Phenomenon of egg activation or events of egg activation
The fusion of acrosonal
and egg membranes is the beginning of egg activation. Egg activation is a
series of biochemical changes in the egg that ensures the completion of
fertilization and initiates embryonic development. Biologists have extensively
studies of events and some of the findings of egg activation in echinoderms
that work are discussed here.
Membrane events:
Some of the
earliest changes in the zygote at the plasma membrane and in the outer region
of the cell cytoplasm (called cortex). These early changes ensure fertilization
by only single sperm.
Single sperm
fertilization is important because multiple fertilization usually results in
genetic imbalances and a nonviable embryo.
After contract
by sperm, microvilli from the plasma membrane of the ovum wrap around single
sperm. Contraction of micro filaments in egg’s cytoplasm then draws the sperm
into the egg.
Second series of
events defends against multiple fertilization within milliseconds of
penetration by a sperm, ionic changes make the plasma membrane unresponsive to
other sperm and initiate the formation of protective envelope around the egg
called the fertilization membrane.
Cortical eventa:
The
fertilization membrane forms as granules in the cortex discharge into the
region between the egg plasma membrane and the vitelline layer. The cortical
granules release enzymes that loosen the vitelline layer’s contact with plasma
membrane. The granules allow water to enter the space between the vitelline
layer and the egg plasma membrane, causing the vitelline layer to lift off the
egg. Proteins of the cortical granules thicken and strengthen the vitelline
layer. Al of these reactions is completed in 1 to 2 minutes following
fertilization.
Other important
changes occur in the egg cortex. After sperm penetration, the cortical layer
thickens and rotational and sliding movements of the outer egg cytoplasm begin.
In amphibians these cortical changes result in the formation of a grey crescent
on the egg, opposite the point of sperm penetration. The grey crescent has
important influence on later development.
Metabolic and
Nuclear events:
Prior to nuclear
fission, other nuclear events usually occur. Post fertilization changes help prepares
the zygote for ensuring mitotic divisions. Ionic changes raise the intra
cellular pH and initiate changes in zygote physiology. DNA replication occurs
and rates of protein synthesis increase to meet the needs of new cells for
enzymes and structural proteins that make up the mitotic spindle and contribute
to chromosome structure.
Maternal
Dominance:
Little mRNA is
synthesized in the zygote, instead, existing maternal (egg) mRNA is activated
and it directs the formation of the bulk of proteins synthesized in early
stages of embryonic development. This influence is called maternal dominance.
The region of
the egg referred to as the animal pole contains less yolk, more mitochondria
and more ribosomes, and is more metabolically active than the opposite vegetal
pole of the egg.
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