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Mónica Marques, Ana Paula Sousa, Artur Paiva, Teresa Almeida-Santos, and João Ramalho-Santos

We have applied the mitochondria-specific superoxide fluorescent probe MitoSOX Red (MitoSOX) to detect mitochondria-specific reactive oxygen species (mROS) production in human sperm samples using flow cytometry. We show that human ejaculates are heterogeneous in terms of mROS production, with three subpopulations clearly detectable, comprising sperm that produce increasing amounts of mROS (MitoSOX−, MitoSOX+, and MitoSOX++). The sperm subpopulation producing the lowest amount of mROS represented the most functional subset of male gametes within the ejaculate, as it was correlated with the highest amount of live and non-apoptotic sperm and increased both in samples with better semen parameters and in samples processed by both density-gradient centrifugation and swim-up, both known to select for higher quality sperm. Importantly, the MitoSOX− subpopulation was clearly more prevalent in samples that gave rise to pregnancies following assisted reproduction. Our work, therefore, not only describe discreet human sperm heterogeneity at the mROS level but also suggests that mROS may represent a strategy to both evaluate sperm samples and isolate the most functional gametes for assisted reproduction.

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Ana Filipa Ferreira, Maria Soares, Sandra Almeida Reis, João Ramalho-Santos, Ana Paula Sousa, and Teresa Almeida-Santos

Mitochondrial supplementation was proposed as a complementary treatment to assisted reproductive technologies to improve oocyte competence and support post-fertilization development. This strategy is based on the fact that poor-quality/aged oocytes contain lower and dysfunctional mitochondria. However, the efficacy and safety of mitochondrial supplementation are still controversial. Therefore, this review summarizes the clinical/biological outcomes of mitochondrial supplementation, aiming to improve oocyte competence or explore the safety of this technique, and was based on an online search using PubMed and Web of Science, until September 2019. The studies included reported outcomes related to the efficacy and safety of mitochondrial supplementation either in human or animal models (bovine, porcine and mouse). Extracted data were organized according to study objective, the mitochondrial source and the main outcomes: fertilization/pregnancy rates, embryo development and adverse outcomes. Clinical pregnancy was not improved in the only randomized controlled trial published, although an increase was demonstrated in other non-randomized studies. Fertilization rate and embryo development were not different from control groups in the majority of studies, although performed in different contexts and using diverse sources of mitochondria. The safety of mitochondria transfer is still a concern, however, the euploid rate and the absence of reported congenital malformation from the clinical studies are reassuring. In summary, mitochondrial supplementation does not seem to cause harm although the benefit of improving oocyte competence is still unclear due to the diversity of methodological approaches and low-quality of the data available. Analyzed data support the need to investigate further, in both pre-clinical and clinical contexts.