Tunicate Embryos and Cell Specification

Abstract

Tunicates are marine invertebrate chordates and closest relatives of vertebrates. The most common tunicates are ascidians. The fertilised egg of ascidians develops quickly into a tadpole‐type larva. The larva is composed of approximately 2600 cells and has distinct organs including the nervous system, endoderm and mesenchyme in the trunk, and muscle and notochord in the tail. The larval surface is covered by an epidermis. This configuration represents a basic body plan of chordates. Every blastomere of early embryos up to the gastrula stage is distinguishable, and invariant lineage of embryonic cells is well documented. Cell fate restriction occurs relatively early in ascidian embryos. Maternal factors are responsible for differentiation of muscle, endoderm and epidermis, whereas the notochord and nervous system are specified by cellular interactions. Genes that are involved in the specification of embryonic cells have been identified. The tunicate is an emerging model for studies of embryonic cell specification. Key Concepts: In ascidians, invariant lineage of embryonic cells is completely described up to the gastrula stage. Developmental fates of most embryonic cells are determined during the blastula formation. Maternal factors are responsible for differentiation of muscle, endoderm and epidermis. The notochord and nervous system are specified by cellular interactions. Genes that are responsible for the specification of embryonic cells have been identified. The tunicate is an emerging model for studies of embryonic cell specification.