Population based quantification of dendrites: evidence for the lack of microtubule-associate protein 2a,b in Purkinje...

The high molecular weight isoforms (a and b) of microtubule-associate protein 2 (MAP2a,b) are widely believed to be specific markers for neuronal somata and dendrites. We analyzed and quantified MAP2a,b stained dendrites of the cerebellar molecular layer using a novel approach that segmented and 3D reconstructed them, and the results have been compared with those obtained by other methods, including single-cell reconstruction and analysis of electron micrographs. Our results show that the molecular layer dendritic volume fraction is lower than in the neocortex (10% compared to neocortical 29%). The low total volume fraction of dendrites in the molecular layer is best explained by the majority of the afferents to the dendrites being from the very densely packed parallel fibers, which allows the dendritic fields of individual neurons to be smaller and more compact than in the cerebral cortex. However, the MAP2a,b dendritic volume fraction is even lower (5.2%) than the total volume fraction of dendrites in the molecular layer (10%). Analysis of the material shows that this difference between the two results is due to the unexpected finding that there were few MAP2a,b stained Purkinje cell spiny dendrites.