Origin of the ectoplacental cone and secondary giant cells in mouse blastocysts reconstituted from isolated trophoblast and inner cell mass.

1. Inner cell mass (ICM) and trophoblast tissue were isolated from 3^-day post-coitum mouse blastocysts that were homozygous for different electrophoretic variants of the enzyme glucose phosphate isomerase (GPI). Blastocysts were reconstituted from these tissues, transferred to pseudo-pregnant recipients and allowed to develop to the early somite stage. 2. The embryo plus membranes and trophoblast were dissected and typed separately for GPI. 3. Contamination of trophoblast with maternal decidual tissue was quantified. 4. The trophoblast of the implanted embryos was almost exclusively of the trophoblastdonor GPI type. The embryos plus membranes were mainly of the ICM-donor type but most also showed a substantial proportion of trophoblast-donor type. 5. It is argued that the ICM controls trophoblast proliferation by inhibiting giant cell transformation of adjacent trophoblast cells rather than through making a significant cellular contribution.