and cells in
the center of the Primitive node also disappear
disappear
right and you can say now this cell
layer has become like this and this
there's a gap here
foreign
this longitudinal Line This longitudinal
line where cells are deficient
write this line which is present within
the Primitive straight this line is
called primitive Groove what is it called
called
primitive Groove now if you look at this diagram
diagram
what you really see that within the
center of the Primitive state
right there is a longitudinal depression
and what is this depression called primitive
primitive grow
grow
in the same way from this area also
cells disappear and the central area of
the Primitive node
has also a depression and this Central
depression the central depression in the
Primitive node is called primitive
primitive paint
paint
primitive pad
so what we can say that what is
happening that when you are looking at
the epiblast layer initially it was a
flat you can look at one end there is
pro pre-cordial plate
and on the other end which is called the
land you find in the central axis of the
disc many cells proliferate and they
make primitive straight and most
interior part of the Primitive state is
swollen and a larger circular end which
is called primitive node then cells
within the Primitive streak disappear
and there's a longitudinal depression
there right this depression is called primitive
primitive
Groove and in the Primitive node there
is a circular depression there and that
depression is called primitive
now you must be thinking why there's
Groove and why there's pet why nature is
making such longitudinal Groove and then
making a circular depression there is a
purpose in that now what is the purpose
purpose is
that actually the cells from epiblast
look now carefully cells of the epiblast
they will start proliferating and they
will start migrating towards yes where
to the Primitive group what really
happens let me tell you
that cells right from the epiblast which
are on the cells which are on the sides
of the
this operators newly farmed operators
these cells and these cells start
proliferating and the start migrating
towards the primitive
streak and groove a question is that why
this is formed there and why cells are
moving to that direction
that's a question
answer is first of all why cells are
moving to that direction actually these
primitive streak strict cells and
primitive node cells they are producing
special chemical substances which are
attracting the cells
towards A Primitive streak and towards
the primitive
pet right so these cells right these
cells are producing a special type of
substance which is called fibroblast fibro
fibro blast
blast
growth factor eight
so it means in these cells
in these cells right genes are
expressing and those genes are making a
growth factor
eight this growth factor is produced by
these cells and it will diffuse
laterally and
cells which are present in the
neighboring area under the influence of
this growth factor start proliferating
rapidly and start migrating towards the
street and start migrating towards the
Primitive North
so what's going on
right here was your what was there
this group of cells are primitive
primitive
right
this group of cells now what is being
produced by these cells yes green cells
what they are producing fibroblasts
growth factor they are diffusing laterally
laterally
when these substances are diffusing laterally
laterally
they are acting as growth factor and
chemo attractant and chemote return for
the surrounding cells of course don't
forget notochord procodile pre-cardial
plate the cell from this side will start
moving towards
primitive streak from other side also
cells will start proliferating moving in
this direction
so what is happening these cells are
start proliferating and moving in this
direction from both sides
right when they reach here these cells
from here because they're the Gap here
they will come down they will come down
down
now you understand why that Gap is
produced through that Gap
first cells on the surface are
proliferating then they're migrating
there then through that Gap they are
coming down
right okay these cells which are coming
down I I give them a different color
when they come down let's suppose these
cells are they become flask shaped cell and
and
I'm showing them by blue color when
these cells come down they
they
mix with the cells of hypoblast
and hypoblast cells start degenerating
and these cells come down and down and
they start taking the place off hyper
hyper blast
blast
so as time passes by hypoblast
disappears and a new cell
the area where previously what was present
present
previously there was
are you understanding what has happened really
really
right again
let's suppose
that here was epiblast this was epiblast
and what is this
hypoblast they were put together first
of all you press them together what is this
then you produce a longitudinal
depression here and a circular
depression here then
cells from this side will move to the
street cells from this side will move
through the streak and then they will
jump down right and they will come down
and they start moving laterally
it means the cells from epiblasts are
moving where
to the medial side in the median they
reach to the Primitive Groove from there
those cells go down and then they come
down and spread laterally
and when these cells are making a new
layer which layer disappears hypoblast
and this new layer which is made there
this is called endoderm what is this layer
layer
this is called yes
endoderm so now you know from where the
endoderm come
endoderm is derived from the epi
blast is that right endoderm is
derived from the
every blast right
okay now
as these cells were moving
from lateral side to the medial side
they come down and detach and then they
go down and they take the place of
hyperblast hyperblast disintegrate and
disappear and this new cells layer which
has been derived from epiblast and which
has come down through the
primitive streak and primitive node
write that area rather through the
Primitive Groove and through the
Primitive pad they have come down and
they spread down and they are making a
new layer that layer is called endoderm
once this endoderm layer is made then
another generation of cells start migrating
migrating
when another generation of cells start
migrating now this
they also start moving in which
direction the same way
and they will also come down
and when they will start coming down
look at them
right this middle layer of cell which
has been derived from the epiblast but
its position itself below the epiblast
and above the endoderm this layer is
called misoderm what is it measure
measure
darn and now the lower most area what is this
this yes
yes endo
endo
and what we have learned up to now that
mesodermine endoderm both are derived from
from
IP blast
am I clear
there's no problem in this now you can
understand why we need to produce this
line and groove the cause teller to move
towards the groove and through the
groove they go down deep
and convert into endoderm and then
convert into
is that right
in this way
now after endoderm is developed right
and mesoderm has also been developed now
whatever is left at the top these cells
whatever is left at the top
the remaining part of
epiblast this layer of the remaining
part of the epiblast
write is called
actoner what is it called ectoderm
so what is actor
ectoderm is the remaining part of the
epiblast right
which has already made
endoderm and mesoderm and
Noto card I will explain what is Noto
card right for a while you just trust me
it's so simple look here attention
this is biliminar disc at the top we have
have
what is this
epivlast what is here
hypoblast right let's suppose this is
the disc right
right
this is the front end
this is the disc this is the front end
spelling part and what is this
collateral part right now in the
epiblast this is a longitudinal
streak what is the streak called
primitive streak and in the most
interior part of that there is a
circular swelling what is that primitive
node so there is primitive streak and
primitive node the cells disappear then
now within the Primitive Street the
longitudinal Groove is called primitive
Groove and within the Primitive node
there's a depression produced which is
called primitive pet then what happened
these cells of the Primitive streak and
the Primitive node the start producing
certain substances like fibroblast
growth factor these substances move
laterally and they move laterally this
make the surrounding neighboring cells
to proliferate and migrate towards the
Primitive streak and to the node mclared
when the cells are my another thing
fibroblast growth factor inhibits the
production of
sticky molecules there are some molecules
molecules
look here these cells
are held together very tightly through
special sticky molecules what are those
sticky molecules which help to stick the
cells they are called each adherence e adherence
adherence
eak adherence so normally these cells
are held tightly with each other through
each adherence so when
fibroblast growth factor comes laterally
not only it forces the cell to
proliferate and move towards it but
fibroblasts growth factor also reduces
the concentration of each adherence so
cells should not remain sticky to each
other so they migrate more
easily they migrate more easily is that
right so what we can say the word
fibroblast growth factor are doing from
this primitive streaks they come laterally
laterally
make the cell proliferate make the cell
migrate towards the Primitive uh streak
and primitive node and they also Force
the cell to down regulate the expression
of each adherent molecules when cells
are having less ecardians they are less
sticky to each other they migrate more
easily am I clear and once the cells
from this side from both sides they're
moving towards this they come down the
first generation of cell which comes
down they replace the hypoblast and make
endoderm then second generation of cells
which are coming down they yes
they insert themselves between the
epiblast and Endo Derm and they make a
layer of the cells which is called
and after that the remaining part of the
epiblast right is called ectoderm is
that right there is no problem up to
this fine now you have seen that
biliminar disc has been converted into
try eliminate disc
as I told you the process of conversion
of bilaminar disc into trilominar disc
is called process of
gastrulation now
there are more important point to be discussed
discussed
I did not discuss
that what happens to the cells which are
going down through primitive
node I only told the cell that moving
through the primitive straight come down
make endoderm and make measured
but I did not tell what happened to
these cells but before I go to that I
must mention that cells are moving
what is this medially go down and then
the move laterally not only they go down
and move laterally they also move forward
forward
let me tell you more clearly this concept
concept
okay I will make this
is the disk what is this three chordal
plate and now you are so intelligent you
should be knowing what is this primitive
primitive straight
straight
and what is this
primitive node right and there's no fun
in telling that what's happening on the sides
now
and there is of course hyperblast there
if I show the movement of the cells
through the
red color okay
through the red color
cells were moving like this from here
they go down and after going down I'm
showing in the broken line they're
moving like this
from here also cells move
move
they go down and then they are moving
like that
the point which I want to make cells not
only move
as they go down not only they move
laterally they also move
forward right
they are also moving interiorly
now
if you really look at this process
you will find that primitive streak is
present only in the
outer side of the plate
this is embryonic plate or disc the
street is present not in the
front area it is only in the back area
so it means the epiblast cells are
moving only from the back area they go
down and they move laterally and then
they also move to spell it Direction
it means that
that
cephalic side of the disc is only
gaining the cells only receiving the
cells but not losing the cells because
if we really see from this part of the
disk interior part or left asphalic part
and caudible part of the disc so the
swelling part of the disc there's
nothing moving like this and nothing
going down and nothing coming there's
nothing like but from this side cells
are going
going down and then turning
because this is the back of the disk a
spelling a countable part of the disc
which is forcing the cell to proliferate
and moving and then going down and then
coming laterally it means formation of
me endoderm and mesoderm or the process
of gastrulation is more active on the
sphilic powder cuddle part cuddle part
but then once cell go down they convert
into endodermal sell the miserable cells
then those cells move forward
so it means the mesoderm which is
present interior layers follicularly
that has been derived from
now I'll make the three layers
this is the epiblast layer
here is the
middle layer what is the middle layer
yes please
and what is uh
endo
dermal layer right
now look
there is wizard on present here as well
as here but this has been derived from
the south of the
total part and we slipped into
front uh spelling part it means that
even though cells are proliferating a lot
lot
on the lateral sides of the Primitive straight
straight
right or do you think this disc will
really expand much no because
sell the proliferating going to the
medial side coming down and then moving
laterally and forward
so actually this area is truly losing
the cell and this area is a truly
gaining the South
Julio am I clear or teaching myself
you understand it not sleepy
due to this reason as time passes by
right the disc shape
what has happened
which part has really broadened spelling
and kaudulent remains narrow so this is
very important point to understand
that this is the Cowardly end in which
the Primitive streak and primitive node
node is formed and from there the cells
are moving down the contributing
mesoderma androdum formation not only in
the caudible part but also in the
sphalic part so caudible part of the
disc remain narrow But as time passes by
the asphalic part of the disc become
broad end
am I clear
even though in the very beginning this
was almost
rounded then it became longitudinal then
it became something like a sleeper right
you know most of the sleepers have
one end which is more wide where you put
your five fingers and
for the heel there's a narrow area
slipper shift is that right
but I did not tell you
what happened to the cells which are
going down through primitive node and pit
pit
right now those cells right I will make
them with the pink color another color
those cell there are some cells which
are going like this from here
okay I'll make a very different color
let me give it a black color
there are a group of cells which are
pet
now first understand it
here let us suppose this is primitive
what if this was the street and what is
this primitive node and what is this
primitive fat now look
this is the spelling end this is the
cowderland again this is our desk like
this this is the
cowderland and this is spelican here is what
what recoil
recoil
plate this is primitive node here was
primitive State we have already know the
cells move like this they come into primitive
primitive
streak and they move like that and from
here also cell move like this and then
right we have to look at the cells here
now concentrate on this cells from this
side will go down from this side will go
down from here they will go down from
all sides they will go down so it looks
like there's a circular area and cell
the moving down in a circular pattern
in a circle and all those cells when
they go down and they're being pushed
forward they cannot go down because down
there is
hypoblast so when cells go from the
Primitive pit down this is a circular
group of cells so they move like a tube
these cells will move like a
tube and this tube will move from the primitive
primitive
node or primitive pad downward and then
forward it is going like a from here you
know look at this hand now this tube
start like this and they start moving
forward it will keep on growing this
tube of cells which are derived from the
Primitive node and
write these cells will keep on moving
forward in the central part of the this
from the Primitive node up to
Pro cardinal pre-cardinal plant because
pre-cardial plate is an area where upper
layer and lower layer of cells are so
strongly held together that this tube
cannot grow forward
now again what was this primitive
node cells go down from the Primitive
node look here and this is all going
going going this is a tube going what is
this tube
yes what is this tube
don't tell me this is my R what is this tube
tube
yeah again look at the tube cell coming
here here here here the tube form then
it goes down down down here here the
pre-codile plate tube cannot extend it
stopped here so this is what primitive
now from there this tube from the
Primitive node it is extending this is
derived from the cells from primitive
node and this tube is extending forward until
regardless of the plate what is this
tube called
this is called notocardial tube
or notocordal canal
canal
immacula so it means during the process
of gastrulation
from the Primitive node and primitive
fit what is derived notocardial tube
notocardial plate now
how you will show in diagrams look here
if in the diagram I show
these solid lines show the cells which
down from here right
now this dotted line
tube some people call it simply
notocodal process
so notochodal process
is derived from the south of the
Primitive node and these cells go down
so the Primitive pad and then the move
forward below the epiblast
and above the endoderm Because by that
time and hypoblast may be replaced by
endoderm so between the endoderm and the
epiblast this nodochordal process
extends cephalically or forward until it reaches
reaches
yes it reaches free chordal plane is
that right if I really
make a section here now look
if I make a section here
cross section
uh showing like this let's see how it
looks in the diagram
now I'm going to show you the
development of
yes notocoidal process
process
right no to chordal process now this
notochordal process this is very
important process this neutral Control
process will act at the act as the
central axis of the body around which
vertebra will develop and on one side
there will be right side of the body on
the other side there will be left side
of the body so this will be the central
axis of the embryo this will be the
central axis of the embryo right
notochordal process now this notohodal
process which is extending from the
Primitive node up to the yes
plate and it is derived from the cells
of primitive node which went down
through the Primitive pad and this
process is placed between the epiblast
and underlying Android right
right
make a longitudinal section and then
make a
transfer section and we'll see the
diagram for similar longitudinal section
in longitudinal section what you really
blast right and here there was your
friend what was this primitive
primitive right
right
and cells went down from here
from here and now they are extending
what is this notoch order
tube or Canal or process of course this
is also made of
yes please cells
is that right and
and yeah
yes endoderm
right here the ectoderm or you can say
hyperblast and underlying endoderm they
are put together so this area is called
you remember what was this area free
free
harder plate I'm trying to make some
section through that right so we have
made the section here
and this is and of course what is here
here is also mesoderm
now here if you have to remember one
thing let's come back to the measure
done mesoderm goes down
look that is present throughout between the
the
ectoderm and endoderm except three
places this is very important to make a
concept actually there is suppose in the
end this will be actor down right this
is endoderm and below the acrodum and
above the endoderm or in between the
ectoderm and endoderm what is this
mesoderm this mesoderm is present
throughout between the Ecto and Endo
except three places three places there
is no measure Dom number one the
sticky together like this area so there
cannot be witherum in between these two
is that right number two
on the very back of
okay let me stand it more backward
this disk
on the very very posterior area
right there is another area like what is
this what was this
pre-cardinal plate this on the very back
area outer area there's another
area where ectodermal cells are very
much sticky with
what is this
and because here also they're very
sticky mesoderm cannot come in between
so now we know two areas in the
trilometers where mesoderm is not
present between not present one is
regardle plate
and the other is what is the name of this
this
plate
another thing I think is worth
mentioning here just more
more
cephalic to pre-codile Plate here is a
special area and this is called yeah
this is called buccopharyngeal membrane bucko
bucko pharyngeal
pharyngeal [Music]
right this is pre chordal area and just
adjacent to that and more sphilically
there is another area which is called
Buco pharyngeal membrane in both areas
picardo plate and book of Angel membrane
between the endoderm and ectoderm
there's no mesoderm
is that right
and if you go to the most comfortable
area there is vehicle plate and
chlorical plate does not have any
so it means listen listen carefully it
means in this point octodom and
mesoderma are coming in acrodome and
endoderm are in contact and here also
ectoderm and endoderm are in contact
you know this is the point in the future
where mouth will form
this is the point I will not show you
NS will form right
you're moving your tongue on your lips
you are confirming that there is a
bucopharyngeal membrane it is no more
there now you are now not an
inter-embryonic life right uh yeah now
you are having well defined
mouth and oral cavity right now anyway
look in the oral cavity attention look
here in the oral cavity the skin is
ectoderm and this mucosa is derived from
endoderm and then meeting each other
and of course the other end
in US
outside the skin and inside the mucus of
the j8 is coming acroderman androdum
fusing so basically this is going to be
Buckle Fringe of membrane and future
osteum or mouth area and this is going
to be
where eventually anus will be placed
is that right and of course
notochord is made
if now we can show here
notochordal processes
present over here what is this
nodo model process around it vertebrae
there's no problem in this okay
okay
now there is something good this will
bend later on you will learn this area
will bend like this this will also Bend
like that and then openings will be made
right anyway
we'll study that later now come back
I was telling the three areas where
between the ectoderm and endoderm
mesoderm is not present one area is
crocodile plate and bucopharyngeal
plate
is that right what is the third area
where between the ectoderm and endoderm
mesoderm is not present something else
is present yes foreign
tube is going right this tube is present
between the ectoderma and endoderm or
you can say IP blast and endoderm right
so mesoderm cannot come into this place
right so what we can really see that
but ton cannot come into place of
clerical plate it cannot come at the
and of course it cannot come at the
place where the notocardial process
process is present any question up to
this there's no question okay
okay
now another thing
if we look at this diagram right
what was the fluid above the disk
do you remember that
you don't remember the fluid okay let me
make a very simple diagram here a
smaller diagram I will make it as a part
of bigger diagram
diagram look
Le plate yes crocodile plate primitive
straight primitive node
node
what was this process
process
is that right
and what was at the top what was this
amniotic fluid you forget that
here you were
looking down
right and what is this jock
definitive job set you understand this
as in this diagram I was making a
section here so I'll continue the
section here I will make a section here
if I make a section here then
then
what will be the structure under it if I
active Dam or epiblast right
and what was under it if you have made a
right and of course you know what is
here there is
a dumb but it cannot come visit them
here and here is also these are dumb but
now
this is for capricornal plate and all of
this is
epiblast or ectoderm is that right
and this was primitive streak and this
was primitive node and through the
Primitive pit what has gone down
process now let's concentrate and focus
this is the cells which are making what notocardon
