Cell allocation in preimplantation mouse chimeras

Abstract

Abstract The mechanism of cell allocation in the preimplantation mouse chimera was studied in aggregation chimeras of 4‐ and 8‐cell stage embryos. To follow the fate of cells, some embryos were labeled by a 16‐h incubation (at the 2‐cell stage) with 3 H‐thymidine. At the 4‐8‐cell stage both labeled and unlabeled embryos were treated with pronase to remove the zona pellucida and then aggregated in various combinations: one labeled 8‐cell and three unlabeled 4‐cell embryos (labeled 8‐cell chimeras); one labeled 4‐cell and three unlabeled 8‐cell embryos or two labeled 4‐cell and two unlabeled 8‐cell embryos (labeled 4‐cell chimeras). Autoradiography of air‐dried preparations of inner cell masses isolated from the chimeras at the blastocyst or late blastocyst stage by immunosurgery revealed that labeled 8‐cell embryos contributed a disproportionately large number (over 60%) of their descendant cells to the inner cell mass of labeled 8‐cell chimeras. By contrast, in labeled 4‐cell chimeras only 8% (with one labeled 4‐cell embryo) or 31% (with two labeled 4‐cell embryos) of inner cell mass cells were derived from the labeled embryos. These results suggest that cell allocation in preimplantation mouse chimeras is not a random process and that cells of embryos developing ahead of others are preferentially allocated to the inner cell mass.