Differential binding of X- and Y-chromosome-bearing human spermatozoa to zona pellucida in vitro

The sex of human offspring has been associated with the day in the mother's menstrual cycle on which insemination occurs, with male zygotes being formed earlier in the fertile period than female zygotes. Using an in vitro environment designed to mimic the in vivo milieu, we tested the hypothesis that Y-chromosome-bearing spermatozoa survive functionally longer than X-chromosome-bearing spermatozoa, and that this differential functional survival is a contributing factor to the in vivo phenomenon. Donor semen was processed by swim-up and incubated at 37 degrees C in culture medium for 0, 24 and 48 h, with human zona pellucida (hemizona, HZ) being used to select functional spermatozoa. A second set of in vitro storage conditions, 4 degrees C in test-yolk refrigeration buffer, was used to determine whether changing the incubation conditions alters the process. The sex chromosome of the spermatozoa was determined using fluorescent in situ hybridization (FISH). For spermatozoa incubated at 37 degrees C, the percentage of functional (HZ bound) Y-bearing spermatozoa was significantly increased (P < 0.05) at 48 h (55.4 + 2.9%) but not at 0 h (50.5 + 0.7%) or 24 h (52.8 + 3.1%) compared to swim-up spermatozoa (50.6 + 0.3%). No difference in the percentage of functional Y-spermatozoa was observed at any time-point with storage at 4 degrees C in refrigeration buffer. Thus, we demonstrated that significantly more Y-bearing spermatozoa were capable of zona binding than X-bearing spermatozoa at 48 h at 37 degrees C incubation, with an observed Y : X ratio of 1.15 for these zona-bound spermatozoa compared to 1.02 for post-swim-up spermatozoa. We conclude that a differential functional survival appears to exist between X-bearing and Y-bearing spermatozoa, with the latter exhibiting a longer functional survival under in vitro conditions.