Induction of trunk lateral cells, the blood cell precursors, during ascidian embryogenesis.

The tadpole larvae of the ascidian Halocynthia roretzi have trunk lateral cells (TLCs) in their trunk. TLCs give rise to adult blood cells after metamorphosis. TLCs are exclusively derived from the A7.6 cell pair of 64-cell embryos. When prospective TLC blastomeres were isolated from embryos before the 16-cell stage, they failed to express TLC-specific antigen, a molecular indication of differentiation of TLCs. Isolates after the 32-cell stage, however, autonomously expressed the antigen. Results of experiments involving coisolation and recombination of blastomeres at the 16-cell stage showed that the inductive influence emanating from cells of animal hemisphere (presumptive epidermis blastomeres) is required for TLC formation. The inductive interaction takes place at the 16-cell stage, two cell cycles before the developmental fate becomes exclusively restricted to TLC formation. The inducing activity is distributed widely in animal hemisphere. By contrast, only presumptive TLC blastomeres have competence to be induced to form TLCs.