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Fertilization

Fertilization is the union of a haploid sperm and haploid egg to form a single diploid zygote. This cell will develop into an embryo.

Unfertilized EggThe egg is quiescent -- it performs no DNA synthesis, cell division nor RNA transcription. There is a reduced rate of translation until fertilization. The egg stores a multitude of macromolecules, precursors and yolk. To exit this dormancy, the egg must be activated by sperm.
Finding EggSperm are sometimes chemotactically guided to the egg; other times, the sperm and egg meet randomly. However, the region where the sperm and the egg meet is guided by behavior.
Sperm BindingThe sperm binds to the egg outer covering (mammalian zona pellucida; urchin egg jelly) which causes the outer covering to depolarize. This depolarization blocks further sperm entry and initiates activation.
Acrosome Reaction

In the acrosome reaction, the sperm moves from the egg's outer covering to the egg's cytoplasmic membrane, then the egg's acrosomal vesicle fuses with the sperm's cytoplasmic membrane via Ca2+-mediated exocytosis. The tip of the sperm has an acrosomal process coated with a membrane-bound protein called bindin. Different species have different kinds of bindin molecules. The egg has species-specific bindin receptors extending through the vitelline envelope. Binding of a bindin with its species' bindin receptor stimulates the fertilization cone to form from the egg plasma membrane, engulfing the sperm head and bringing it into the egg cytoplasm.

In sea urchins, the acrosomal reaction is triggered when the sperm reaches the egg jelly. In mammals, the sperm must first enzymatically digest through follicle cells attached to the egg; the acrosomal reaction is only triggered once the sperm contacts a non-cellular layer of fibrous glycoprotein (the zona pellucida) that surrounds the egg. In many invertebrates (including sea urchins), the acrosome reaction also causes polymerization of actin monomers into the long protruding acrosome process.

The acrosomal reaction depends on Ca2+, evidenced by: its inability to occur without Ca2+; and its induction by A23187, a Ca2+ ionophore which inserts in the membrane to allow a flux of Ca2+ into the cell. Other Ca2+-dependent processes include the breakdown of cortical granules, release of neurotransmitters and the secretion of insulin precursors from pancreatic cells.

Fast BlockThe fast block to polyspermy (aka depolarization) occurs within 1-3 seconds of sperm entry. The egg membrane depolarizes from -70mV to 0mV due to an influx of Na+ and efflux of K+. This disallows further sperm entry. Laurinda Jaffe showed that: eggs held at -70mV are susceptible to polyspermy; eggs held at 0mV are impermeable to sperm; and that a Na+-influx/K+ efflux is responsible for this depolarization.
Sperm-Egg FusionAbout thirty seconds after initial contacts, the sperm has penetrated the vitelline membrane (in sea urchins) or the zona pellucida (in mammals) and its cytoplasmic membrane fuses with the egg's cytoplasmic membrane. In mammals, this fusion is mediated by the sperm membrane protein fertilin, which binds a specific egg membrane receptor. The fertilization cone rises around the sperm (due to egg cortex microfilaments) and pulls the sperm nearer, allowing the egg and sperm membranes to fuse and the merging of sperm and egg cytoplasm.
Slow BlockThe slow block to polyspermy (aka cortical reaction) permanently blocks sperm entry to the egg by raising the fertilization membrane (forming an impenetrable layer). It begins within ~1 minutes of sperm entry.
Ca2+ InfluxThe endoplasmic reticulum releases a huge quantity of Ca2+ into the cytoplasm.
DegranulationThe Ca2+ influx makes cortical granules between the egg membrane and egg outer layers release their contents. Cortical granules are Golgi-derived membrane-bound structures that contain proteolytic enzymes, polysaccharides and protein; there are thousands of cortical granules per egg.
Outcome

Release of cortical granule contents between egg outer layers and membrane does:

  1. Polysaccharides absorb water and swell.
  2. Proteolytic enzymes break bonds egg membrane and outer layers.
  3. Proteolytic enzymes strip off any remaining sperm.

Evidence that degranulation of cortical granules is Ca2+-dependent is confirmed by:

  1. Cortical vesicles break down just after the Ca2+ wave.
  2. Cortical vesicles break down in response to A23187, which creates a Ca2+ influx.
  3. Cortical vesicle breakdown is inhibited by procaine, which blocks release of bound Ca2+.
  4. Injection of EGTA (which chelates Ca2+) into the egg blocks the cortical reaction.
ActivationNow that the egg is fertilized, the egg ceases to be arrested at meiosis I or meiosis II, followed by fusion of the sperm and egg nuclei. Fertilization leads to an influx of Na+ and Ca2+ and an efflux of H+, creating acidic conditions that activate the rates of: protein synthesis (from stored maternal mRNA); RNA synthesis (in mammals); metabolism; and rapid DNA synthesis and cleavage. These processes are similar as in cells stimulated by growth factor, and by cancer cells.
  1. ↑ Protein synthesis.
  2. Completion of meiosis and pronuclei fusion.
  3. ↑ DNA synthesis.
  4. ↑ RNA synthesis.

Activation of Translation

Some mRNAs in the cytoplasm of egg cells are not translated until the egg is fertilized. The same process regulates the translation of some mRNAs in the dendrites of neurons so that they are not translated until that dendrite receives synaptic input from an associated axon of another neuron. Translation can be controlled by cytoplasmic polyadenylation. Certain mRNAs are made to have short poly(A) tails and thus no translation initiation. These are localized at synapses, far away from that cell's nucleus. Synaptic activity will stimulate polyadenylation in the immediate region of the synapse. It has been determined that this process is utilized by the egg, and is likely similar to that in neurons:

Translational DormancyThe dormant state is maintained by binding of cytoplasmic element binding protein (CPEB) to the cytoplasmic polyadenylation element (UUUUAU) which causes polyadenylation to halt. CPEB binds Maskin, which then binds EIF4E.
Translational ActivationPhosphorylation of CPEB causes Maskin to fall off. All the poly(A) factors are now free to bind, extend the poly(A) tail and thus make the mRNA translationally active.
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