The role of vertical and planar signals during the early steps of neural induction.

The classical Einsteck-test (Spemann and Mangold, Roux Arch. Dev. Biol. 100: 599-638, 1924) and data from total exogastrulae (Holtfreter, 1933) suggest that vertical signals are transmitted between the chordamesoderm (organizer) and reacting ectoderm in the early phase of neural induction. In contrast to these results with Axoloti (urodeles), several authors observed the expression of neural specific genes in Xenopus exogastrulae, isolated dorsal blastopore lip with adjacent ectoderm (open-face explants) and Keller-sandwiches. Our data with Xenopus (anurans) also show that the expression of neural specific genes takes place in exogastrulae. However, when we prepared open face explants and exogastrula-like structures by microdissection at very early gastrula stage, the signal of a class II beta-tubulin, characteristic of terminal neural differentiation, is not found in the ectoderm. These results suggest that planar signals transmitted from the chordamesoderm into the ectodermal part can fairly be excluded under these experimental conditions. In similar experiments with Triturus alpestris we could not observe either the differentiation of neural structures in the ectodermal part of exogastrulae. These results confirm earlier experiments of Holtfreter performed with Ambystoma mexicanum (Axoltl) embryos. On the basis of the published data of different authors and our results, we cannot exclude the existence of planar signals for early and/or transient expressed genes before the onset of gastrulation in Xenopus, which make the neuroectoderm susceptible for the response to vertical signals during gastrulation. On the other hand our experiments with Triturus alpestris suggest that planar neural signals are unlikely in this species. These differences between Triturus and Xenopus embryos are discussed in the context of the peculiarities in morphological structure, competence and speed of development of the two species.