Allocation of mesendodermal cells during early embryogenesis in the starfish, Asterina pectinifera

The volume of archenteron tissue (mesendoderm) in the early-to-middle gastrula of the starfish Asterina pectinifera was nearly one-quarter of whole embryo. Treatment with LiCl during 7-10 h increased this volume ratio by about 30%, whereas the total volume and the total number of cells of whole embryo remained unchanged. Such a relative increase in mesendodermal part and simultaneous reduction in ectodermal part by LiCl treatment was confirmed by counting the number of constituent cells of these parts at early bipinnaria stage. Pulse treatment with LiCl revealed that the effective period of the treatment is limited from 7 to 10 h of development, when tightly packed blastulae are formed through increase in adhesiveness of blastomeres. These results indicate that a fraction of presumptive ectodermal cells can change its fate to mesendoderm during 7-10 h of development. Cellular interactions during a specific stage of development are suggested to be involved in the determination of mesendodermal tissue.