How the human spermatozoa sense the oocyte: a new role of SDF1-CXCR4 signalling

For fertilization to occur in mammals, ejaculated spermatozoa must reach the egg which, following ovulation has moved from the ovary into the Fallopian tube. Two active mechanisms of spermatozoa guidance have been shown in mammals: thermotaxis and chemotaxis. The identity of most of human spermatozoa chemoattractants is unknown, and herein we tested if SDF1 (chemokine stromal cell-derived factor-1) and its pathway is involved in spermatozoa chemotaxis. We found that SDF1 is expressed in the oocytes, endometrium and follicular fluid, as well as its specific receptor CXCR4 (chemokine CXC motif receptor 4) is expressed in the head of spermatozoa. By SDF1 gradient experiments, we stated that SDF1 is able to induce hyperactivation in spermatozoa leading to accumulation, to give rise to an increase in intracellular calcium concentration, and to preserve the mitochondrial status and not to induce acrosome reaction. Our findings suggest these phenomena could reflect spermatozoa chemotaxis, and that SDF1 action could represent an important event leading to egg fertilization, even if further studies regarding the link between spermatozoa accumulation and chemotaxis are mandatory. These data suggest that the SDF-1/CXCR4 signalling could be used to manipulate the human fertilization, to improve both the outcome of physiological or assisted reproduction, and to develop new contraceptive methods, by development of SDF1 or CXCR4 antagonist.