During fertilisation, the fusion of the sperm and ovum forms a diploid zygote, marking the beginning of the organism's development. The success of fertilisation is ensured by capacitation, the maturation of sperm within the female reproductive system. The entry of HCO3- and Ca2+ and the exit of H+ ions from the head of the sperm, as well as the effects of progesterone, all cause an increase in the intracellular pH and hyperactivation of the sperm, which facilitates its penetration toward the ovum. Sperm guidance within the female reproductive system is aided by estrogens, which promote passive movement, as well as chemotaxis, thermotaxis and rheotaxis - processes that are all dependent on the increase in sperm intracellular Ca2+ concentration. When the gametes meet in the oviduct, the interaction between the SED-1 receptor on the sperm head and the zona pellucida enables the binding of the ZP3 glycoprotein to β1,4-galactosyltransferase-I on the sperm. This interaction triggers the acrosomal reaction, which dissolves zona pellucida. The adhesion of the cell membranes depends on the interaction of IZUMO1 (on the sperm head) and Juno protein (on the oocyte), while the proteins involved in their fusion are still unknown. The increase in intracellular Ca2+ concentration during fertilisation causes a cortical reaction, leading to the formation of a fertilisation membrane and the release of the enzyme ovastacin. Ovastacin alters the structure of the ZP2 glycoprotein, hardening the zona pellucida and preventing polyspermy.