Mammalian gametes and embryos are particularly vulnerable to oxidative stress-induced damage, which is mainly caused by reactive oxygen species (ROS) originating from normal metabolism and/or the external environment.  Several researchers have implicated the role of oxidative stress in the activation of apoptosis, causing peroxidative damage to sperms/oocytes and inducing embryo fragmentation, arrest, or demise.  Melatonin is a tryptophan derivative that is known for its powerful free radical-scavenging activity and broad-spectrum antioxidant property.  Numerous studies have shown that melatonin and its metabolic derivatives can sequentially detoxify ROS in an antioxidant cascade, and modulate various antioxidant enzymes via its receptors to prevent radical-mediated damage.  The identification of melatonin receptors in cumulus/granulosa cells, oocytes, and epididymal tissues implies that melatonin has protective actions on gametes and embryos.  Enriching the semen extender or culture medium with melatonin significantly benefits sperm characteristics, improves oocyte maturation potential and quality, and enhances the developmental competence of preimplantation embryos.  Certainly, further comparative studies are needed to show the unique antioxidant role and the advantage of melatonin in this field.  This review summarizes the harmful effects of ROS and the beneficial role of melatonin against oxidative damage of gametes and embryos. 

    The antioxidant of reduced glutathione (GSH) is the most abundant thiol in cells for the maintenance of the intracellular redox balance. The study aimed to assay the effect of sperm treatment with GSH before incubation with oocytes on the development potential of embryos obtained by in vitro fertilization (IVF). Also the mitochondrial membrane potential (ΔΨm), plasma membrane integrity (viability), DNA fragmentation, reactive oxygen species (ROS) content, superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) activities, methane dicarboxylic aldehyde (MDA) level as indices of lipid peroxidation in sex-sorted and unsorted sperm from three bulls were investigated using flow cytometry and enzyme-labeled instrument individually. The results showed that 2 mmol L^–1 GSH increased significantly the cleavage rate (86.68% vs. 82.78%), 4- to 8-cell rate (82.30% vs. 73.43%) and blastocyst rate (43.15% vs. 35.24%) of IVF embryos compared with untreated group. Furthermore, addition of GSH increased significantly the ΔΨm and viability, decreased the ratio of DNA fragmentation in sex-sorted or unsorted semen (P<0.05), except the sex-sorted semen from bull 019. Similarly, activities of SOD, CAT and GPx were increased significantly. However, the contents of MDA were decreased significantly both in sex-sorted and unsorted semen treated with GSH (P<0.05). These results suggest that sperm pretreatment with GSH during IVF can maintain better the viability and fertility of sperm through reducing apoptosis and increasing the antioxidant capacity, which improves the IVF embryos development.

To our knowledge, no single study has systemically compared cryopreservation efficiencies of bovine blastocysts derived from in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI) and somatic cell nuclear transfer (SCNT) by controlled freezing and vitrification. This experiment, therefore, was designed to compare the cryopreservation of these blastocysts with controlled freezing and OPS vitrification. Adenosine-5´-triphosphate (ATP) content and reactive oxygen species (ROS) level in blastocysts were also analyzed. Firstly, for each type of blastocyst (IVF, ICSI or SCNT), significant differences were observed between the survival rates of the controlled freezing ((81.56±2.33), (68.18±4.72) or (47.89±5.83)%) and OPS vitrification groups ((92.24±4.54), (82.40±3.76) or (78.71±5.91)%; P<0.05). Secondly, for each type of blastocyst (IVF, ICSI or SCNT), ATP content was significantly decreased after controlled freezing or vitrification, and the ATP content in the controlled freezing group (0.43±0.06), (0.35±0.05) or (0.21±0.02) pmol) was significantly lower than that found in the OPS vitrification group (0.62±0.04), (0.46±0.03) or (0.30±0.01) pmol; P<0.05). Thirdly, ROS level in fresh IVF ((47.33±3.56) c.p.s (counted photons per second), ICSI ((36.51±2.58) c.p.s) or SCNT blastocysts ((26.44±1.49) c.p.s) was significantly lower than that found in the OPS vitrification group ((72.14±4.31), (58.89±3.89) or (40.11±5.73) c.p.s; P<0.05), but higher than that of the controlled freezing group (34.41±3.32), (23.13±1.26) or (15.46±2.45) c.p.s; P<0.05). The present study indicated that vitrification is more efficient in the cryopreservation of bovine blastocysts derived from IVF, ICSI or SCNT than controlled freezing. Furthermore, both vitrification and controlled freezing significantly altered the ATP content and ROS level in those blastocysts.