Comprehensive Guide to Cleavage in Early Human Embryonic Development

hello students in the continuation of the development of first week today we are dealing with the second part of the

development that is cleavage so again whenever you are writing in the exam on this question cleavage the first thing

comes is definition of the cleavage now my dear students you have to understand this concept in your mind that we are

dealing with a very big cell of the human body which is your oocyte and we know that oocyte is the largest cell so

the normal somatic cells of our body are not this much in the size so you have to decrease the size of this oocyte to a

normal somatic cell of the human body now how it is possible and it is there is a only one thing which is known as

fast rapid repeated mitosis so by the help of these repeated rapid

mitosis we will decrease the size of the cell of this oocyte or the initial formation of your zygote immediately

after fertilization to a normal semantic cell clear so the need of cleavage is to

decrease the cell size to regain the normal nuclear cytoplasmic ratio of the human somatic

cell so otherwise what will happen if there is a large amount of the cytoplasm is

present and nucleus is small there is a lack of the control of the nuclei on the cellular activity

so that's why the cleavage is required so whenever we are talking about the definition of the cleavage it is defined

as a rapid and repeated mitotic

divisions and these mitotic divisions are taking place in the developing zygote to

gain the smaller sizes of the cell and these are known as blastomere these are known as blastomere

so these are the two things why the cleavage is necessary and what is the definition of cleavage so I hope it is

clear that as soon as there is a fertilization occurs we have to decrease the sizes of the cells to a normal

somatic human cell and that's why there is a necessity of the cleavage and that's why the cleavage is defined as a

repeated rapid mitotic activities taking place in the developing zygote to decrease the cell size and those cells

are known as blastomate now what is the difference between the cleavage and mitosis now we are using

the word cleavage we are not using the word mitosis so there is a one difference that when the mitosis will

take place the cell size will not decrease Whenever there is a mitosis the cell

size will remain same you will have the two cells then you will have further cells so mitosis will not decrease the

size but in the cleavage we are decreasing the cytoplasm in each and every division to regain the normal

nuclear cytoplasmic ratio clear now what time the cleavage begins so the cleavage start immediately after

fertilization as soon as the male and female Pro nucleus fuses with each other and splitting Stars at the same time the

process of cleavage's start so what will happen you will have the two self then three cell four cells six cell eight

cell like so on now what is the duration of cleavage now

cleavage starts from the fertilization and its duration is till implantation till implantation and we know that

implantation taking place at around your seventh day after fertilization so the cleavage is in the whole first week of

your ah development now what is the role of Zona palosoda in cleavage

now why we are talking about this Zona palacida here because when you will have the uterine cavity you know that the

zygote form inside the fallopian tube now this is your employee part where is I got from

now it is start moving towards your uterine cavity and with the transportation from

fallopian tube into this uterine cavity simultaneously the cleavage is going on but this layer of zonopalucida avoid the

abnormal implantation of this developing conceptus into the fallopian tube so if in other term you can say that the

stages which all are covered by zonopalucida are under cleavage clear because as soon as the Zola

pelucida disappears there is implantation and I am saying that what is the duration of cleavage till

implantation so when the zonopolitual disappear there is implantation that means there is a stoppage of cleavage

clear now what are the stages of the your cleavage so first there is a two cell

stage now my dear students you have to understand one thing in the cleavage that there is a one big cell

now when this big cell will divide you will have another unequal distribution

again this will give rise to the two cell one is larger in size with more cytoplasm one is smaller in size

now when the next step will occur the larger cell will show the mitosis first so this will again divide into one

bigger cell one smaller cell so at one stage first you will have two cell then you will have one two three

three cell then it will divide then you will have one two three four four cell so some book says it is two cell stage

then three cell States then four cell clear so why three cell three cell is because

there is a unequal mitosis occurs in the cleavage because one cell is having larger amount of cytoplasm another is

having a smaller amount of cytoplasm and larger cell than will show first mitosis in The Next Step clear so this is how

the cleavage will take place so here when we are talking about the steps of the cleavage we are going like

this 2 4 8 16 32 64 like this now there are two important terms which is import which are your exam point of

view one is modula and second is blastocyst so what is happening how the modula is

different from the eight cell stage and how the modula different from the blastocyst now my dear students what is

happening here when there is a eight cell stage is there now this is the eight cell stage you are having the

eight cell now these all the eight cells are of similar properties and these all the

eight cells are in this stage known as totipotent cell why they are totipotent because these

all cells can form your intramuronic as well as extra embryonic tissues now we have to change the attitude of

these cells into a specified attitude so for that there is a process occurs which is known as

compaction what is that compaction now after this process of compaction these cells will rearrange themselves in the

next cleavage so what is the next stage these are the eight cell in the next stage we will have 16 cell but these 16

cells will arrange themselves into the two part one is known as outer cell Mass one is known as outer cell mass and

another is known as Inner Cell Mass another is known as Inner Cell Mass so in the eight cell stage there is

nothing like outer and inner cell Mass but in the 16 cell stage you will have arrangement of the cells into a outer

cell mass and Inner Cell mass and these cells are not totipotent clear why not totipotent because Inner

Cell mass is now only responsible to form the derivatives of your actronom endoderm and mesoderm that means they

can form your body cells so these Inner Cell Mass behave like a pluripotent cell they are flurry potent cell

so here you have to understand this concept that when we are talking about the cleavage

at the eight cell stage the all cells up to this are totipotent and they can form intra as well as extra

immunic tissue but after the eight cell stage there is a process occurs compaction and in the next cleavage when

you will have 16 cell stage the cells will arrange themselves in a inner and outer cell mass and the Inner Cell mass

cells can produce only and only derivatives of your ectoderm endodom mesoderm they cannot form the placenta

they cannot form the extra ambryonic tissue so my dear students now this 16 cell

stage is known as morula why because it appears like a mulberry shape so now the question comes is what are the features

of modula so the first feature is that it is a 16 cell Steel second feature is modula formed at the

third day of fertilization the third character of the modula is that the cells are arranged into the inner and

outer cell Mass they are not totipotent they are having the Inner Cell Mass which are known as pluripotent so the

Inner Cell mass is going to form the future embryonic intra embryonic tissue future

intra-embryonic tissue and outer cell mass is going to form your future trophoblast cells which are going to

form placenta and there is a one more characteristic that I told you the morula is also covered or lined by

Zona palucida still the zonopalucida is not disappears clear means that morula is not going to implant implantation is

not occurred at modular stage so these are the four features of modula now the next step comes is blastocyst now how

the blastocyst is differentiate from the modula now when you are talking about the migration of your developing zygote

now this is your modula now on the third day this modula is formed and now it is approaching towards

your this uterine cavity now in the uterine cavity you are having the tissue fluid or you can say uterine

fluid now this uterine fluid is also known as milk uterine milk why because this fluid is full of nutrients and this

fluid will start to enter inside the modula now this fluid start to enter inside the

modula and on the fourth day on the fourth day of fertilization what will happen now the scenario will change

so let's see what will happen next so in the developing your zygote now this is your modula which is having outer cell

now these outer cells are known as future trophoblast and you will have Inner Cell

now the uterine fluid which will pierce this outer Zona pellucida of modula enters into these intercellular space of

Inner Cell Mass clear so here you will have the collection of your

uterine milk or uterine fluid into the area between the inner cell Mass now these spaces gradually enlarges and

fuses with each other now if these species will enlarges and fuses with each other what will happen

they shift all these cells at one end of the developing zygote so what will happen if you will remove all these

Inner Cell mass and if you will shift them to the one end so they will come at one end

and now a large cavity will appear inside this is developing zygote now this stage is

now known as blastocyst cyst means any swelling which contains the fluid so now here you can see that there is a

cavities form now this cavity is known as blastocele this cavity is known as

blastocele so how the blasto seal is form the Blastoise seal is formed by the diffusion of uterine milk through the

zonopalucida into the modula where the fluid will go into between the inner cell mass and the species will join

together to form a large cavity and this fluid pushes this fluid push the all inner cells on one end and a large

blastocele will appear now this is the stage after that the implantation will start so at what day

the blastocyst will form so on the fourth day or after fertilization the blastocyst stage is forming and the

cells are around 32 then you will have the next cleavage is 64. clear so the cleavage is still going on you have seen

that 16 cell stage is modular then you will have 32 then you will have 64. so this is what how the blastocyst is

formed but now there are some important terminologies comes in the picture what terminologies now one is this outer cell

mass or outer cell layer is known as trophoblast it is now no more known as outer wall it is now known as

trophoblast the Inner Cell mass is now known as embryoblast

so this is the first thing that once the blastocyst will form there are three names you can see blastocele

this outer layer which is now known as trophoblastic cell layer this Inner Cell Mass which is now known as embryoblast

there is a one more thing that these trophoblasts divided into the two part how the trophoblast which are covering

this area of embryoblast the trophoblasts which are covering this area of embryoblast are known as Polar

trophobia so it is divided into the two part polar trophoblast and mural trophoblasts

so what do you mean by the Polar trophoblast Polar trophoblast or the trophoblast which are just related to

the embryo blast inside the blastocyst and the remaining layer of trophoblast which is not related with embryo blast

cells known as mural trophoblast clear now there is one more nomenclature comes now what is that this embryoblast and

formation of embryoblast divide this ah blastocyst into the two part now this area

is known as embryonic pole of the blastocyst it is known as embryonic pole embryonic pole is also known as animal

pole so this is known as embryonic Pole now this embryonic pole is the area from where the embryo is going to implant

first with the maternal endometrium this remaining part is known as AB

embryonic pole what is that AB embryonic Pole so here now you can see there are few terminologies which is

important when you are writing an exam first there is a two cell stage four cell stage ten eight cell States after

eight cell stage you will have a process of compaction then you will have 16 cell stage now the compaction create the

outer and inner cell Mass which is known as modula then in modular there is a formation of a cavity and this stage is

known as blastocyst and blastocyst the ah cavity pushes the Inner Cell Mass on one end now these cells are now known as

embryoblast the outer lining is known as trophoblast trophoblast divided into the two the trophoblast covering the

embryoblast are known as Polar trophoblast the trophoblast on the remaining part are known as mural

trophoblast now the area of blastocyst which is having the embryo blast inside known as embryonic pole and remaining

part of the area is known as AB embryonic pole so what are the fate of these three part

so what is the fate of hemoroblast embryo blast I told you that these are the pluripotent cells and embryoblast

cells are going to form your tissues of embryo what is the fate of trophoblast the trophoblast is going to form extra

embryonic tissue that means tropho means nutrition so they are going to form placenta and what is the fate of the

Blaster seal the blasto seal is going to form your yolk Sac so these are the some terminologies which are always there in

your mind when you are writing the short note on the cleavage what is blastocele what is uterine milk what is trophoblast

what is polar trophoblast what is neural trophoblast what is your embryonic pole what is AB ammonic pole and what is the

fate of embryoblast clear so now at the end of this session you will have the idea that the cleavage starts

immediately after the fertilization and the purpose of cleavage is to receive the normal size of the somatic cell and

also to receive the normal nuclear cytoplasmic ratio so there is a rapid mitosis will take place then you have to

write down the two then four then a 8 cell stages after the eight cell stages there is a compaction which is going to

formula a moralizes 16 cell stage after the modular there is a invasion of the uterine milk so there is a formation of

a cavity and ultimately the cavity will enlarge you will have a blastocyst and the blastocyst will have a well-defined

prophublastic layer and inside you will have embryo blast so this is all for the lecture thank you