Aspects of the organization of neurons and dendritic bundles in primary somatosensory cortex of the rat

In order to analyze some aspects of the spatial organization in the primary somatosensory cortex of the rat, we have reconstructed the positions of bundles of apical dendrites and neurons in a cortical prisms measuring 0.5 mm × 0.4 mm × cortical thickness, with special reference to a hypothetical columnar organization. Complete series of semithin (0.65 μm) sections were cut, tangentially from the pial surface down to the white matter, stained and digitizalized into a computer and represented as a stack of 2D images. The mean neuron density ( N V-value) was (60 × 10 3 ± 15 × 10 3) neurons/mm 3. The mean number of neurons beneath 1 mm 2 of cortical surface ( N C-value) was (113 × 10 3 ± 8 × 10 3) neurons/mm 2. Well-defined bundles of apical dendrites emanating from layer V pyramidal cells were observed. The bundles consisted of 3–12 (mean 5 ± 2) dendrites. The dendrites within a bundle converged while ascending towards the pial surface and reached a maximal close packing in layer IV. Superficially, the packing density decreased again. The mutual positions of the dendrites within the bundles shifted only slightly along their course towards the pial surface. The occurrence of bundles in tangential sections through layer IV was about 190 bundles/mm 2 and the average number of neurons per bundle was estimated at approximately 600. However, when calculating Voronoi-diagrams, the number of neurons, which with this mathematical technique, is ascribed to each of the reconstructed dendritic bundles, varied between 200 and 1000. The possibility that the dendritic bundles are centers in cortical cell modules is discussed.