Molecular differentiation of inside cells and inner cell masses isolated from the preimplantation mouse embryo.

Clusters of inside cells (ICs) were isolated immunosurgically from morulae and early cavitating blastocysts, and cultured for varying periods in vitro. The ICs responded to their changed position by transformation to a blastocyst-like vesicle and also showed a correlated synthesis of trophectodermal-marker polypeptides. Both changes were inhibited by alpha-amanitin, indicating that transcriptional events are involved in this response to changed position. Inner cell masses (ICMs), isolated immunosurgically from expanded blastocysts, did not undergo a morphological transformation to a blastocyst-like structure and did not show a corresponding and complete change in synthesis of trophectodermal-marker polypeptides. Elements of the change in polypeptide activity were, however, present, suggesting an ineffective response to position by the ICMs.