This book is based on the NATO Advanced Research Workshop on "Mesenchymal-Epithelial Interactions in Neural Development" which was held in Berlin during March 1986. The idea that it may be the time for this workshop arose from a discussion among the organizers who met at a conference in Innsbruck. During the twenties, Spemann demonstrated that, in vertebrates, the formation of neuroepithelium depends on the induction by specific mesodermal derivatives. This was about the first time that mesenchymal-epithelial interactions had been recognized as an important mechanism in the development of an epithelial organ. Since then two things had become clear: 1) The identity and role of molecules, elaborated at mesenchymal-neuroepithelial interfaces had been partially elucidated in spinal cord and peripheral nerve ontogeny but, until recently, had been ignored in studies on the development of the brain. 2) In many-non-neural epithelia, the surrounding mesenchyme regulates such fundamental developmental steps as, for example, cell proliferation, morphogenesis and cell differentia tion, and there is no reason to exclude similar influences and interactions from brain development. It seemed to us that the importance and implications of many of these findings had not been appreciated by many developmental neurobiologists and that an attempt should be made to initiate discussions and exchange of ideas among different groups of scientists involved in extracellular matrix (ECM) research. This turned out to be a formidable task because of the great diversity of disciplines contributing findings to this active field."

At the end of the nineteenth century, controversy arose as to precisely when the first glial cells originate during development of the central nervous system, and to date, the issue has not been satisfactorily resolved. His (1889, 1890) noted that, even in the earliest developmental stages of the germinallayer, there appeared to be two distinct cell types. The cells which he called Spongioblasten were thought to be glial precursors from which all mature glial cells derive; Keimzellen, in contrast, were regarded as forming 1 neurons. His was working on the assumption that the very first preneurons migrate into a preexisting framework of glial eelIs. In contrast to this view, Schaper (1897) regarded both Keimzellen and Spongioblasten as belonging to a common population of proliferating and pluripotent stem cells which begin differentiation into glial and neuronal cells at late developmental stages. It is this latter view which is the basis of the most recent studies on the subject (e. g. , Caley and Maxwell1968a, 1968b; DeVitry et al. 1980). The concept of one common stem cell seemed to be supported both by experiments using 3H-thymidine autoradiography (Fujita 1963, 1965b, 1966; Sauer and Walker 1959; Sidman et al. 1959) and by ultrastructural studies (Fu- jita 1966; Hinds and Ruffet 1971; Wechseler and Meller 1967) indicating that structural differences, which His presumably used to define his two cell types, could be related to different stages of the mitotic cycle.