Size regulation in chimaeric mouse embryos.

When mouse embryos at early stages are aggregated and cultured in vitro, they form a morula and, eventually, a blastocyst which is roughly proportional in size to the number of embryos which contributed to it (Tarkowski, 1961; Mintz, 1965). Cell counts have shown that blastocysts derived from two 8-cell stages contain twice the number of cells of normal blastocysts (Bowman & McLaren, unpublished observations). However, when such a blastocyst is transferred to a 'foster-mother' and allowed to develop, the chimaeric embryo derived from it is normal in size when examined at 16y days (Bowman & McLaren, 1970) or at 10-14 days post coitum (Tarkowski, 1961). The developing embryo evidently undergoes some rather precise process of size regulation. No published data exist as to when or how this process occurs, though Mintz (1970) states that regulation takes place 'soon after implantation'. The same author (1971) suggests that 'the basis for this size regulation may lie in origin of the embryo itself from a small fixed number of cells (possibly as few as three) in the inner cell mass, regardless of the total cell number in the blastocyst'. It is the purpose of this study to investigate more fully the phenomenon of size regulation in chimaeric mice.