Three-dimensional analysis of dendritic spines. I. Quantitative observations related to dendritic spine and synaptic morphology in cerebral and cerebellar cortices.

A total of 212 dendritic spines (108 from the visual and 104 from cerebellar cortices of the mouse) were analyzed in serial sections. Dendritic spines (DS) and synaptic active zones (SAZ) were classified according to their shape, and the following quantitative data were measured: DS stalk and bulb diameters, DS length and volume, number of cisterns of the spine apparatus, DS and SAZ surface areas and their mutual proportions. Quantitative relationships between the spine apparatus and the size of DS and SAZ, between the volume and surface area of DS and between the size of DS and the size of SAZ were studied. Thin, mushroom-shaped and stubby DS with simple (circular or oval), complex (perforated, annulate or horseshoe-shaped) and multifocal SAZ were found on terminal branches of pyramidal cell apical dendrites and club-shaped DS with simple (circular or oval) SAZ on spiny branchlets of Purkinje cells. Statistically significant differences were found between all values measured on various DS types in the visual cortex. Linear dependencies of the DS surface area on DS volume and of the SAZ surface area on the DS surface area were established. Only a limited area of DS plasma membrane (7-10%) was occupied by SAZ. This finding indicates a possible functional importance of the SAZ/DS (and possibly also of the total SAZ/total postsynaptic membrane) surface ratio.