Predominant expression of a cytoskeletal actin gene in mesenchyme cells during embryogenesis of the ascidian Halocynthia roretzi.

During ascidian embryogenesis mesenchymal cells proliferate and have no known function until after metamorphosis, when they give rise to various mesodermal tissues of the adult. Despite their simple lineage, specification mechanisms of the embryonic mesenchyme cells remain to be investigated; this is mainly due to lack of specific molecular markers for this cell type. Here we report that, in Halocynthia roretzi, zygotic expression of a cytoskeletal actin gene (HrCA1) begins at the late gastrula stage and that transcripts were predominantly distributed in embryonic mesenchyme cells, although some expression was observed in special cells of the central nervous system as well as in notochord cells. When HrCA1 expression was examined in cleavage-arrested embryos, it was found only in mesenchyme-lineage blastomeres of arrested 8-cell and later stages, consistent with the predominant expression of HrCA1 being in mesenchyme cells. To examine whether cell-cell contact until the 8-cell stage is required for mesenchyme cell specification, blastomeres were continuously dissociated during the period of 2-8-cell stages and division of the blastomeres was then arrested. Results showed that two of eight dissociated and division-arrested blastomeres from a single fertilized egg (presumably those corresponding to B4.1 cells) expressed HrCA1, suggesting that specification of embryonic mesenchyme cells does not require cell-cell contact until the 8-cell stage.