Steps to Fertilization

Within one hour of insemination, sperm in the vagina enter the cervix, get into the uterine cavity, into the fallopian tube where they get capacitated and remain capable of fertilization for 48 hours.

The sperm membrane undergoes biochemical changes that enhance its ability to bind to and penetrate the egg.

1. Capacitation

Capacitation occurs by removal and/or alteration of the glycoproteins on the sperm plasma membrane.

Sperm hypermotility facilitates sperm passage through the cumulus matrix.

Noncapacitated sperm fail to penetrate the cumulus matrix.

2. The Acrosome Reaction

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Upon reaching the corona radiata layer of the zona pellucida of the egg, the sperm releases releases the enzyme hyaluronidase, from its acrosome (a cap-like structure) to break down the hyaluronic acid contained in the zona pellucida of the egg and penetrate the egg.

Next, the enzyme acrosin digests the zona pellucida and promotes changes in the structure and fluidity of the membranes of the sperm tip and the egg to cause them to fuse.

Once the sperm has passed through the cumulus matrix, proteins on the sperm plasma membrane bind to specific receptor proteins on the zona pellucida.

Following this initial fusion process, a protein molecule on the sperm acrosome binds to a directly interlocking protein, ZP3, on the egg.

This binding is believed to initiate a series of reactions, including the acrosome reaction, a release of enzymes by the acrosome,which allows the sperm to penetrate the zona pellucida and complete the membrane fusion, initiate the cortical reaction in the egg.

The binding of sperm to egg results in a release of calcium ions, which promote fusion of cortical granule membranes with the egg's plasma membrane.

Fusion expresses factors that cause the release of an externally bound, membrane protein on the outer vitellinelayer of the egg. Release of this protein forces the vitelline layer away from the zona pellucida, moving any other sperm away permanently and inhibiting

any other sperm from fertilizing the egg.

The polysaccharide molecule hyalin is also released from the cortical granules to create a layer around the egg that also impedes any further fertilization preventing polyspermy.

Completion of the Second Meiotic Division

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> NEXT: 06. Completion of the Second Meiotic Division

1. Fusion of Membranes:

   • The membranes of the sperm and egg fuse, allowing the sperm's nucleus to enter the egg.

2. Block to Polyspermy:

   • Once a sperm successfully fuses with the egg, the egg undergoes changes to its membrane to prevent other sperm from entering.

3. Nuclear Fusion:

   • The sperm's nucleus and the egg's nucleus combine to form a single diploid nucleus, creating a zygote.

4. Zygote Formation:

   • The zygote begins to divide and develop as it travels toward the uterus, where it may implant and continue growing into an embryo.