The present invention relates to a method and a
system for producing a mammalian pre-
embryo and a
stem cell having a better quality than prior art methods. The
system comprises means for obtaining a mammalian
oocyte, and means for obtaining a mammalian spermatozoa, and an apparatus having at least two separate air-tight chambers, for which the
oxygen tension of one chamber may be changed independent of the
oxygen tension of the other chamber, said at least two separate air-tight chambers constitute a main chamber and at least one
residence chamber. The method for
in vitro producing a mammalian pre-
embryo comprising the steps: a1) providing a mammalian
oocyte, a2) providing a mammalian spermatozoa, b) culturing the
oocyte and the spermatozoa, c) fertilizing the oocyte with the spermatozoa obtaining a fertilized oocyte, and d) allowing
cell-division of the fertilized oocyte obtaining a multicellular pre-
embryo wherein at least one of the steps a1) or a2) is conducted at an
oxygen tension below 15%, or e) allowing
cell-division of the fertilized oocyte obtaining a multicellular pre-embryo, wherein the culture is performed at an
oxygen tension allowing cultivation of the cells and wherein at least one of the steps comprises a change in the
oxygen tension. Stem cells are produced from the multicellular pre-embryo.