Nuclear transplantation in mus musculus: Developmental potential of nuclei from preimplantation embryos

Genomic exchange of a mammalian cell has been attempted by entirely replacing the nuclear genome of the mouse egg with a nucleus from an embryonic cell via nuclear transplantation. Its biological applicability to experimental embryology has first been probed on the mouse blastocyst. After mechanical isolation of the trophectoderm (TE) and inner cell mass (ICM) from LT/Sv or CBA/H-T6 blastocysts and subsequent dissociation into single cells, their nuclei were transplanted singly into fertilized C57BL/6 eggs from which the female and male pronucleus were removed after nuclear injection. Of the 179 eggs injected with TE nuclei, 68 (38%) survived microsurgery and 34 cleaved, but most of them became arrested during early preimplantation development. Biochemical analysis of 25 TE nuclear-transplant embryos showed that nuclei from TE cells were not capable of initiating nor supporting development beyond preimplantation without participation of the egg genome. Of the 363 eggs injected with ICM nuclei, 142 (39%) survived micromanipulation and were cultured in vitro. From the 96 cleaving eggs, 48 embryos reached the late preimplantation stage. Biochemical or karyotypic analyses of 54 ICM nuclear-transplant embryos at various preimplantation stages for either strain-specific variants of glucosephosphate isomerase (GPI) or chromosomal markers (T6 translocation) revealed that, in most instances, the transplanted nuclei successfully promoted development of the recipient eggs. A total of 16 nuclear-transplant embryos originating from ICM nuclei were transferred into the uteri of pseudopregnant ICR/Swiss females in order to allow development to term. Three liveborn mice, two females and one male, were derived exclusively from the transplanted ICM nuclei as judged by their coat color, karyotype and GPI enzyme pattern. Furthermore, in subsequent matings, two of the nuclear-transplant mice, one female and one male, gave rise to several normal progeny all of which exhibited the nuclear-donor phenotype. Our results demonstrate that nuclei from ICM cells of the mouse blastocyst are not yet restricted in their differentiation capacities but rather remain developmentally equivalent to the totipotent zygote nucleus.