Apoptosis in Parthenogenetic Preimplantation Porcine Embryos1

Parthenogenesis (PA) of the oocyte is essential to a number of oocyte- or embryo-related technologies such as intracytoplasmic sperm injection and cloning by nuclear transfer. This study investigated the onset and frequency of apoptosis in PA- porcine embryos and the morphological changes that conform to the general criteria of apoptotic cell death by using a terminal deoxynucleatidyl transferase-mediated deoxyuridine 5-triphosphate nick-end labeling (TUNEL) assay. PA embryos had a higher degree of apoptotic cell death during in vitro culture, a lower cleavage rate (45% vs. 71%), and a lower development rate to the blastocyst stage (16% vs. 29%), relative to in vitro fertilization (IVF). The earliest positive TUNEL signal in the PA embryos was detected on Day 6, 1 day later than that in IVF embryos. Apoptosis in PA embryos increased from 15% of the embryos on Day 6 to 29% on Day 8. The mean level of apoptosis of the PA embryos was statistically higher than that of IVF embryos, except on Day 5. In particular, apoptosis in PA embryos was twice that of IVF embryos on Day 6 (15% vs. 6.7%) and Day 8 (29% vs. 13%). The mean cell number in PA blastocysts was significantly lower than that of IVF blastocysts, whereas the percentage of apoptosis in PA blastocysts was significantly higher than that of IVF blastocysts. There was a high percentage of haploid (62.5%) PA blastocysts. The ploidy may contribute to a high level of apoptosis. These results may help to explain the mechanism of parthenogenetic developmental failure and may lead to methods that will improve parthenogenetic development.