Factors affecting human blastocyst formation in vitro and freezing at the blastocyst stage.

Whatever the culture medium, embryo culture generally leads to a major loss of viability in mouse, rabbits even if the morphological development of the embryo is preserved. Moreover, Embryo metabolism is commonly depressed in culture media. The protein turnover is accelerated and the quality of the metabolites transport systems is impaired. Various coculture systems have been designed to avoid this loss of viability and in some animal species, to overcome the so called "embryo developmental arrest" usually observed at the approximate time of genomic activation. Moreover, it is clear now that cocultured embryos have usually higher cells numbers than those observed for embryos cultured in classical culture media. In the human, the problems seem less complicated because embryos can be transferred into the uterus on the second day post fertilization, at a time when they would normally be in the Fallopian tube: this is not possible in animal species. Also, blastocysts can be obtained, even at low rates, in conventional culture media and there is no apparent block of development. In this paper, we will present an overview of Cocultures in different species. Then, we will focus on the Human including the blastocyst formation rate and freezing at the Blastocyst stage. At the beginning of the Story, For coculturing, 2 ideas were put forward: The use of embryonic tissue (trophoblast) to help the embryo through an autocrine effect. The use of female genital tract cells, to assist the embryo through a paracrine effect.