Andrology: The influence of sperm morphology and the acrosome reaction on fertilization outcome after sub-zonal injection (SZI) of human spermatozoa

The usefulness of sub-zonal injection (SZI) for the treatmentof severe male factor infertility has been restricted by lowand unpredictable fertilization rates and the high risk of polyspermyafter the injection of multiple spermatozoa. In this prospectivestudy, we have evaluated whether sperm morphology and the percentageof acrosome-reacted spermatozoa at the time of injection canbe used to predict SZI fertilization outcomes. Populations ofmotile spermatozoa equivalent to those injected were collectedfrom the medium/oil interface immediately after SZI of eachcohort of oocytes. Morphology was assessed using the World HealthOrganization 1987 criteria and the acrosomal status of spermatozoawas determined after staining with rhodamine-conjugated Pisumsativum agglutinin. A fertilization index (FI) was calculatedto express the actual fertilizing potential of the spermatozoainjected. In all, 67 patients underwent 72 SZI cycles. The overallfertilization and polyspermy rates were 36 and 47% respectively,and a clinical pregnancy rate per transfer of 22% was achieved.Linear regression analysis demonstrated a statistically significantrelationship between morphology and the FI (r = 0.506, P <0.0001). Patients with <10% normal morphology always hada FI < 10%, and this was reflected by low fertilization andpolyspermy rates and the high number (32%) of cycles with completefailure of fertilization in this group. In patients with >10% normal morphology, there were two patterns: low (10% FI)or high (>10% FI) fertility. This was evident in the fertilization(23 and 85%, respectively) and polyspermy (25 and 68%, respectively)rates of these two patient sub-groups. While the percentageof acrosome-reacted spermatozoa at the time of injection wasweakly correlated with the FI (r = 0.292, P < 0.05), it couldnot be used to predict differences in fertilization potentialbetween patient sub-groups. We conclude that sperm morphologyand acrosomal status at the time of injection are of limiteduse in predicting SZI fertilization outcomes, although patientswith poor morphology ( 10% normal) have lower fertilizationand polyspermy rates.