Describe the main events that occur during human fertilization.

Fertilization is an example of a typical cell-to-cell recognition process between the small, assymetrical and motile sperm cell and the large, spherical, immobile egg. As soon as the sperm is released inside the vagina, we have the first event of fertilization which is referred to as capacitation. That is the preparation of the sperm for fertilization, by becoming diluted to the semen present in the vagina. This dilution results in the activation of the sperm's surface and its cilia, making it even more motile. Then, the sperm-egg binding follows, during which the head of the sperm binds to egg-surface receptors. That binding induces, what is called, the acrosome reaction, during which the acrosomal contents (genetic material) of the sperm enter the egg. This overall, process results in the next step in the process, which is known as the sperm-egg fusion. During this stage, all the sperm genetic material will enter the egg, as mentioned above, and will trigger a cascade of events influencing the egg cell. Firstly, there will be an alteration in the ionic gradients across the egg membrane (Na+ and H+), which will subsequently induce an increase in the egg's pH. The final step is the activation of the egg, often referred to as "the egg's response". The goal of that stage is the prevention of polyspermy, which is the fertilization of a single egg by more than one sperm. Cortical granules which are normally located near the egg's surface will immediately fuse with the membrane and exocytosize their contents (cortical reaction). The above contents include a number of enzymes and lipida which will form a protective "layer" around the fertilized egg, to prevent other sperm cells from penetrating. In parallel, subsequent protein synthesis will begin inside the cell to stimulate the development of the zygote.