[Preimplantation embryo development: current status and perspectives in clinical embryology]

Preimplantation embryo development is one of the key features with implantation itself to achieve a pregnancy. Assisted Reproductive Technologies (ART) both in human and animal have improved our knowledge on these events, although predicting embryo potential to give a baby remains elusive. However data from the last 10 years have allowed either to hierarchize the available parameters or to open some new perspectives to predict embryo developmental potential. Both kinetics and morphological parameters belong to noninvasive quality embryo assessment for many years, although recent data on microvideographic analysis and multivariate analysis led to reduce their biological meaning. Moreover, new technical detection of meiotic spindle birefringency or zona pellucida anisotropy has improved the oocyte quality assessment with a deep impact for countries with restrictive legislation. Beyond such morphological criteria, more functional approaches concerned the oocyte (embryo) or its environment. Direct transcriptomic analysis, while invasive and therefore experimental, brought important data on embryo "quality". However, noninvasive metabolomic or proteomic analysis of embryo media gave promising results as well as respirometry. The environment of the oocyte has focused a specific attention, either based on regulatory proteins or cytokines present in follicular fluid, or involving genes or proteins from cumulus cells, as oocyte-cumulus dialog is a key factor in oocyte maturation. Whereas it is not possible for the time being to predict which parameter(s) will be implemented routinely, all data obtained underline that the ability to develop and implant is not based on embryo superlatives (more rapid, expressing more genes or proteins, larger metabolites uptake) but rather on a quiet state, as claimed by Leese some years ago, where a lot of resources would not be mobilized by any stressful situation.