88 SELECTION OF A CRYOSURVIVAL SPERM POPULATION DOES NOT IMPROVE THE IN VITRO PENETRATING ABILITY OF FROZEN–THAWED BOAR SPERMATOZOA

Abstract

Boars can be classified as good or bad sperm freezers according to their sperm cryosurvival. Different sperm selection techniques, such as PureSperm^® (PS; MidAtlantic Diagnostics, Inc., Mount Laurel, NJ, USA), have been developed to improve functional competence of spermatozoa. The aim of this experimental study was to assess the ability of PS for improving the in vitro penetrating ability of frozen–thawed boar spermatozoa from good and bad sperm freezers. The sperm-rich fractions from two boars, good (Boar A) and bad (Boar B) freezers, were extended in a lactose/eggyolk/ glycerol/Equex Stem (Noba Chemical Sales, Inc., Scituate, ME, USA) mixture (1 × 10⁹ sperm mL^–1), dispensed into 0.5-mL straws, and frozen using a programmable cell freezer. After thawing (1.200°C min^–1), semen from each boar was split into two aliquots of 500 µL. One aliquot was used as the control. The second was placed into a tube of PS gradient (90%/45%) and centrifuged at 425g for 20 min; the pellet re-suspended in 1 mL of BTS and re-centrifuged at 320g for 10 min (PS sample). Control and PS samples were diluted in supplemented TCM-199 (TCMm; Roca et al. 1998 Reprod. Fertil. Dev. 10, 479–485) at 200 × 10⁶ sperm mL^–1. Sperm survival (SV) was assessed afterTCMm dilution according to progressive sperm motility (PSM, %) using a computer-assisted sperm analysis (CASA) system (ISAS^®), and plasma and acrosome membrane integrity (PMI; %) by flow cytometry (SYBR^®-14/PE-PNA/PI; Molecular Probes, Leiden, The Netherlands). A homologous in vitro penetration (hIVP) assay, using immature oocytes (20 oocytes/2 mL TCMm supplemented with caffeine and calcium lactate), was used to assess sperm penetrating ability (Martinez et al. 1993 Theriogenology 40, 547–557). A total of 960 immature oocytes were inseminated (200 × 10³ sperm/oocyte) in 3 batches. After 18 h of co-incubation at 39°C under 5% CO₂ in air, the oocytes were washed, mounted on slides, fixed with ethanol:acetic acid (3:1, v/v) for 48 h, stained with 1% lacmoid, and examined under a phase contrast microscope (×400). Oocytes with swollen or unswollen heads of sperm found in the vitellus were considered as penetrated. Sperm penetrability ability (SPA) was assessed according to penetration rate (PR) and the mean number of sperm per oocyte (S/O). Data were analyzed using a PROMIXED model and expressed as mean ± SEM. Boar A showed better (P ≤ 0.01) results for both SV and SPA parameters than boar B, independent of sperm treatment. PureSperm improved (P ≤ 0.05) PSM and PMI in both boar A (control v. PS: 48.0 ± 5.8 v. 66.5 ± 3.6 and 63.1 ± 7.7 v. 88.4 ± 1.3, respectively) and boar B (12.3 ± 1.2 v. 22.2 ± 3.7 and 44.3 ± 3.5 v. 58.7 ± 7.0, respectively). However, no differences (P ≥ 0.05) were observed in PR and S/O in either boar A (71.2 ± 3.4 v. 78.3 ± 3.1 and 5.0 ± 0.4 v. 5.2 ± 0.4, respectively) or boar B (34.3 ± 3.6 v. 37.3 ± 3.9 and 1.5 ± 0.1 v. 1.5 ± 0.1, respectively). In conclusion, under our laboratory conditions, PureSperm selection improves sperm quality but not in vitro penetrating ability of frozen–thawed spermatozoa of both good and bad sperm freezers.

This work was supported by CICYT (AGF2005-00760), Madrid, Spain.