Molecular markers of fertility in cattle oocytes and embryos progress and challenges

In order for assisted reproduction technologies to improve, better methods to discern eggs and embryos according to their level of developmental competence are urgently required to substitute or complement the subjective morphological selection criteria still broadly in use. Objective and reliable molecular markers of viability have been studied during the last decades as robust options to select the best oocytes and embryos for embryo transfer programs. These molecular methodologies rely mostly on the novel “OMICS” technologies. Among these, transcriptomics is the primary platform applied so far in animal breeding research mainly due to the possibility to amplify small samples. In addition, biomarkers of competence have not only been instrumental to select the best oocytes and embryos for reproductive technologies, but have also shed light on the intricate molecular physiology leading to the acquisition of developmental capacity within the ovary and on how the embryo manifests this potential during culture. Nevertheless, such molecular profiling usually implies the destruction of the oocyte/ embryo hereby preventing the practical use of biomarkers in in vitro embryo production and transfer systems. Alternatively , encouraging results have been lately obtained from non-invasive technologies based on biopsies of follicular somatic cells surrounding the developing egg, as well as from metabolic analysis of follicular fluid or spent culture media. This work summarizes the achievements of recent years in the field of biomarkers of competent bovine embryos. The main challenges will be exposed, while the future guidelines will help to comprehend why biomarkers of developmental competence appear promising to take us steps forward in the amelioration of reproductive technologies.(AU)

Molecular markers of fertility in cattle oocytes and embryos progress and challenges

In order for assisted reproduction technologies to improve, better methods to discern eggs and embryos according to their level of developmental competence are urgently required to substitute or complement the subjective morphological selection criteria still broadly in use. Objective and reliable molecular markers of viability have been studied during the last decades as robust options to select the best oocytes and embryos for embryo transfer programs. These molecular methodologies rely mostly on the novel “OMICS” technologies. Among these, transcriptomics is the primary platform applied so far in animal breeding research mainly due to the possibility to amplify small samples. In addition, biomarkers of competence have not only been instrumental to select the best oocytes and embryos for reproductive technologies, but have also shed light on the intricate molecular physiology leading to the acquisition of developmental capacity within the ovary and on how the embryo manifests this potential during culture. Nevertheless, such molecular profiling usually implies the destruction of the oocyte/ embryo hereby preventing the practical use of biomarkers in in vitro embryo production and transfer systems. Alternatively , encouraging results have been lately obtained from non-invasive technologies based on biopsies of follicular somatic cells surrounding the developing egg, as well as from metabolic analysis of follicular fluid or spent culture media. This work summarizes the achievements of recent years in the field of biomarkers of competent bovine embryos. The main challenges will be exposed, while the future guidelines will help to comprehend why biomarkers of developmental competence appear promising to take us steps forward in the amelioration of reproductive technologies.

Improving oocyte quality in cows and heifers - What have we learned so far?

The major challenge in assisted reproduction technologies remains producing oocytes with optimal developmental competence. Such developmental competence can be defined as the ability of the oocyte to fulfill maturation, undergo su ccessful fertilization, reach the blastocyst stage and yi eld a viable and healthy progeny. The main follicular parameters linked to oocyte competence are presented in this review: follicle size, health/atresia status, effects of superstimulation and level of differentiation. In a commercial environment, exogenous FSH superstimulation combined with FSH starvation (referred to as the coasting period) and ovum- pick up followed by in vitro maturation and fertilization, leads to maximal blastocyst rates in Bos taurus heifers and cows. This coasting period increases the proportion of medium-to-large size follicles and may induce slight atresia in the follicular cells, which improves developmental competence. The optimal period for oocyte retrieval in this context has been recently characterized in cows and is re lated to follicular size and state of differentiation. Apoptosis related signals have been identified in various stud ies as implicated in antral folliculogenesis. Follicular somatic cell transcriptome studies are currently used to identify competence related markers. Finally, oocyte competence acquisition is a complex and dynamic process. The best oocytes are obtained from follicles of the optimal size and differentiation status, which can be manipulated with the right hormonal regimen.

Improving oocyte quality in cows and heifers - What have we learned so far?

The major challenge in assisted reproduction technologies remains producing oocytes with optimal developmental competence. Such developmental competence can be defined as the ability of the oocyte to fulfill maturation, undergo su ccessful fertilization, reach the blastocyst stage and yi eld a viable and healthy progeny. The main follicular parameters linked to oocyte competence are presented in this review: follicle size, health/atresia status, effects of superstimulation and level of differentiation. In a commercial environment, exogenous FSH superstimulation combined with FSH starvation (referred to as the coasting period) and ovum- pick up followed by in vitro maturation and fertilization, leads to maximal blastocyst rates in Bos taurus heifers and cows. This coasting period increases the proportion of medium-to-large size follicles and may induce slight atresia in the follicular cells, which improves developmental competence. The optimal period for oocyte retrieval in this context has been recently characterized in cows and is re lated to follicular size and state of differentiation. Apoptosis related signals have been identified in various stud ies as implicated in antral folliculogenesis. Follicular somatic cell transcriptome studies are currently used to identify competence related markers. Finally, oocyte competence acquisition is a complex and dynamic process. The best oocytes are obtained from follicles of the optimal size and differentiation status, which can be manipulated with the right hormonal regimen.(AU)