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The Spatial Distribution of Inductive Capacities in the Neural Plate and Archenteron Roof of Urodeles

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

1. In order to contribute to a better understanding of the neural induction process in amphibians, an analysis was made of the distribution of inductive capacities in the neural plate and the archenteron roof of the open neural plate stage of Triturus alpestris. The results were interpreted on the basis of the activation-transformation hypothesis of NIEUWKOOP, and the "field" concept. 2. Median, paramedian, and lateral areas were isolated from four successive cranio-caudal zones of the neural plate (including the neural folds) and of the archenteron roof (up to the inner borders of the overlying neural folds). The inductive capacities of the isolated pieces were tested in sandwiches of competent ectoderm from early gastrulae of Ambystoma mexicanum. 3. To characterize the activating capacities of the various inductors the numbers of positive cases as well as the medians of the volumes of the induced neural masses were determined. The latter was achieved by measuring the total surface area of all sections of the neural masses concerned. To make them comparable the volumes of the induced neural masses were expressed per 100 volume-units of the inductor. The volumes of the inductors were measured in transverse sections of a number of neurulae fixed at the operation stage used. Differences between series as regards the volumes of the induced neural masses were tested statistically with Wilcoxon's two sample rank test. 4. In all cases where the induced neural masses could be analysed qualitatively, the volume of their deuterencephalic and spinal cord portions was expressed as a percentage of their total volume. This percentage was used as the main measure of the transforming capacity of the various inductors. It is of restricted significance only. 5. The distribution of the activating capacity in the neural plate and the archenteron roof, as found in experiments made with isolated inductors, may be called a morphogenetically active diffusion field. This can be used to derive conclusions concerning the activation field involved in the neural induction process. The production of the hypothetical activating factor takes place in the presumptive notochord. At the open neural plate stage the centre of production is found at the anterior spinal cord level. In the median region, where intimate contact between the two germ layers exists, a transfer of the activating factor from the archenteron roof to the overlying neural plate takes place. In the median region of the neural plate a temporary accumulation of the factor occurs, followed by a diffusion in cranial and medio-lateral directions. During diffusion the factor exerts a morphogenetic action upon the substrate, and is consumed as a result. Since at the open neural plate stage the competence for activation has disappeared, the continued presence of the activating factor may be assumed to play a part in the further regional organisation of the central nervous system. Although there certainly is some medio-lateral diffusion of the activating factor through the mesodermal mantle, the regions of the archenteron roof outside the presumptive notochord very probably play only a subsidiary role in the activation process. In studying the distribution of the activating capacity the fact has to be taken into account that the production of the activating factor probably continues in isolated pieces of the notochord. Evidence is presented that at the open neural plate stage the activation field is already regressing. 6. The distribution of the transforming capacity, as found in experiments made with isolated inductors, suggests the existence of a transformation field. This field has its centre in the posterior part of the presumptive notochord. The analysis of the self-differentiation of the isolated pieces of the neural plate used as inductors, as well as the qualitative analysis of the induced neural masses, shows that at the open neural plate stage the transformation phase of the induction process is not yet completed. It may even be assumed that the transformation field still has to be built up further in the neural plate. 7. Taking into account the morphogenetic movements taking place during gastrulation, the presumable genesis of the induction fields was described. A comprehensive picture of the normal neural induction process was presented. From this the dominant role of the presumptive notochord becomes evident. The maintenance of both the activation field and the transformation field depends upon the production of the inductive factors in the presumptive notochord, upon the continuity of the latter with the rest of the mesodermal mantle, and upon its contact with the overlying dorsal ectoderm. 8. The subsidiary role of the parachordal mesoderm in the induction process is confirmed by the fact that the inductive capacities of combinations of presumptive notochord and adjacent parachordal mesoderm do not differ from the inductive capacities of the presumptive notochord alone. On the other hand, the presence of notochordal mesoderm markedly enhances the self-differentiation of the parachordal mesoderm. Doubling the amount of parachordal mesoderm affects neither its inductive capacities nor its self-differentiation. 9. The quantitative results exhibit a rather high degree of variability. The importance of several uncontrollable variables was discussed. There is evidence that the results are clouded by activating influences from the surrounding medium (artificial activation). This very probably is a consequence of the fact that the competent ectoderm of the axolotl is rather susceptible in this respect under the present experimental conditions. The results of a small reciprocal series of experiments, in which isolated parts of the neural plate and archenteron roof of neurulae of A. mexicanum were combined with competent ectoderm of gastrulae of T. alpestris, indicate that the effect of artificial activation can be prevented by using less susceptible ectoderm. However, the picture emerging from the present study is consistent, and may contribute to a better understanding of the neural induction process. 10. Homoeogenetic neural induction has no special significance in the normal induction of the neural plate. It is a property which is achieved automatically by the presumptive neural material during the diffusion of the inductive factors through the dorsal ectoderm.

Affiliations: 1: (Hubrecht Laboratory, Utrecht, The Netherlands


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