Altered establishment of cell lineages in theCaenorhabditis elegans embryo after suppression of the first cleavage supports a concentration-dependent decision mechanism.

In the early embryo ofCaenorhabditis elegans five somatic cell lineages and a germ cell lineage are established by a series of unequal cleavages in the germline. We suppressed first cleavage by means of cold, mechanical pressure or centrifugation. Thereafter, with the second attempt of the zygote to divide, four blastomeres were generated simultaneously in a tetrapolar cleavage. Cell division pattern, segretation of germline-specific granules, and terminal differentiation of such manipulated embryos were analysed. Instead of six, only from one to five visible cell lineages were established before the germline prematurely aborted from its typical pattern of unequal cleavage. The absence of germline-specific cleavage appears to accompany the abnormal segregation of germline-specific granules. While muscle differentiation was detected even in embryos expressing only one cell lineage, in general, gut differentiation became visible only if a separate gut lineage had been generated. We hypothesize that the potential for differential cleavage is lost in manipulated embryos because a cytoplasmic control factor is diminished as a result of the retarded soma/germline separation. According to this hypothesis, after manipulation, a concentration-dependent decision mechanism leads to: a reduced number of unequal germline cleavages or even none at all, the establishment of fewer distinct cell lineages, and limited cellular differentiation.