Friday, April 29, 2011

Cortical Reaction

Fertilisation, no it is not a typo for fertilization, which is more so directed towards humans. Fertilisation identifies the fusion of gametes to produce a new organism. Fertilisation can be used in reference to anything from flowering plants to humans. In humans the process involves the fusion of a sperm to an ovum, which is the first step in initiating the cortical reaction.

The cortical reaction begins when the sperm unites with the eggs plasma membrane, leading to an influx of intracellular calcium, resulting to the exocytosis of cortical granules. The cortical granules are secretory vesicles, composed of contents that vary with species and are released into the extracellular space. The exact roll in the excretion of these vesicles containing varying contents is not quite fully understood. Upon the release of the cortical granules the vitelline layer is then altered. In mammals that layer is identified as the zona pellucida. The zona pellucida, or vitelline layer, is an external membrane to the plasma membrane of an egg cell. The alteration of the vitelline layer is an effect of an increase of osmotic pressure causing water to fill the space between the layer and the plasma membrane. This single event is responsible for the successful fertilization of an egg, because it raises all of the other non-fertilizing sperm from the plasma membrane. This is so important because it prevents polyspermic fertilisation, a lethal fate to an oocyte.

The reaction is represented in the shown diagram. A sort of signal transduction cascade is initiated upon uniting the sperm to the egg. Evidence supports the theory that the PIP2 cascade mediates the cortical reaction. The PIP2 cascade generates two second messengers, IP3 and DAG. DAG is responsible for inducing the release of intracellular Ca2+ from the sarcolemma and various storage organelles. This sudden influx in calcium leads to the exocytosis of the cortical granules. The granules then induce the alteration of the zona pellucida, which prevents polyspermic fertilisation and allowing only one sperm to fertilize the egg leading to the completion of the cortical reaction.

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