Monospermy vs. Polyspermy

Prevention of Polyspermy

• Monospermy occurs when only one sperm enters the egg.
• In monospermy, a haploid sperm nucleus combines with a haploid egg nucleus to form the diploid nucleus of the zygote, restoring the appropriate chromosome number for the species.
• The sperm's centriole divides to form the two poles of the mitotic spindle during cleavage.
• Polyspermy occurs when multiple sperm enter the egg.

Consequences of Polyspermy

• In sea urchins, fertilization by two sperm results in a triploid nucleus and three copies of each chromosome.
• The centriole of each sperm divides to form two poles, resulting in a mitotic apparatus with multiple poles.
• Triploid chromosomes divide into four cells or poles total.
• Since there is no mechanism to control chromosome number distribution, chromosomes are distributed unequally.
• Some cells receive extra copies of chromosomes, while others lack them, causing the cells to either die or develop abnormally.

Block to Polyspermy

• Species evolved mechanisms to prevent union of more than two haploid nuclei.
• The most common way to block polyspermy is to prevent the entry of more than one sperm into the egg.

Block to Polyspermy in Sea Urchins

• Sea urchin eggs avoid polyspermy through two mechanisms: fast and slower reactions.
• Fast reaction is established by an electric change in the egg plasma membrane.
• Slower reaction involves exocytosis of the cortical granules.

Fast Block to Polyspermy

• Electric potential of the egg plasma membrane changes.
• Membrane potential is the voltage, charge, or ionic difference across two sides of the membrane (outside and inside).
• The egg plasma membrane is a selective barrier between the egg cytoplasm and outside environment.
• Ionic concentration of Na+ and K+ is different inside and outside the plasma membrane.
• Difference in concentration of Na+ and K+ is important for sodium and potassium ions.
• Seawater contains a high sodium ion concentration.
• Potassium ions are higher inside the cell.
• Plasma membrane regulates this difference by inhibiting Na+ entry and blocking K+ leaking.
• The resting membrane potential is generally about -70 mV, due to the inside of the cell being negatively charged with respect to the exterior.
• Fast block to polyspermy occurs one to three seconds after the first sperm binds.
• Membrane potential changes to +20mV as a result of a small influx of sodium ions into the egg.
• Sperm can fuse if the resting potential is -70 mV ,but it cannot fuse if the membrane potential is positive.
• No more sperm can fuse with the egg.
• Polyspermy can be induced if sea urchin eggs are artificially supplied with an electric current to keep their membrane potential negative.
• Conversely, fertilization can be prevented entirely using artificially maintaining the membrane potential positive.
• Fast block to polyspermy can be prevented by lowering sodium ion concentration.
• If supply of sodium ions is insufficient to cause the shift in membrane potential, polyspermy can occur.
• It is unknown how changes in membrane potential of the egg blocks secondary fertilization.
• Perhaps, sperm contains a voltage-sensitive component, which might be a positively charged fusogenic protein.
• Insertion of a special component into the egg plasma membrane is regulated by the electric charge across it.
• In frogs, the electric block to polyspermy occurs, but its probably occurs more rarely in mammals.

Slow Block to Polyspermy in Sea Urchins

• Sea urchin eggs have a second mechanism, to ensure multiple sperm cannot enter the egg cytoplasm.
• Fast block to polyspermy is transient, as the membrane potential remains positive for only about a minute.
• A mechanical block to polyspermy that occurs after 1 minute, to prevent multiple sperm.
• The slow block is accomplished by the cortical granule reaction.
• Directly beneath the sea urchin egg's plasma membrane are about 15,000 cortical granules, each about 1 μm in diameter.
• Upon sperm entry, these cortical granules fuse with the egg plasma membrane and release contents into space between the plasma membrane and vitelline envelope.

The Slow Block to Polyspermy: Released Proteins

• Contains several proteins released during cortical granule exocytosis:
• Proteases
• Mucopolysaccharides
• Peroxidase
• Hyaline

The Slow Block to Polyspermy: Role of Proteases

• Dissolves the protein that connect the vitelline envelope proteins to the cell membrane.
• It removes the bindin receptors and any sperm attached to it.

The Slow Block to Polyspermy: Role of Mucopolysaccharides

• Produces an osmotic gradient that causes water to rush into the space between the plasma membrane and the vitelline envelope.
• Envelope expands becoming the fertilization envelope.

The Slow Block to Polyspermy: Role of Peroxidase

• It hardens the fertilization envelope by cross-linking of tyrosine residues on adjacent proteins, starting at the sperm entry site.
• Expansion of the fertilization envelope continues around the egg.
• After 20 seconds, the process begins, after sperm is attached. It is completed by the end of the first minute of fertilization.

The Slow Block to Polyspermy: Role of Hyaline

• Forms a coating around the egg, that creates elongated microvilli that extend from plasma membrane.
• Their tips then attach with hyaline layer.
• This layer provides support for the blastomeres during cleavage.

Slow Block to Polyspermy in Mammals

• Cortical granule reaction does not create a fertilization envelope, but the ultimate effect is the same.
• Released enzymes modify the zona pellucida sperm receptors in a way that prevent binding with sperm.
• This process is called the zona reaction, and both ZP3 and ZP2 are modified.
• The cortical granules of mouse eggs contain an enzyme that removes the terminal sugar residues of ZP3.
• Leads to the release of of remaining sperm and prevention of remaining sperm being able to attach to the egg.
• Cortical granules of mouse eggs contain N-acetylglucosaminidase enzymes.
• Cleaves N-acetylglucosamine from the ZP3 carbohydrate chains.
• N-acetylglucosamine binds with sperm.
• Proteases Clip ZP2 (zona protein 2)
• The zona protein 2(ZP2) loses its ability to bind with sperm
• Once a sperm has entered the egg, no other sperms can bind
• Their binding to the zona pellucida is rapidly shed

Calcium's Role in the Cortical Granule Reaction

• The mechanism of the cortical granule reaction is similar to the acrosomal reaction.
• Intracellular calcium ions are released from their intracellular stores, or the endoplasmic reticulum, after fertilization.
• In a high-calcium environment, the cortical granule membranes fuse with the egg plasma membrane and release the cell contents.
• The fusion of the cortical granules begins near the point of sperm entry.
• A wave of cortical granule exocytosis propagates around the cortex to the opposite side of the egg.
• Located in the cortex and surrounds, the endoplasmic reticulum and cortical granules are present in sea urchins and frogs.