An investigation of inner cell mass and trophoblast tissues following their isolation from the mouse blastocyst

1. Inner cell mass (ICM) and trophoblast were isolated from 3½-day post-coitum mouse blastocysts by microsurgery. 2. Trophoblastic fragments formed vesicles in culture but did not aggregate with other such fragments. They proved as effective as intact blastocysts in inducing decidua in recipient uteri, but thereafter failed to proliferate. 3. Isolated ICMs remained as solid balls of cells that readily aggregated in pairs or groups in culture but failed to induce implantation changes in receptive uteri. 4. Possible explanations for the failure of isolated trophoblast to proliferate after implantation are discussed. It is argued that presence of ICM tissue is necessary for trophoblast proliferation, and suggested that the ICM exerts its effect by controlling development of the ectoplacental cone.