Cell interactions influence the fate of mouse blastomeres undergoing the transition from the 16- to the 32-cell stage

Newly formed polar and apolar 1/16 blastomeres were isolated and cultured singly, or in various combinations, through division to form 32-cell blastomeres. The morphology of the resulting cell cluster appeared to depend upon the nature and composition of the cell combination used. In most polar + apolar couplets, the polar cell enveloped the apolar cell, and following division, a 4/32 cluster was thereby generated containing two trophectoderm-like external cells derived from the polar cell and two ICM-like internal cells derived from the apolar cells. A polar cell cultured in isolation divided to give either two trophectoderm-like external cells or a trophectoderm-like cell and an ICM-like cell. Two polar cells cultured together generated clusters in which the ratio of trophectoderm-like:ICM-like cells was 4:0 or 3:1. Most apolar cells cultured together in couplets polarized, and generated 4/32 clusters containing either purely trophectoderm-like or a mixture of trophectoderm- and ICM-like cells. The results are consistent with the notion that continuing interactions between polar and apolar cells are necessary to maintain their respective fates as trophectoderm and ICM, and that in the absence of these interactions polar cells can generate ICM cells by a differentiative division and apolar cells can generate trophectoderm cells by polarizing in response to asymmetric cell contacts.