ABSTRACT
Tadpole larvae of ascidians have a brain and two kinds of sensory pigment cells within the brain. These neural tissues are formed through folding of the neural plate and, as in vertebrates, neural induction is involved in the formation of the nervous system. The brain and pigment cells are derived from anterior-animal (a-line) blastomeres, and inductive interaction with anterior-vegetal (A-line) blastomeres is necessary for formation of neural tissues from the a-line cells. In order to investigate the characteristics of this process in detail, blastomeres were isolated from embryos of Halocynthia roretzi at the 110cell (late blastula) stage, and then their developmental capacity was examined. In the first set of experiments, brain- and pigment-lineage cells, isolated prior to neural induction, failed to develop three specific features of epidermis (epithelial morphology, secretion of larval tunic and expression of epidermis-specific antigen). These observations suggested a process different from amphibian neural induction, indicative perhaps of a permissive type of induction in ascidian embryogenesis.
Next, in order to identify which type of tissueprecursor cell is the inducer of the sensory pigment cells, various sets of blastomeres were isolated from 110-cell embryos, in which the fates of most of the blastomeres are restricted to a single type of tissue. The results suggested that spinal-cord precursor blastomeres, which are cells of the A-line, are sufficient to induce the a-line cells to form pigment cells. In addition, embryos from which all spinal cord precursors had been ablated, failed to form sensory pigment cells as well as a brain structure. Therefore, it is probable that the inducers of the ascidian sensory pigment cells are the spinal-cord precursor cells.
