[Sequential media: why and how?].

Blastocyst transfer is highly recommended for the following indications to avoid repeated failure of implantation: related to maternal, paternal and cytogenetic aspects on the embryonic side and/or uterus motility. Sequential media have now replaced coculture in order to produce blastocysts. The basic idea of sequential media is to follow embryo's need. There are 2 phases during preimplantation period: the first one corresponds to a development through maternal stores, gathered during maturation, before genomic activation. This requires a complete protection against free radicals (through EDTA), and a decrease in the concentration of glucose and phosphate: mitocondrial function is impaired. Regulation of the endogenous pool is not perfect. Then at the time of genomic activation, there is an increased need for numerous metabolites. Insulin can be added as the I-Receptor appears at the 8-cell stage. The medium used during this phase has to be rich.