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The number of spines on an individual Purkinje cell in the cerebellar cortex of the rat was determined by stereological methods. Investigations were based on thin section electron micrographs, freeze fracture replicas, and horseradish peroxidase labeled cells. Purkinje cell dendritic spines in our embedded material had a mean length of 1.4 +/- 0.05 micron and mean neck and head diameters of 0.22 +/- 0.01 micron and 0.45 +/- 0.02 micron, respectively. From these dimensions, an estimate of spine volume in embedded material of 0.132 micron 3 was obtained. The density of dendritic spines in our fixed material was 8.15 x 10(8) or 7.24 x 10(8) per microliters of molecular layer from volume fraction and density per mm2, respectively. The number of spines per linear micron of Purkinje cell spiny branchlet was 17.2 from freeze fracture and 17.6 from horseradish peroxidase labeled dendrites. These all indicate that there are between 154,000 and 175,000 spines on the dendritic tree of each Purkinje cell, considerably more than previously reported for the rat.
Napper et al. (Mon,) studied this question.