Induction of the trophoblastic giant-cell transformation after ovariectomy in the mouse.

INTRODUCTION The trophoblastic giant-cell transformation of mouse blastocysts (Duval, 1891; Dickson, 1963) is blocked by ovariectomy carried out on the fourth day of gestation (Dickson, 19666)Such blocked blastocysts have a greater overall length and a shorter inner cell mass, and hence a larger blastocoele, thanuntransformed blastocysts from mice that have not been ovariectomized (Dickson, 19666, c). Since implantation can be induced in lactating and ovariectomized mice by the injection of ovarian hormones (Bloch, 1958, 1965; Smithberg & Runner, 1960), it may be assumed that the giant-cell transformation is brought about by such treatment, for this differentiation of the ovum seems to be an integral part of the implantation process (Snell, 1956). It is nonetheless well worth demonstrating that the transformation can be induced by the injection of ovarian hormones, for the morphological changes then become valuable as indicators of response to experimentation which are detectable before implantation sites can be identified visually. The initiation of development in the blastocyst may then be amenable to investigation separately from implantation, which is an interaction between egg and uterus. With this end in view, ovariectomized mice have been treated with progesterone and the morphology of the blastocysts observed.