The ascidian embryo: An experimental system for studying genetic circuitry for embryonic cell specification and morphogenesis

Ascidians have served as an appropriate experimental system in developmental biology for more than a century. The fertilized egg develops quickly into a tadpole larva, which consists of a small number of tissues including the epidermis, central nervous system with two sensory organs, nerve cord, endoderm, mesenchyme, notochord and muscle. Lineage of these embryonic cells is completely described up to the gastrula stage. These features of the ascidian embryo provide an opportunity to study the mechanisms underlying the differentiation of tissues in development. To understand the molecular basis of ascidian embryogenesis, cloning of various genes has been performed, including those that exhibit a lineage-associated expression pattern and those encoding transcription factors, which are expected to be involved in differentiation of tissues, lineage specification, axis formation and regionalization in developmental fields. Here, we present recent advances in the isolation and characterization of these genes. We emphasize the advantages of the ascidian embryo as an experimental system to study genetic circuitries that are required for cellular differentiation and morphogenesis.