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 of 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 not due to compression, sectioning artifact, or to inaccuracy of the reconstruction method. 2/3 of spines are emitted from the relatively narrow, ridge‐like regions of the flattened shafts, causing the dendritic cross‐section to resemble bitufted cell bodies that also are elongated along the poles that give rise to their major projections. Apical dendrites of pyramidal cells also exhibit a flattened morphology and preferential distribution of spines. The flattened shape may reflect the in vivo condition, or be an artifact of fixation. If the latter, flattening 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.