Dendritic morphology of pyramidal neurones of the visual cortex of the rat: II. Parameter correlations.

This study concerns the correlations between the various morphometric parameters obtained for the dendrites of neocortical pyramidal cells. The primary aims were to uncover underlying design principles in dendritic morphology, to see if these differed between different types of dendrite, and to see if estimates of parameters such as total dendritic shaft membrane area could be obtained from a limited number of measurements, avoiding the need to measure every dendritic segment. The data were from a sample of 39 pyramidal neurones, from layers 2/3 and 5 of the visual cortex of the rat, that had been injected with horseradish peroxidase, reconstructed, and measured with the light microscope as part of an earlier study (Larkman and Mason, '90: J. Neurosci. 10:1407-1414). Correlations between the somal area or the combined diameters of the stem segments and measures of the overall size of the dendrites were generally weak. For basal dendrites, the size of a tree was correlated with both its number of tips and the diameter of its stem segment, but these correlations were weaker for apical dendrites. Within individual cells, the diameter of any basal segment was closely related to the size of the tree arising from it, and quantitatively similar relations applied to apical oblique trees from the same cell. Terminal arbor trees showed relations that were similar in pattern but differed quantitatively, whereas apical trunk segment diameter correlations were often weak. In all cases, the number of tips in a tree was closely related to its size. Segment lengths, however, were not closely related to the size of the trees arising from them. It appears that at least some aspects of pyramidal dendritic morphology obey simple design rules. There was heterogeneity between trees of different types, although basal and oblique trees were very similar in most respects. It should prove possible to make use of correlations to estimate the sizes of basal, oblique, and terminal arbor trees from a limited number of measurements, but this does not seem to be possible for apical trunks.