Synaptic density on the axonal tree of a pyramidal cell in the cortex of the mouse.

The aim of this study was to investigate the density and distribution of synapses on the axonal tree of a pyramidal cell in the mouse cortex. The method used was a new variation of Golgi-electron microscopy involving the modification of Colonnier, the photochemical method of Blackstad and the use of phosphotungstic acid. A segment of axon collateral of 87 microns carried 17 synapses, the proximal 27 microns being free of synapses. The main axon was postsynaptic down to a length of 30 microns. On the following 163 microns there were six presynaptic elements. On the proximal parts of four other collaterals, five synapses were found; a few synapses might, however, have been missed on these ramifications. Most of the synapses were located on thickenings already visible in the light microscope. The synaptic size varied by a factor of 2.7. The results are compared to estimates derived from former statistical investigations with other light and electron microscopical methods.