Lithium induces dorsal-type migration of mesodermal cells in the entire marginal zone of urodele amphibian embryos.

The effect of lithium (Li) on gastrulation movements was investigated during the development of the urodele amphibianPleurodeles waltl. Attention was focused on mesodermal cell migration. Under conditions of Li treatment providing a maximal enhancement of dorsoanterior structures, it was found that the dorsoventral polarity of gastrulation was abolished. In particular, vital staining and scanning electron microscopy observations on embryo fractures showed that mesodermal cells migrated radially after Li treatment, which led to the formation of rounded embryos. Epiboly movements thus were accelerated. Nevertheless, contrasting with the precocious disappearance of the early-formed yolk plug, archenteron invagination was constantly retarded and commenced with a delay of several hours as compared to control gastrulae. Cell-lineage analysis of the progenies from ventral or dorsal equatorial blastomeres of 32-cell-stage embryos provided evidence that both dorsal and ventral mesoderm contributed to notochordal tissue after Li treatment. Dorsalization of the entire marginal zone was confirmed by the ability of the entire mesoderm rudiment to behave as a dorsal organiser after Li treatment. Comparison of the migratory behaviour of isolated animal hemispheres from Li-treated or control embryos cultured on fibronectin-coated substrate indicated that all marginal cells acquired the autonomous capacity for migration of dorsal marginal cells under the action of lithium.