FERTILIZATION - CLEAVAGE & BLASTULATION

STD-12 UNIT-6 CHA-3

HUMAN REPRODUCTION

FERTILIZATION - CLEAVAGE & BLASTULATION

CLEAVAGE

Cleavage is defined as the conversion of single celled zygote into many celled morula by continuous repeated and rapid mitotic divisions known as cleavage .
Cleavage is completed in fallopian tube within 72 hours of fertilization smaller and smaller cells produced during cleavage are known as blastomeres , divisions are so rapid that there is no time for growth .
Zona pellucida is tough and intact size also doesn't change i.e. number of cells increase but cytoplasm quantity remains same , DNA replicates everytime therefore number of DNA also increase .
Cleavage is holoblastic because entire zygote divides due yolk in them is less .

Ist cleavage :
1st cleavage starts after 3 hours of fertilization and completed in 30 hours of fertilization.
In first division zygote nucleus divided mitotically into two cells , first cleavage is vertical, meridional and passes from animal pole to vegetal pole , through centre .
Two equal sized blastomeres are formed surrounded by zona pellucida .
First cleavage is long process because competed in 30 hours .
IInd cleavage
IInd cleavage is completed in 40 hours of fertilization in which 2 celled stage is converted into 4 celled stage
IInd cleavage is perpendicular to Ist cleavage i.e. right angle to Ist cleavage .
During IInd cleavage 2 cells do not divide at same time so intermediate 3 celled stage is formed .
IIIrd cleavage :
IIIrd cleavage is longitudinal and equatorial at right angle to 1st cleavage and IInde cleavage , In III cleavage blastomeres divide mitotically and 8 celled stage is formed .
III cleavage competed within 55 hours of fertilization .
In this way cleavage process proceed on further cleavages are fast and in IVth cleavage 8 celled stage converted into 16 celled stage then 16 celled stage is converted into 32 celled stage .
16 celled stage in called as early morula .
32 celled stage is called as late morula .

Blastulation

It is a process of formation as blastula from the morula is called as blastulation
blastula or blastocyst is a spherical multicellular hollow fluid filled structure .
Process of blastulation is competed in fallopian tube in 3 days after cleavage .

Morula in fallopian tube absorb surrounding uterine fluid .
Uterine fluid in the morula separate the cells of morula separated some cells become flat and form the outer covering called as trophoblast , no any future organ is derived from the trophoblast Remaining cells become more and more spherical and form cell mass , which is attached to the trophobia towards animal pole called as embryoblast or inner cell mass .
Future embryonic development is from the cells of embryoblast .
Cavity within the trophoblast and embryoblast is called as blastocoel cavity , blastocoel cavity is filled uterine fluid .
Whole structure with trophoblast , embryoblast , blastocoel is called as blastula .
Significance of Blastulation : Embryoblast develops into embryo proper .
Trophoplast gives rise extra embryonic membranes amnion and chorion .
Trophoblast is useful for the formation of placenta .

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