313 APOPTOSIS IN PARTHENOGENETIC PIG EMBRYOS PRODUCED BY DIFFERENT METHODS

Abstract

Apoptosis or programmed cell death is a process during which cells die in a controlled fashion in response to a variety of stimuli. Apoptosis has been demonstrated to occur during pre-implantation development both in vivo and in vitro and it is believed to contribute to early embryonic loss. It is also believed that parthenogenetic embryos generally have a lower developmental potential compared to those derived from fertilization. In the present study we investigated the rate of apoptosis in parthenogenetic pig embryos produced by activating oocytes through various methods. Pig oocytes were collected from slaughterhouse ovaries and matured in vitro. Parthenogenetic development was induced by three different methods. In Group 1, oocytes were activated by two consecutive electrical pulses. In Group 2, oocytes were electroporated and then incubated for 4 h in 5 mM butyrolactone I, a specific inhibitor of cyclin dependent kinases. In Group 3, electroporated oocytes were incubated for 5 h in cycloheximide, a protein synthesis inhibitor. Activated oocytes were cultured for 7 days in NCSU-23 medium. At the end of the culture period the embryos were fixed in 4% paraformaldehyde and permeabilized in 0.1% Triton X-100 with sodium citrate. They were then incubated in a TUNEL reaction medium that specifically identifies apoptotic nuclei by labeling fragmented DNA with a fluorochrome. Blastocysts produced by in vitro fertilization and DNase I-treated embryos were used as controls. The proportions of apoptotic cells were compared using ANOVA. Forty-three blastocysts were analyzed for apoptotic activity in the electroporation group. These embryos had a blastocyst rate of 33.6 ± 8.7%, total cell number of 31.9 ± 13.2, and an average of 2.7 ± 2.2 apoptotic cells per embryo; the rate of apoptosis was 9.1 ± 7.1%. Twenty-eight blastocysts were used for the TUNEL reaction in the group where activation was induced with the combined treatment of electroporation and butyrolactone (Group 2). On average, the blastocyst rate was 54.5 ± 6.4% and blastocysts contained 27.4 ± 9.6 cells of which 2.8 ± 3.9 were apoptotic; the percentage of apoptosis for this group was 10.0 ± 12.1%. In the cycloheximide treated group (Group 3), the onset of apoptosis was investigated using 29 blastocysts. The blastocyst rate was 38.2 ± 15.9% with an average total cell number of 27.2 ± 11.4%. The TUNEL assay revealed that the mean number of apoptotic cells per embryo in these blastocysts was 2.1 ± 1.5, representing 9.0 ± 7.6% apoptotic cells. The blastocysts (n = 14) produced by in vitro fertilization had a blastocyst rate of 18.0 ± 5.1% and 29.9 ± 12.0 cells; 9.2 ± 8.1% of these cells (2.6 ± 2.2 cells per embryo) showed signs of apoptosis. All nuclei in the DNase I-treated embryos showed positive signals following the TUNEL reaction. The results confirm previous findings that apoptosis occurs in blastocyst stage embryos. There was no difference in the percentage of apoptotic cells between embryos whose development was triggered by different oocyte activation methods; the rate of apoptosis in parthenogenetic blastocysts was similar to that in blastocysts produced by in vitro fertilization.