Stereological approach to the study of synapse morphometry with particular regard to estimating number in a volume and on a surface.

The paper presents a system of stereological formulae which are suitable for the quantitative ultrastructural characterization of synapses. Attention is given first to the question of deciding which synaptic feature to adopt as the basis for measurement. Three counting units can be recognized on electron micrographs: the bouton, the total zone of apposition of pre- and postsynaptic membranes, and the zone of paramembranous density. These may provide different numerical results. Possible units of reference are considered also and whilst emphasis is given to a volume of nervous tissue and a membrane surface of postsynaptic element, interesting alternatives are presented for the sake of completeness. Stereological parameters available for quantifying synapses (for example boutons per tissue volume, paramembranous dense zones per soma surface) are described. In addition, some new and previously neglected possibilities are included and some errors found in the literature are corrected. Worked examples of the estimation of synaptic densities are provided.