THE ASCIDIAN EMBRYO AS A PROTOTYPE OF VERTEBRATE NEUROGENESIS

Ascidian tadpole larvae, composed of only about 2500 cells, have a pri mitive nervous system which is derived from the neural plate. The ster eotyped cell cleavage pattern and well characterized cell lineage in t hese animals allow the isolation and culture of identified blastomeres in variable combinations. Ascidian embryos express cell-type-specific markers corresponding to their cell fates, even when cultured under c leavage-arrest by cytochalasin B. This system provides us with a uniqu e opportunity to study the roles of cell lineage and cell contact in e arly neuronal differentiation in the absence of events associated with complex morphogenesis. In addition, the isolated, cleavage-arrested b lastomeres are ideally suited to electrical recording, permitting the use of ionic channels as specific markers for differentiation. In the cleavage-arrested embryos, suppression of one type of K+ channel, and induction of two types of Na+ channels, occur following cell contact w ith the vegetal blastomere. The combination of molecular and electroph ysiological analyses on this simple animal system may provide insights into the nature of the cell interactions important in early neurogene sis, both in ascidians and in vertebrates.