Sequential interval micro-droplet loading in closed hemi-straw carrier system: A convenient and efficient method for ultra-rapid cryopreservation in extreme oligozoospermia.

Cryopreservation of human spermatozoa with low concentration while maintaining adequate post-thawing motility remains a major challenge for male fertility preservation. A convenient and efficient ultra-rapid freezing method for small amounts of human spermatozoa in a closed Hemi-Straw carrier system (CHS) was developed. Spermatozoa from 60 healthy men were involved in a parameter refining test and another 15 extreme oligozoospermic specimens were assigned to a verification test. A commercialized sperm freezing medium, Quinn's Advantage® Sperm Freeze medium (glycerol and sucrose as the cryoprotective agent) was used in the study. The results showed that the highest recovery rates would be obtained via the method of 2 μl single droplet sequential interval loading, by placing the straw at 1 cm above the liquid nitrogen (LN) surface for 60 s during freezing and 2 cm above the LN for 2 min during thawing. This method was applied in cryopreservation for the normozoospermic specimens and compared with a conventional slow freezing method. The results were better than those in the control group in the total motility recovery rate (77.8 ± 11.2% vs 56.6 ± 11.9%, P < 0.01), progressive motility recovery rate (77.6 ± 13.2% vs 47.7 ± 14.6%, P < 0.01), 24 h survival index (60.9 ± 13.4% vs 42.1 ± 14.1%, P < 0.01) and the sperm DNA fragment index (4.2 ± 3.7% vs 5.8 ± 3.7%, P = 0.126). This method was applied to the oligozoospermic specimens. Motile spermatozoa could be found in 12 of 15 cases in the ultra-rapid freezing group, while only in 7 cases in control group. The results indicated that this freezing method was simple, convenient and bio-safe for cryopreservation of severe oligozoospermic specimens.