Egg-penetration ability and structural properties of human sperm prepared by Percoll-gradient centrifugation

Human sperm with a high zona-free hamster egg-penetration ability were obtained by centrifuging freshly ejaculated sperm through a discontinuous two-step (47% and 90%) Percoll gradient at 600g at room temperature for 30 min. Highly motile sperm with good penetration ability were recovered in the pellet fraction (Per-sperm), whereas those with low penetration ability were in the gradient interface. The increased penetration ability of Per-sperm, as compared to sperm capacitated by other methods such as a single-tube swim-up or multiple-tube swim-up preparation, was not due to an increased proportion of acrosome reacted sperm. Rather, transmission electron microscopy indicated that Per-sperm were devoid of coating envelopes, which were present around both the head and tail regions of noncapacitated and single-tube swim-up sperm. Changes to the surface of Per-sperm were demonstrated by their decreased interaction with UEA I lectin, which binds specifically to fucose residues. Removal of the coating envelopes as well as other changes on the sperm surface may lead to an enhanced binding of Per-sperm to the oocyte. In addition, 99% of Per-sperm contained chromatin that was fully condensed. By contrast, about 15% of swim-up sperm still possessed incompletely condensed chromatin. With a higher penetration ability, "clean" appearance, and homogeneity of condensed chromatin, Per-sperm are recommended for insemination and studies of human sperm capacitation.