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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