Denritic Morphology

Abstract

The common assumption that dendrites are cylindrical is incorrect. Analyses of spiny stellate cell dendrites reconstructed from serial thin sections through mouse barrels shows these dendrites to be flattened; the ratio of long to short axes in cross‐section is typically 2:1. Planes of flattening are unrelated to the orientation of dendritic segments, are not related to plane of section, and are unchanged for various estimates of section thickness. Thus, the flattened shape is due neither to tissue compression, sectioning artifact, nor to inaccuracy of reconstruction. Two‐thirds of dendritic spines are attached at the narrow, ridge‐like regions of the flattened shafts. Apical dendrites of pyramidal cells are also flattened and exhibit a preferential distribution of spines. These phenomena may be related to the distribution of cytoskeletal elements. 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 cytoskeletal elements within certain compartments. Alternatively, the shape 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. Israel Science Foundation grant 52/00‐4