Enzymatic characterization of zona pellucida hardening in human eggs and embryos

To characterize possible hardening of the human zona pellucida (ZP) and evaluate the effect of culture duration, patient age, and ZP thickness, ZP of unfertilized eggs (experiment 1, n = 367; experiment 2, n = 174) and abnormal embryos (experiment 1, n = 52) were randomly designated for alpha-chymotrypsin treatment after 0, 24, 48, 72, 96, 120 h (experiment 1) and 48 h, 72 h, and 1 week (experiment 2) of in vitro culture in HTF medium supplemented with 0.5% human serum albumin. Mean ZP thickness was predetermined in experiment 2.

The dispersion of the ZP glycoproteins was assessed, and the duration of time for complete digestion was recorded as an index of ZP hardness.

In experiment 1, enzyme digestion duration increased (P < 0.05) in the first 24 h in vitro from 18.0 +/- 2.0 to 34.6 +/- 2.5 min, and tended to decrease over the next 4 days in culture (25.2 +/- 1.3, 29.4 +/- 0.9, 27.3 +/- 0.6, 26.6 +/- 1.1, and 20.7 +/- 1.5 min on Day 2-6 ZP, respectively). Zona hardening of fertilized eggs was revealed by a longer (P < 0.01) digestion time (32.2 +/- 1.8 vs 25.8 +/- 0.6 min).

There were significant patient-to-patient variation (16.4 +/- 0.7 to 39.6 +/- 2.2 min); however, age was not correlated to enzyme digestion duration. In experiment 2 we determined that ZP thickness (range 8.4-21.6 microns; mean 14.6 +/- 0.2 microns) was not correlated (r = 0.09) to the digestion interval (mean 24.3 +/- 0.8 min). Based on our enzymatic ZP digestion measurements, it is apparent that spontaneous zona hardening does occur within 24 h of in vitro culture, similar to levels achieved postfertilization. The data do not support, however, the concept that additional, abnormal hardening of the ZP occurs during extended culturing.