UNBIASED STEREOLOGICAL ESTIMATION OF THE TOTAL NUMBER OF SYNAPSES IN A BRAIN REGION

Modern stereological methods have been used to make unbiased estimates of the total number of synapses in the striatum radiatum of the hippo campal CA1 region of five rabbits. The approach used involved a two st age analysis and is generally applicable to all parts of the nervous s ystem. During the first stage of the analysis, the reference volume wa s estimated by point counting, at the light microscope level, accordin g to the Cavalieri principle. During the second stage, the numerical d ensity of synapses was estimated with disectors at the electron micros copic level. The total number of synapses was calculated as the produc t of the numerical density and the volume of the region. The sampling with points and disectors was carried out in all three dimensions of t he entire CA1 region in a manner that ensured that all parts of the re gion and all synapses within it had equal probabilities of being sampl ed. An analysis of the precision of the estimate of total synapse numb er has been performed in terms of the variances of volume and synaptic numerical density at different levels of sampling, i.e. at the level of points, sections, individual animals and group of animals. Detailed descriptions of the procedures used to estimate the total number of s ynapses, evaluate the precision of the estimates, and optimize the sam pling scheme are provided.