Establishment of pigment cell lineage in embryos of the sea urchin, Hemicentrotus pulcherrimus

In an attempt to estimate the number of pigment precursor cells in sea urch in embryos, DNA synthesis and cell divisions were blocked with aphidicolin from various stages of development. Interestingly, pigment cells differenti ated on a normal time schedule, even if the embryos were treated from late cleavage stages on. In most of the embryos treated from 10 h on, 10-15 pigm ent cells differentiated. Thereafter, the number of pigment cells in the ap hidicolin-treated embryos further increased, as the initiation of the treat ment was delayed. On the other hand, total cell volumes in the pigment line age, calculated from the averaged number and diameter of differentiated pig ment cells, were almost the same irrespective of the time of the initiation of aphidicolin treatment. This indicated that the increase in the number w as caused by divisions of the pre-existing cells in the pigment lineage. Th us, the founder cells that exclusively produce pigment cells could be ident ified. They are nine times-cleaved blastomeres and specified by 10 h post-f ertilization. The obtained results also clarified the division schedule in the pigment lineage; the founder cells divide once (10th) until hatching, a nd divide once more (11th) by the end of gastrulation.