In Xenopus embryos, neural induction results from a specific tissue interaction occurring between the dorsal mesoderm (Spemann's Organizer) and the dorsal ectoderm. The Organizer region is known to secrete a number of inductive signals, one of which - noggin - has been shown to induce neural tissue of an anterior character. Noggin has also recently been shown to induce an influx of calcium through L-type calcium channel activation. An increase in intracellular calcium in the dorsal ectoderm has been reported to be a necessary and sufficient event to trigger conversion of ectoderm from an epidermal to a neural fate. There is, therefore, good evidence to link calcium signalling to neural induction via the secretion of noggin. Exploring such interactions in intact embryos is complicated by...[ Read more ]

In Xenopus embryos, neural induction results from a specific tissue interaction occurring between the dorsal mesoderm (Spemann's Organizer) and the dorsal ectoderm. The Organizer region is known to secrete a number of inductive signals, one of which - noggin - has been shown to induce neural tissue of an anterior character. Noggin has also recently been shown to induce an influx of calcium through L-type calcium channel activation. An increase in intracellular calcium in the dorsal ectoderm has been reported to be a necessary and sufficient event to trigger conversion of ectoderm from an epidermal to a neural fate. There is, therefore, good evidence to link calcium signalling to neural induction via the secretion of noggin. Exploring such interactions in intact embryos is complicated by the fact that the mesoderm involutes around the dorsal lip into the interior of the embryo, positioning itself underneath the dorsal ectoderm. Thus, in intact embryos inductive and/or restrictive signals generated by the mesoderm can impinge on the dorsal ectoderm by either a planar or a vertical (i.e., passing up from below) route. In order to simplify the experimental system, Keller open-face explants, including both dorsal ectoderm and mesoderm portions were made at Stage 10 (i.e., before mesoderm involution) in order to eliminate vertical signalling, and thus explore the still-complex interaction between planar signals, calcium transients and neural induction. Explants are shown to generate calcium transients that are similar in some aspects to those from intact embryos, but different in ways that can be hypothetically linked to the elimination of vertical signalling. Here I also present data suggesting a link between patterns of calcium transients in explants with those of the expression of Zic3, one of the earliest expressed neuralizing genes identified so far, and one known to be upregulated by increases in intracellular calcium.