The preparation of graphical and three-dimensional reconstructions of the developing central nervous system

In the development of the central nervous system morphogenesis and histogenesis are two strongly interdependent aspects. Since the histogenesis can only be investigated on microscopical sections the spatial relations of the various structures are lost. Thus the availability of reliable three-dimensional reconstructions is indispensible. The advantage of the bilateral symmetry of the central nervous system is that it creates the possibility to use the median plane as a natural plane of reference in the piling of the reconstructions, provided the material is sectioned perpendicular to that plane. Such a direction of sectioning is the more advisable, since during the histogenesis the outwards migration of the young neuroblasts mainly is radially orientated. The reconstruction method presented here is based upon a comparison of the median sections of two specimens, which are graphically reconstructed from two different series. Both series are sectioned at right angles to the median plane and also in mutually perpendicular direction. Each resulting optimalized graphically reconstructed median section is used as a reference in the correct piling of the three-dimensional reconstruction of the corresponding series, which consists of polystyrene plates. A prerequisite of the method is an exact orientation of the material during embedding, of which the procedure is described. Secondly the two specimens which yield both series must show a high degree of conformity. In embryological terms this means, that the various developmental stages must be accurately defined. The use of criteria which can only be appropriate when referring to the organ under investigation is argumented. In a discussion on the possibilities and limitations of the method a comparison is made with the reconstruction technique which is based upon the use of artificially introduced outer points of reference.