Capillary Density and Neuronal Homeostasis in Human Primary Visual Cortex

Hypoxia is a critical determinant of neuronal function and vulnerability in the brain. While neuronal density has been extensively characterized across cortical regions, the spatial relationship between neurons and the microvasculature represents an additional structural constraint on oxygen diffusion and metabolic support. In particular, the distance between neurons and nearby capillaries influences local oxygen availability required to sustain neuronal activity and maintain ionic homeostasis.

In this study, we quantified neuron-capillary spatial relationships across cortical layers in the human primary visual cortex. Postmortem tissue from three neuropathologically normal individuals (ages 55-75 years) was analyzed using dual-label immunohistochemistry for NeuN (neurons) and CD34 (vascular endothelial cells). Regions of interest corresponding to neocortical layers were delineated by a neuropathologist. Automated image analysis in QuPath was used to segment neurons using object-based classification and capillaries using pixel-based classification. Distances between neuronal and vascular centroids were corrected for tissue shrinkage using a linear scaling factor of 1.20. Centroids of segmented objects were extracted, and nearest-neighbor relationships were computed using a quad tree spatial search algorithm.

Across more than 155 000 neurons and 84 000 vascular objects, we observed a consistent laminar gradient in neuron-capillary distance. Neurons in superficial and deep layers were located farther from capillaries, whereas neurons in the central layers exhibited shorter distances, with layers III and IV demonstrating median distances of ~30-45 μm.

These findings demonstrate that neuron-capillary spatial relationships vary systematically across cortical depth, suggesting that cortical microvascular architecture is consistent with layer-specific metabolic demands.