Mechanisms of Specification in Ascidian Embryos

Abstract

Ascidians (subphylum Urochordata, class Ascidiacea) are ubiquitous marine animals. Since the work of Chabry (1) in 1887, which described the first blastomere destruction experiments in the history of embryology, ascidian eggs and embryos have served as an experimental system in developmental biology. The fertilized egg develops quickly into a tadpole larva (about 2600 component cells) consisting of a small number of tissues including epidermis, central nervous system with two sensory organs, nerve cord, endoderm, mesenchyme, notochord, and muscle (2). The lineage of these embryonic cells is described almost completely. In addition, the recent isolation of cDNA clones of various tissue-specific genes by our laboratory provides molecular probes with which to monitor the differentiation of each type of tissue (3). The advantageous features of the embryo, together with tissue-specific molecular probes, allow us to study the mechanisms underlying the specification and subsequent differentiation of embryonic cells (2, 3).