This series has addressed a constituency of scientists possessing biochemical background with the goal of providing them with specialized reviews of neu­ robiological interest. Since its initiation, neurochemistry and neuroscience have come of age, and the editors initiate with this volume the concept of a central theme. It is planned that each subsequent volume will also be topical. We note with sadness the passing of Dr. Henry Mahler who served as an advisory editor since the initiation of this series. He played a major role in building the bridge between biochemistry and neurochemistry and will be missed. Weare pleased to welcome two new editors, Drs. William Norton and Bruce McEwen. B. W. Agranoff M. H. Aprison vii FOREWORD Oligodendroglia constitute one of the three principal cell types of the central nervous system. These cells, together with their elaborated membranes, account for at least 25 percent of the dry mass of an adult rat brain and an even greater percentage of the central nervous system of larger animals.