Effect of in vitro fertilization medium on the acrosome reaction, cortical reaction, zona pellucida hardening and in vitro development in pigs

Abstract

Physiological events at the time of fertilization of pig oocytes may differ in vitro depending on the in vitro fertilization (IVF) medium. This hypothesis was tested by in vitro maturation of pig oocytes for 44 h in NCSU-37 medium and thereafter fertilization with frozen-thawed ejaculated spermatozoa. Three different IVF media (TCM-199, Tyrode's albumin lactate pyruvate (TALP) and Tris-buffered medium (TBM)) were used. For the acrosome reaction test, spermatozoa were incubated for 0-150 min in the three IVF media, and the proportion of live acrosome-reacted and acrosome-intact cells was determined by fluorescein isothiocyanate-labelled peanut agglutinin (FITC-PNA) and propidium iodide (PI) staining. The cortical granule density of oocytes was evaluated by confocal microscopy, 2.5 and 5.0 h after culture in each medium in the presence or absence of spermatozoa. Zona pellucida resistance to pronase digestion was also determined in the same groups. The percentages of penetration, monospermy, male pronucleus formation, cleavage and blastocyst formation, and the number of cells per blastocyst after culture were determined. The results indicate that the acrosome reaction occurred much faster in TBM than in TCM-199 or TALP medium. Continuous cortical granule synthesis was observed in the three media when oocytes were incubated in the absence of spermatozoa. The presence of spermatozoa triggered the cortical reaction in a large proportion of oocytes fertilized in TCM-199 and TALP media. On the basis of the duration of pronase digestion, the zona pellucida of oocytes incubated in TCM-199 was harder (407.7 +/- 35.5 s) than that of oocytes cultured in TALP (235.4 +/- 18.2 s) or TBM (189.1 +/- 16.8 s). No zona pellucida hardening was noted in oocytes after insemination in any of the media. The percentages of penetration and cleavage were higher in oocytes cultured in TCM-199 and TALP than in TBM. The percentage of monospermy was higher in TCM-199 and TBM than in TALP. No effect of the medium was shown on the percentage of blastocyst formation or on the number of cells per blastocyst. In conclusion, the results highlight how differently the fertilization events take place in each IVF medium and how far these IVF media still are from achieving biological properties of gametes close to those observed in the physiological setting.