Neuroanatomical tracing at high resolution

Most techniques used for the study of the fiber connectivity in the central nervous system produce results which are visualized in the conventional li ght microscope or fluorescence microscope. Although in some cases this may be sufficient, often proof is necessary that fibers which enter a particula r brain area indeed terminate here. Alternatively, it may be necessary to d etermine whether the axon terminals of traced fibers form synapses with spe cific processes of specific neurons. With the latter neurons all cellular e lements are meant which can be labeled in some way. Evidence of synaptic co nnectivity necessitates visualization at a higher level of resolution, that is at the electron-microscopic level. In this contribution to the Special Issue we discuss several methods currently available to visualize individua l tracers, and methods developed to visualize two different markers, that i s one marker attached to a fiber or an axon terminal, and the second marker attached to a presumed pre- or postsynaptic neuronal element. (C) 2000 Els evier Science B.V. All rights reserved.