Distribution of spine classes shows intra-neuronal dendritic heterogeneity in mouse cortex.

Significance: Neuronal dendritic spines are central elements for memory and learning. Their morphology correlates with synaptic strength and is a proxy for function. Classic light microscopy cannot resolve spine morphology well, and techniques with higher resolution (electron microscopy and super-resolution light microscopy) typically do not provide spine data in large fields of view, e.g., along entire dendrites. Therefore, it remains unclear if spine types are organized on mesoscopic scales, despite their undisputed importance for understanding the brain. Aim: Recently, it was shown that the distribution of spine type is dendrite-specific in the turtle cortex, suggesting a mesoscopic organization, but leaving the question open if such a dendrite specificity also exists in mammals. Here, we determine if such a difference in spine-type distribution among dendrites also exists in the mouse brain. Approach: We used super-resolution stimulated emission depletion microscopy of complete dendrites and advanced morphological analysis in three dimensions to decipher morphological differences of spines on different dendrites. Results: We found that spines of different shapes decorate different dendrites of the same neuron to a varying extent. Significant differences among the dendrites are apparent, based on spine classes as well as based on quantitative descriptors, such as spine length or head size. Conclusions: Our findings may indicate that it is an evolutionarily conserved principle that individual dendrites have distinct distributions of spine types hinting at individual roles.