Development of rat x mouse hybrid embryos produced by microsurgery

Experimental production of hybrid embryos between Mus musculus L. and Rattus norvegicus L. was achieved by nuclear transplantation using both ovulated oocytes in metaphase II and pronuclear zygotes. Recipient egg-cells were of mouse origin in all cases. The developmental potential of hybrids was examined in vivo. Nucleo-cytoplasmic hybrids resulting from the introduction of rat metaphase II chromosomes into enucleated mouse oocytes, which were subsequently activated, were regularly blocked at the 1- or 2-cell stage. Nuclear (genetic) hybrids produced by transfer of a rat nucleus (in the form of metaphase II chromosomes or a pronucleus) into a nucleated mouse recipient (oocyte or zygote) were capable of development to the 5- to 8-cell stage. Transplantation of rat cytoplasm alone to intact metaphase II oocytes, followed by oocyte activation, generated cytoplasmic hybrids which developed to the morula stage. In control experiments (nuclear transfer between mouse oocytes or zygotes), a high proportion of embryos formed morulae and blastocysts. These results demonstrate that the rat nucleus is incapable of functioning in mouse cytoplasm, that introduction of the rat genome into intact mouse egg-cells impairs normal development, and that transfer of foreign (rat) cytoplasm into mouse egg-cells affects preimplantation development of manipulated embryos.