An in situ cell marker system has been developed which allows identification of Mus caroli and Mus musculus cells in interspecific chimaeras. A radioactively labelled, cloned DNA probe to M. musculus satellite DNA was hybridized in situ to sections of M. musculus and M. caroli adult tissues. Autoradiography revealed high levels of hybridization to the nuclei of M. musculus cells, but little or no label bound to M. caroli cells. The DNA probe could also distinguish M. musculus and M. caroli cells in the same tissue section. Patches of labelled and unlabelled cells were clearly identified in sections of adult chimaeric tissues and also in the embryonic ectoderm of 6·5-day embryonic chimaeras.

The ability to recognize M. musculus and M. caroli cells in sections of chimaeras should provide a powerful new tool in analyses of cell lineages in both embryonic and adult mouse chimaeras. The marker system has several advantages over other marker systems so far developed, the most important of which is its ubiquity. Since it is a nuclear marker, only cells without nuclei should be unsuited to its use. The potential of the marker system has been shown by its use in demonstrating directly for the first time the postimplantation derivatives of inner cell mass and trophectoderm in blastocysts ‘reconstituted’ with M. musculus trophectoderm and M. caroli inner cell mass.

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