oocyte. Progesterone binds to the CatSper receptor on the sperm membrane and increases intracellular calcium levels, causing hyperactive motility. The sperm will continue to swim towards higher concentrations of progesterone, effectively guiding it to the oocyte. Around 200 of 200 millions spermatozoa reach the ampulla. Sperm preparation At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize. The sperm must undergo capacitation in the female's reproductive tract, which increases its motility and hyperpolarizes its membrane, preparing it for the acrosome reaction, the enzymatic penetration of the egg's tough membrane, the zona pellucida, which surrounds the oocyte. Corona radiata The sperm binds through the corona radiata, a layer of follicle cells on the outside of the secondary oocyte. The corona radiata sends out chemicals that attract the sperm in the Fallopian tube to the oocyte. It lies above the zona pellucida, a membrane of glycoproteins that surrounds the oocyte. Cone of attraction and perivitelline membrane Where the spermatozoan is about to pierce, the yolk (ooplasm) is drawn out into a conical elevation, termed the cone of attraction or reception cone. Once the spermatozoon has entered, the peripheral portion of the yolk changes into a membrane, the perivitelline membrane, which prevents the passage of additional spermatozoa. Zona pellucida and acrosome reaction After binding to the corona radiata the sperm reaches the zona pellucida, which is an extracellular matrix of glycoproteins. A ZP3 glycoprotein on the zona pellucida binds to a receptor on the cell surface of the sperm head. This binding triggers the acrosome to burst, releasing acrosomal enzymes that help the sperm penetrate through the thick zona pellucida layer surrounding the oocyte, ultimately gaining access to the egg's cell membrane. Some sperm cells consume their acrosome prematurely on the surface of the egg cell, facilitating the penetration by other sperm cells. As a population, mature haploid sperm cells have on average 50% genome similarity, so the premature acrosomal reactions aid fertilization by a member of the same cohort. It may be regarded as a mechanism of kin selection. Recent studies have shown that the egg is not passive during this process. In other words, they too appear to undergo changes that help facilitate such interaction. Fusion Cortical Reaction After the sperm enters the cytoplasm of the oocyte, the tail and the outer coating of the sperm disintegrate.