Acrosomal and cortical reactions in animal development
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| Acrsomal Reaction | When the head of a sperm cell encounters the jelly coat of the egg cell
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| Acrsomal Reaction step 1 | A specialized vesicle at the tip of the sperm, the acrosome, discharges hydrolytic enzymes.
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| Acrosomal Reaction Step 2 | These enzymes digest the jelly coat, enabling an elongating sperm structure called the acrosomal process to penetrate the jelly coat
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| Acrosomal Reaction Step 3 | A protein on the tip of the acrosomal process then adheres to molecules of a specific receptor protein on the egg's surface.
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| Acrosomal Reaction Step 4 | These receptors extend from the plasma membrane of the egg cell through the vitelline layer, a meshwork of extracellular matrix molecules lying under the jelly coat.
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| Acrosomal Reaction Step 5 | Contact of the tip of the acrosome with the egg membrane leads to the fusion of sperm and egg plasma membranes and the entry of the sperm nucleus into the cytoplasm of the egg.
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| Acrosomal Reaction Step 6 | Either contact or fusion of the membranes cause ion channels to open in the egg cell's plasma membrane, allowing sodium ions to flow into the egg cell and change the membrane potential.
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| Acrosomal Reaction Step 7 | The change in membrane potential is depolarization, only occurs for 1-3 seconds and blocks additional sperm from fusing with the egg's plasma membrane.
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| Acrosomal Reaction | Fast- block to polyspermy
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| Cortisomal Reaction | slow block to polyspermy
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| Cortical Reaction Step 1 | Sperm binding appears to activate a signal transduction pathway involving two second messengers; IP3 and DAG. These second messengers cause calcium ions to be released from the egg cell's ER into the cytosol.
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| Cortical Reaction Step 2 | The calcium release from the ER begins at the site of sperm entry and then propagates in a wave across the fertilized egg.
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| Cortical Reaction Step 3 | Within seconds, the high concentration of calcium ions brings about the cortical reaction, fusion with the egg's plasma membrane of numerous vesicles lying in the egg's cortex.
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| Cortical Reaction Step 4 | These cortical granules now release their contents into the perivitelline space, the space between the plasma membrane and the vitelline layer.
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| Cortical Reaction Step 5 | Enzymes released from the granules degrade proteins holding the vitelline layere to the plasma membrane, while mucopolysaccharides produce an osmotic gradient, drawing water into the perivitalline space and swelling it.
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| Cortical Reaction Step 6 | The swelling pushes the vitelline layer away from the plasma membrane and their enzymes harden it.
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| Cortical Reaction Step 7 | These change transform the vitelline layer into the fertilization envelope and this resists the entry of additional sperm.
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