Observations on the Shapes of Dendrites

The common assumption that dendrites are cylindrical is incorrect. Analyses of spiny stellate cell dendrites reconstructed from serial thin sections through mouse barrels indicates that these dendrites are flattened; the ratio of long to short axes in cross‐sectioned dendritic profiles is typically 2:1. Planes of flattening are unrelated to the orientation of dendritic segments within the brain, bear no relationship to the plane of section, and are unchanged for various estimates of section thickness. Thus, the flattened shape is due neither to tissue compression, nor to sectioning artifact, or to inaccuracy of the reconstruction method. Spine necks are attached preferentially along the relatively narrow, ridge‐like regions of the flattened shafts; the ridges bear two‐thirds of the spine attachment points. Both spiny stellate cell dendrites as viewed in cross‐section, and bitufted cell bodies are elongated along the poles that give rise to their major projections. These morphological phenomena are suggested to have a common basis: the non‐homogeneous distribution of cytoskeletal elements associated with structural stability and/or intracellular transport. Published work on the apical dendrites of pyramidal cells indicates that they too exhibit a flattened morphology and preferential distribution of spines. The flattened shape may be an artifact of fixation, and if so, could be imagined to result from the shrinking of cell membrane around the cytoskeleton and/or from the collapse of particularly delicate cytoskeletal elements within certain dendritic compartments. Alternatively, the shape reported here may reflect the in vivo condition. Consequences for various aspects of electrical activity within dendrites of the possible compartmentalization of the dendroplasm and of the observed irregular distribution of spines remain to be determined. Supported by Israel Science Foundation grant 52/00‐4 ELW.