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Since the early days of classical neurohistology, the nervous system of the leech has been and still is a favorite object of investigation. In particular, the neurofibrillar bundles or the neurofibrils have attracted much attention because they represent a characteristic and ubiquitous component of the neuron (1). The term neurofibrillar bundle has been introduced recently to designate the structures in the electron micrographs corresponding to the neurofibrils of light microscopy (2 4). In spite of the continuous efforts of numerous investigators the structure, chemical composition, and the functional significance of the neurofibrils remain obscure (5). ApAthy (1) showed that the neurofibrils of the leech neuron are about 0.5-0.75 gu thick and have a helical course in the axon. Furthermore, he demonstrated with the light microscope that the neurofibrils consist of elementary filaments; when the neurofibril is highly coiled, the filaments could separate. It is noteworthy that already in 1897 ApAthy (6) was able to isolate the neurofibrils which appeared as rigid rods and showed, in polarized light, positive uniaxial birefringence. Recent electron microscopic studies of thin sections of leech nervous system (2-4) are in agreement with these observations. In transverse sections, the neurofibrillar bundles present a coherent structure of reticular or cribriform subunits. These subunits correspond to transversely sectioned filaments about 60 A in diameter bound together laterally. However, the subunits do not show as segregated dots, in contrast to the glial filaments, tonofilaments, and also the neurofilaments in vertebrates. According to Gray and Guillery (2), the angular structure of the subunits of the neurofibrillar bundles is probably produced by three or more closely packed finer filaments, and these filaments may have the same structure as the vertebrate neurofilaments. Hagadorn et al. (4) suggest a tubular structure of the subunits of the neurofibrillar bundles; the walls of these tubular subunits consist of "protofibrillae" about 60 A in diameter. It is evident that methods of specimen preparation other than thin sectioning need to be applied in order to clarify the intricate, fine structure of the neurofibrillar bundles of the leech and to relate properly the neurofibrillar bundles to the vertebrate neurofilaments. Electron microscopy of isolated and negatively stained neurofibrillar

HELICAL ARRANGEMENT OF THE SUBUNITS OF THE NEUROFIBRILLAR BUNDLES ISOLATED FROM LEECH NERVOUS SYSTEM