3.4.4. Blastocyst Competence Profiling in ARTs

Although many of the molecular players involved in the complex process of implantation have been characterized, the selection of competent embryos remains one of the major challenges in ART. A parallel and complementary morphological and molecular profiling analysis of the embryo may represent a successful approach for embryo selection, thus improving IVF outcome. Although morphological characteristics have been significantly associated with euploidy and competence of the embryo [294,295], their evaluation for good quality embryo selection has some limitations, such as operator subjectivity, variability linked to the timing of laboratory observation, culture medium and other culture conditions, hence combined different approaches might be useful [296]. In this respect, metabolomic and proteomic analyses of embryo spent media have been proposed as complementary, non-invasive tools to select embryos with higher implantation ability [297–299]. Limitations derive from the variability of commercial culture media, high metabolic plasticity of the embryos which can adapt to different culture conditions and from the fact that embryo development and metabolism vary under different culture conditions [300]. Recently, novel strategies based on gene expression profiling of trophectoderm biopsies have been developed and have linked gene expression patterns

with developmental competence [301–303]. Although complementary approaches may be used to select the best embryos to be transferred, it should be considered that it has been recently proposed that it is the endometrium itself that selectively discriminates between high-and low-quality embryos in order to guarantee a successful implantation. Based on this, it could be envisioned a test in which the endometrium might be used as a "bio-sensor" to avoid transfer of low-quality embryos, which if implanted would be possibly later rejected resulting in a miscarriage [304,305].
