Expression of an antigen specific for trunk lateral cells in quarter embryos of the ascidian, Halocynthia roretzi

Cell-lineage analysis has demonstrated that a pair of the right and left A7.6 cells of a 64-cell embryo of the ascidian Halocynthia roretzi, descendants of A4.1 cells of an 8-cell embryo, give rise to trunk lateral cells (TLCs). In this study, in order to investigate cellular mechanisms involved in the specification of TLCs, we have examined the expression of a TLC-specific antigen in cleavage-arrested embryos and in quarter partial embryos. Although cleavage arrest of embryos by treatment with cytochalasin B at early stages, prior to and including the 16-cell stage, inhibited expression of the TLC-specific antigen, embryos arrested at the 32-cell stage and at later stages developed the antigen. The only blastomeres exhibiting expression of the antigen were the presumptive TLCs, as predicted by cell-lineage assignments. When the developmental potential of quarter embryos that originated from four isolated blastomere-pairs (a4.2, b4.2, A4.1, and B4.1 pairs) of an 8-cell embryo was examined, the A4.1 quarter embryos, which are developmentally fated to give rise to TLCs, rarely showed evidence of expression of the antigen. Expression of the antigen was not observed in a4.2 and b4.2 quarter embryos, which are not associated with the TLC fate. By contrast, expression of the antigen was detected in about a half of the B4.1 quarter embryos which are also not associated with the TLC fate. These results are discussed with reference to the relationship between TLCs and mesenchyme cells.