An investigation of the fate of cells transplanted orthotopically between morulae/nascent blastocysts in the mouse

A technique has been devised for selectively replacing some inside cells (ICs) or outside cells (OCs) of late morulae/nascent blastocysts with corresponding cells from genetically dissimilar, synchronous embryos. The main purpose was to determine whether the inner cell mass (ICM) contributes cells to the overlying polar trophectoderm at any stage during the blastocyst phase of development. Notwithstanding the high incidence and level of chimaerism in ICM derivatives of postimplantation conceptuses obtained in the IC transplantation experiments, trophoblast tissue was composed entirely of host cells in the majority of cases. Even where a donor IC contribution to trophoblast was detected there were strong grounds for suspecting that it was due to tissue contamination rather than genuine chimaerism. Thus, not only did such contributions differ in both level and distribution from those produced by transplantation of OCs but they also varied markedly in frequency according to the day of gestation on which conceptuses were dissected. The possibility that ICs regularly colonize the polar trophectoderm but fail to persist there was excluded by the results of short-term transplanta tion experiments using an in-situ genetic marker. These findings offer no support for the hypothesis that the ICM serves as a population of stem cells for the trophectoderm as well as the primitive endoderm and ectoderm during normal development. The frequency of chimaerism was lower in OC than IC transplantation experiments. Nevertheless, in a substantial proportion of chimaeras, OCs colonized derivatives of the ICM. This is consistent with evidence from other studies that some outside cells divide differentially at 5th cleavage to produce an OC plus an IC rather than two OCs.