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 axonaltree of apyramidalcell in the mousecortex. 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 μm carried 17 synapses, the proximal 27 μm being free of synapses. The main axon was postsynaptic down to a length of 30μm. On the following 163 μm 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