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First published online February 9, 2007
doi: 10.1242/10.1242/dev.02752

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Sperm binding to the zona pellucida is not sufficient to induce acrosome exocytosis

¹ Laboratory of Cellular and Developmental Biology, NIDDK, National Institutes of Health, Bethesda, MD 20892, USA.
² Department of Cell Biology and Neuroscience, University of California, Riverside, CA 92521, USA.

^* Author for correspondence (e-mail: borisb{at}mail.nih.gov)

Accepted 21 November 2006

At fertilization, spermatozoa bind to the zona pellucida (ZP1, ZP2, ZP3) surrounding ovulated mouse eggs, undergo acrosome exocytosis and penetrate the zona matrix before gamete fusion. Following fertilization, ZP2 is proteolytically cleaved and sperm no longer bind to embryos. We assessed Acr3-EGFP sperm binding to wild-type and huZP2 rescue eggs in which human ZP2 replaces mouse ZP2 but remains uncleaved after fertilization. The observed de novo binding of Acr3-EGFP sperm to embryos derived from huZP2 rescue mice supports a `zona scaffold' model of sperm-egg recognition in which intact ZP2 dictates a three-dimensional structure supportive of sperm binding, independent of fertilization and cortical granule exocytosis. Surprisingly, the acrosomes of the bound sperm remain intact for at least 24 hours in the presence of uncleaved human ZP2 regardless of whether sperm are added before or after fertilization. The persistence of intact acrosomes indicates that sperm binding to the zona pellucida is not sufficient to induce acrosome exocytosis. A filter penetration assay suggests an alternative mechanism in which penetration into the zona matrix initiates a mechanosensory signal transduction necessary to trigger the acrosome reaction.

Key words: Molecular mechanisms of mouse fertilization, Sperm acrosome exocytosis, Mechanosensory signal transduction, Zona pellucida matrix, Sperm-egg recognition, Postfertilization block to polyspermy

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