Influences of Meningeal Cells on the Development of the Brain

Most epithelia are unable to develop in the absence of their surroundings mesenchyme, and the latter influences such fundamental events in epithelial development as cell proliferation, branching morphogenesis and functional differentiation (reviews in Grobstein, 1955; Wessells, 1977; Bernfield and Banerjee, 1978; Hay, 1981, 1984; Cunha et al., 1983; Bernfield et al., 1984). Already the earliest stages of neural development, i.e., the formation of the neural plate in the dorsal ectoderm of the gastrula, are induced by the underlying mesoderm (reviews in Spemann, 1938; Jacobson, 1984). In the spinal cord this neural induction is followed by a series of tissue interactions between neural plate and neural tube on the one hand and the adjacent chorda dorsalis and the somites on the other hand which result in the characteristic shape of the developing spinal cord (see van Straaten and Drukker, this volume). Little is known about similar influences on the development of the brain, yet the mesenchymal partner in such tissue interactions would be the meninges, and the occurrence and significance of a postulated mesenchymal-epithelial interaction during brain development can be assessed if the meningeal cells are eliminated in different stages of CNS ontogeny. In this article we will summarize the evidence collected in our laboratories over the last few years on the influences of meningeal cells on brain development, which we have obtained utilizing a pharmocological method for the selective destruction of meningeal cells. These findings show that in addition to secreting as yet unidentified, probably diffusible molecules stimulating cell proliferation (see Gensburger et al., and Pehlemann et al. this volume), the meningeal cells stabilize the external surface of the cerebellum and hippocampus by producing components of both the interstitial matrix and the basal lamina and thus form the foundation of the radial glial scaffold presenr in these regions which, is involved in establishing the pattern of folia and laminae by guiding neuronal migration.