Quantitative reconstruction of neuronal mitochondrial network in neurites and somata in rat hippocampus and...

Mitochondrial networks exhibit striking heterogeneity in their morphology and distribution across different neuronal compartments, reflecting the diverse metabolic demands of these structures.

In this study, we used automated tape-collecting ultramicrotome scanning electron microscopy (ATUM-SEM) to reconstruct and quantify mitochondrial networks in the somata and neurites of neurons in the rat prefrontal cortex (PFC) and hippocampus (HPC; CA1 stratum radiatum). We developed an automated segmentation pipeline based on an attention-enhanced 3D U-Net to extract all mitochondria from volumetric EM data.

Our quantitative analyses revealed pronounced regional and subcellular heterogeneity. In the PFC, the mitochondrial volume fraction was higher in neurites (7.2%) than in somata (2.9%; 7.1% when nucleus was excluded). Mean individual mitochondrial volume was 0.11 μm³ for neuritic and 0.33 μm³ for somatic mitochondria in the PFC, with similar results observed in the HPC (0.13 μm³ in neurites, 0.31 μm³ in somata). In both regions, the vast majority of mitochondria (~91%) assumed an oval or rod shape, with few displaying branched or donut-shaped structures (~1%). Notably, elongated linear mitochondria (~8%) were mostly confined to neurites, and approximately 90% of these comprised up to 120 nanotunnels-thin segments (<220 nm) connecting enlarged, oval-shaped structures (>350 nm) in tandem.

These data provide a detailed quantitative characterization of mitochondrial network architecture in the adult rat cortex and hippocampus, revealing significant regional and subcellular differences in mitochondrial morphology and distribution.