PLANAR AND VERTICAL INDUCTION OF ANTEROPOSTERIOR PATTERN DURING THE DEVELOPMENT OF THE AMPHIBIAN CENTRAL-NERVOUS-SYSTEM

In amphibians and other vertebrates, neural development is induced in the ectoderm by signals coming from the dorsal mesoderm during gastrul ation. Classical embryological results indicated that these signals fo llow a ''vertical'' path, from the involuted dorsal mesoderm to the ov erlying ectoderm. Recent work with the frog Xenopus laevis, however, h as revealed the existence of ''planar'' neural-inducing signals, which pass within the continuous sheet or plane of tissue formed by the dor sal mesoderm and presumptive neurectoderm. Much of this work has made use of Keller explants, in which dorsal mesoderm and ectoderm are cult ured in a planar configuration with contact along only a single edge, and vertical contact is prevented. Planar signals can induce the full anteroposterior (A-P) extent of neural pattern, as evidenced in Keller explants by the expression of genes that mark specific positions alon g the A-P axis. In this review, classical and modern molecular work on vertical and planar induction will be discussed. This will be followe d by a discussion of various models for vertical induction and planar induction. It has been proposed that the A-P pattern in the nervous sy stem is derived from a parallel pattern of inducers in the dorsal meso derm which is ''imprinted'' vertically onto the overlying ectoderm. Si nce it is now known that planar signals can also induce A-P neural pat tern, this kind of model must be reassessed. The study of planar induc tion of A-P pattern in Xenopus embryos provides a simple, manipulable, two-dimensional system in which to investigate pattern formation. (C) 1993 John Wiley & Sons, Inc.