Relations Between Mesodermal and Neural Segmentation

Abstract

The importance of the early embryonic mesoderm for peripheral nerve segmentation was amplified some five years ago at the NATO Advanced Workshop on Somite Development (Keynes and Stern, 1986). In the course of normal development, motor and sensory axons emerge in register with the somites, and are arranged in a repeating pattern along the longitudinal axis of the embryo. Earlier this century, studies of spinal nerve development had not been particularly concerned with identifying exactly where they arise in relation to individual somites. More recently, Keynes and Stern (1984), from an examination of whole-mounted embryos stained with zinc iodide - osmium tetroxide, drew attention to the fact that the developing axons cross only the anterior half-sclerotome. This not only confirmed the descriptions of the last century but was important because, by means of a series of surgical manipulations performed on the developing embryo (reviewed by Keynes and Stern, 1986), it was shown that peripheral nerve segmentation results from differences between anterior and posterior half-sclerotome cells. Keynes and Stern (1986) could only speculate, however, on what these differences are likely to be in molecular terms. The purpose of the present contribution is to survey what is now known about molecular differences existing between the two populations of cells that may explain why axons are confined to the anterior half of the somite.