Figure 3.

Schematic drawing showing the most critical steps during the sperm’s transit through the female genital tract before fertilizing the egg. The gamete interactions are a critical step on reproduction. Mammalian fertilization comprises: (i) sperm migration through the female reproductive tract (rheotaxis, thermotaxis and chemotaxis), (ii) biochemical and morphological changes to sperm (capacitation) and (iii) sperm-egg interaction in the oviduct (fertilization) (A). In the female reproductive tract, specifically in the isthmus of the uterus, the mammalian sperm must undergo a series of important modifications, such as tyrosine phosphorylation, cholesterol depletion, hyperpolarisation and finally hyperactivation. These complex priming processes, by which sperm become competent to fertilize an egg, are all together termed “capacitation” (B). Chemotaxis permit sperm to move into the ampulla and locate the egg, organized in a cell complex (C). The ovulated oocyte is covered by a multicellular cumulus oophorous. The fertilization takes place after specific steps: (i) binding of zona pellucida, (ii) acrosome reaction, (iii) penetration of zona pellucida and (iv) final membrane fusion (D).