Acrosome‐reacted human sperm in insemination medium do not bind to the zona pellucida of human oocytes

Summary In the literature there is still confusion whether acrosome‐reacted sperm in medium can initiate primary binding to human zona pellucida (ZP). The ability of acrosome‐reacted sperm to bind to ZP in vitro can be deduced by measuring the acrosome reaction (AR) of ZP‐bound sperm compared with sperm in medium after incubation under different conditions inhibiting the ZP‐induced AR. Motile sperm from fertile men, normospermic men and infertile men diagnosed with disordered ZP‐induced AR (DZPIAR) were selected by swim‐up (2 × 10 6 in 1 mL medium) and incubated for 1–2 h with four oocytes from failed in vitro fertilization (IVF). The acrosome status of sperm was assessed using pisum sativum agglutinin labelled with fluorescein. The ZP‐induced AR was inhibited in experiments using sperm from DZPIAR patients, hyper‐osmotic medium (400 mOsm/kg) and medium containing soybean trypsin inhibitor (SBTI; 4 mg/mL). Pre‐treatment with calcium ionophore was used to create a sperm population with elevated AR. In all experiments with factors inhibiting the ZP‐induced AR, the AR was significantly lower for ZP‐bound sperm compared with sperm in medium: DZPIAR patients 4% vs. 15%, hyper‐osmotic medium 3% vs. 12%, SBTI 2% vs. 12% and SBTI 3% vs. 23% after treatment with calcium ionophore. In conclusion, acrosome‐reacted sperm in vitro have significantly reduced, in fact probably zero ability to bind to the ZP.