Altered expression of spatially regulated embryonic genes in the progeny of separated sea urchin blastomeres.

We have examined the importance of the extracellular environment on the ability of separated cells of sea urchin embryos (Strongylocentrotus purpuratus) to carry out patterns of mRNA accumulation and decay characteristic of intact embryos. Embryos were dissociated into individual blastomeres at 16-cell stage and maintained in calcium-free sea water so that daughter cells continuously separated. Levels of eleven different mRNAs in these cells were compared to those in control embryos when the latter reached mesenchyme blastula stage, by which time cells in major regions of the intact embryo have assumed distinctive patterns of message accumulation. Abrogation of interactions among cells resulted in marked differences in accumulation and/or turnover of the individual mRNAs, which are expressed with diverse temporal and spatial patterns of prevalence in intact embryos. In general, separated cells are competent to execute initial events of mRNA accumulation and decay that occur uniformly in most or all blastomeres of the intact embryo and are likely to be regulated by maternal molecules. The ability of separated cells to accumulate mRNAs that appear slightly later in development depends upon the presumptive tissue in which a given mRNA is found in the normal embryo. Messages that normally accumulate in cells at the vegetal pole also accumulate in dissociated cells either at nearly normal levels or at increased levels. In one such case, that of actin CyIIa, which is normally restricted to mesenchyme cells, in situ hybridization demonstrates that the fraction of dissociated cells expressing this message is 4- to 5-fold higher than in the normal embryo. In contrast, separated cells accumulate significant levels of a message expressed uniformly in the early ectoderm but are unable to execute accumulation and decay of different messages that distinguish oral and aboral ectodermal regions. These data are consistent with the idea that interactions among cells in the intact embryo are important for both positive and negative control of expression of different genes that are early indicators of the specification of cell fate.