Fertilization involves the union of haploid gametes: a sperm and an oocyte to give rise to a diploid zygote. Fertilization is a unique process, which requires a precise series of cell-cell interactions. It starts when the sperm locates the egg and binds to it in a species-specific manner. The gametes then fuse resulting in the presence of two haploid pronuclei in the egg cytoplasm, following which the egg completes meiosis and embryonic development begins. Various aspects of fertilization have been studied extensively in numerous model systems.
In both vertebrates and invertebrates, for successful fertilization to take place sperm must compete with each other to fertilize the oocyte. Only a fraction of the large number of sperm ejaculated in the female reproductive tract is capable of reaching and fertilizing the egg. Within the reproductive tract where the sperm are stored, only a few become capacitated and are able to fertilize the oocyte. In marine species where fertilization is external and occurs in an aqueous environment, chemo attractants released by the female gamete in its vicinity help sperm navigate chemotactically.
Chemotaxis towards the oocyte occurs in a species-specific manner with sperm being attracted to only specific female chemo attractants. In mammals, sperm deposited in the vagina have to travel to the oviduct. Sperm that successfully migrate to the oviduct are stored and subsequently undergo capacitation. The spermatozoa are then released from the oviduct epithelium and guided to the oviduct ampulla in which the egg is released. This journey through the oviduct to the egg takes place along an increasing chemical gradient and can occur in a straight or curved line based on the movement of sperm flagella. The speed of swimming also increases as the sperm get nearer to the chemoattractant source.
Some known examples amongst the few identified chemoattractants are Resact, secreted by sea-urchin egg jelly. In mammals, Olfactory Receptor (OR) 17-4 in sperm binds to a chemo attractant. The exact mechanisms by which sperm sense these chemical molecules are unknown. It is however, plausible that the varying concentrations of different chemical attractants may serve as spatial and temporal coordinates for the passage of spermatozoa towards the egg. Recent studies on the molecular mechanisms of sperm chemotaxis have implicated either a guanylyl cyclase (sea-urchin) or an adenylyl cyclase (mammalian) as the sperm chemoreceptor and thus involve increases in CGMP or cAMP. Activation of these pathways either by Resact or OR 17-4 leads to changes in intracellular calcium, which then in turn alters flagellar movement of sperm. Sperm movement towards the egg involves certain basic principles of cell migration and is specific to each species. It is the first and most critical step of fertilization.