Extracellular Vesicles, MicroRNA and the Preimplantation Embryo: Non-invasive Clues of Embryo Well-being

Elective single embryo transfer is rapidly becoming the standard of care in assisted reproductive technology for patients under the age of 35 years with a good prognosis. Clinical pregnancy rates have become increasingly dependent on the selection of a single viable embryo for transfer, and diagnostic techniques facilitating this selection continue to develop. Current progress in elucidating the extracellular vesicle and microRNA components of the embryonic secretome is reviewed, and the potential for these findings to improve clinical embryo selection discussed. Key results have shown that extracellular vesicles and microRNAs are rapidly detectable constituents of the embryonic secretome. Evidence suggests that the vesicular population is largely exosomal in nature, secreted at all stages of preimplantation development and capable of traversing the zona pellucida. Both extracellular vesicle and microRNA concentrations within the secretome are elevated for blastocysts with diminished developmental competence, as indicated either by degeneracy or implantation failure, whereas studies have yet to firmly correlate individual microRNA sequences with pregnancy outcome. These emerging correlations support the viability of extracellular vesicles and microRNAs as the basis for a new diagnostic test to supplement or replace morphokinetic assessment.