Strategies to Overcome OS in Assisted Reproduction

ROS may originate from the male or female gamete or the embryo or indirectly from the surroundings, which includes the cumulus cells, leucocytes, and culture media. In human IVF/ICSI procedures, the clinical pregnancy rates have remained unchanged at 30-40% [218].

It is hypothesized that the altered redox state in in vitro conditions may play a role in poor ART outcomes, and controlling OS may improve ART outcomes. Fertilization and embryo development in vivo occur in an environment of low oxygen tension [46].

It has been noted that blastocyst development in vitro always lags behind blasto-cyst development in vivo as there is a variation in the ability of IVF media and its components to scavenge ROS and prevent DNA damage and apoptosis [219]. During ART procedures, it is important to emulate in vivo conditions by avoiding conditions that promote ROS generation. Accomplishing that has been shown to lead to a reduction in blastocyst degeneration, increased blastocyst development rates; increased hatching of blastocysts, and reduction in embryo apoptosis, and other degenerative pro-oxidant influence has been reported [19]. The available strategies include the following:

1. Ensuring in vitro culture under low oxygen tension conditions: During culture, low oxygen tension conditions improve the implantation and pregnancy rate better than high oxygen tension [45] .

2. Metal ion culture media supplementation: It has been shown that metal ions may enhance the production of oxidants. As a result, it was suggested that it may be useful to add metal ion chelating agents to culture media to decrease the production of oxidants [45] .

3. Enzymatic and nonenzymatic antioxidant culture media supplementation: Higher implantation and clinical pregnancy rates are reported when antioxidant-supple-mented media is used rather than standard media without antioxidants. Various nonenzymatic antioxidants including beta-mercaptoethano [220] , protein [219] , vitamin E [149], vitamin C [221], cysteamine [222], cysteine [223], taurine and hypotaurine [224], and thiols are added to the culture media with the purpose of improving the developmental ability of the embryos by reducing the effects of ROS. Also, the addition of the enzymatic antioxidant, for example SOD, to the culture media prevented the deleterious effects of OS on sperm viability and on the embryo development both in vivo and in vitro. This was demonstrated by increased development of the two-cell stage embryos to the expanded blastocyst stage in the SOD-supplemented media. Mechanical removal of ROS in IVF/ET has been studied as a method to improve IVF outcome [225] . The rinsing of cumulus oophorus has been shown to overcome the deleterious effects of ROS in patients with ovarian endometriosis [225] .

4. Control of sperm ROS production and sperm chromatin damage: Spermatozoa are particularly susceptible to ROS-induced damage because their plasma membranes contain large quantities of PUFA and their cytoplasm contains low concentrations of the scavenging enzymes [193].

5. Reducing sperm-oocyte coincubation time: Reports suggest that a prolonged sperm-oocyte coincubation time (16-20 h) increases the generation of ROS. Two prospective randomized controlled studies have advocated using a shorter sperm-oocyte coincubation time [64] . Coincubation times of 1-2 h resulted in better quality embryos and significantly improved fertilization and implantation rates [226],

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