X chromosome inactivation studied by injection of a single cell into the mouse blastocyst.

Abstract

CHIMAERISM in mice, whether naturally or artificially caused, is a valuable tool in the study of development. Much information has already been gained from chimaeras made by fusing eggs at cleavage stages1–3. Chimaeras can also be made by the injection of cells into the mouse blastocyst4. Because this method gives chimaerism at a later stage of development it potentially yields different information5, and refinement of the technique to inject a single cell into each blastocyst further enhances its scope. The injected cell divides to form a single-cell clone, which can be traced through development. In the past cloning has been used for studying X chromosome inactivation in cultured cells; we have used this new method of single cell injection for studying X inactivation in the embryo in vivo. This communication reports the birth of several mice made chimaeric by the injection of a single cell into the blastocyst, and presents preliminary results concerning the time of X chromosome inactivation in the mouse embryo.