Differentiation of notochord cells and mesenchyme cells of the ascidian Halocynthia roretzi requires interactions with neighboring endodermal cells and previous experiments

Patterning of complicated animal body plans requires interactions between constituting cells, and signaling cascades play important roles in the interactions. Examples of some well-characterized signaling cascades include the Wnt family (Moon et al., 1997; Dierick and Bejsovec, 1999), the TGFβ family (Piek et al., 1999), the FGF family (Nugent and Iozzo, 2000; Ornitz, 2000) and the EGF family (Riese II and Stern, 1998; Shen and Schier, 2000). Each of these signaling cascades is established by specific interactions between ligands and receptors followed by a pathway in which these signals eventually activate genes that encode transcription factors. For example, in vertebrate embryos, FGFs function as differentiation factors and play key roles in mesoderm induction (Slack, 1994; Yamaguchi and Rossant, 1995). FGFs can induce muscle, mesenchyme, lateral plate mesoderm and blood islands in Xenopus. Inhibition of FGF signaling caused by the truncation of FGF receptors resulted in embryos with reduced mesoderm (Amaya et al., 1991; Yamaguchi et al., 1994; Griffin et al., 1995). During embryogenesis of ascidians that produce tadpole larvae, a notochord consisting of exactly 40 cells develops within a muscular tail (Satoh, 1994). Developmental processes required for notochord formation have been investigated in detail. First, the entire cell lineage responsible for generating the 40 notochord cells has been completely documented (Conklin, 1905; Nishida, 1987); the anterior 32 cells are derived from a pair of right and left A4.1 (anterior and vegetal) cells of the bilaterally symmetrical 8-cell embryo, while the posterior 8 cells are descendants of the pair of B4.1 (posterior and vegetal) cells. At the 32-cell stage, A-line endodermal cells (A6.1 and A6.3) produce a signal(s) that induces the neighboring presumptive notochord cells (A6.2 and A6.4) to be specified along the notochordal pathway (Nakatani and Nishida, 1994). At the 64-cell stage, the developmental fates of A7.3 and A7.7 are restricted to give rise to notochord cells only. Simultaneously, the Brachyury gene is activated (Yasuo 2711 Development 128, 2711-2721 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV5433