Sperm head decondensation, pronuclear formation, cleavage and embryonic development following intracytoplasmic injection of mitochondria-damaged sperm in mammals

The objective of this study was to investigate the influence of sperm mitochondrial destruction on sperm head decondensation, male pronuclear formation, cleavage and embryonic development. In the study two models were used: heterologous (hamster ICSI assay: human sperm injected into a hamster oocyte) for evaluation of sperm head decondensation and pronuclear formation, and homologous (mouse model) for the study of fertilisation and development. Destruction of mitochondria of the sperm was achieved by exposure to cyanide, a respiratory poison. Rhodamine 123 was used to evaluate the functional integrity of mitochondria. Sperm head decondensation was found to be not statistically significantly affected by mitochondrial damage (p = 0.8), with 62.8% and 67.9% condensation in the experimental and control groups respectively. Male pronucleus formation was seen in 40.2% and 44.4% of the injected oocytes in the experimental and control groups respectively. In the mouse experiments 45.5% and 49.7% of the injected oocytes were fertilised in the mitochondria-damaged and live-intact sperm groups respectively (p = 0.53). Development to blastocyst was achieved in 53.5% and 59.4% of the experimental and control groups respectively; the difference was not significant (p = 0.71). Inner cell mass (ICM) cell number were 15.7 +/- 4.12 and 43.1 +/- 11.3 respectively in the mitochondria-damaged group; the equivalent numbers were 14.12 +/- 4.02 and 39.3 +/- 12.6 in the control group. However, the differences in ICM and total cell counts between these two groups were not significant. Of the blastocysts transferred to pseudopregnant mice, 51.3% (20/36) implanted and 33.4% (12/36) developed to live fetuses in the mitochondria-damaged group. These rates were 60.5% (23/38) and 39.5% (15/38) in the control group. In conclusion, this study shows that functional integrity of the sperm mitochondria is not necessary in the process of fertilisation and development when the sperm is deposited into the ooplasm. Fertilisation and development can be achieved by injection of sperm at the very early stage of necrosis in which only the mitochondria have been destroyed and the rest of the cell including the plasma membrane is still intact.