Area- and layer-specific vascular density in the macaque striate and extrastriate cortex

Introduction: Most functional neuroimaging methods, including fMRI, rely on the hemodynamic response following neural activation. Consequently in order to correctly interpret the results of neuroimaging experiments both the functional and the structural neurovascular coupling must be well understood. The former includes the degree and density of cortical vascularization. Here we studied the layer and area specific vascular differences in areas V1, V2, V3, V4 and V5 in the macaque monkey. Methods: Formalin-fixed frozen sections of 4 animals (M. mulatta) were processed for double fluorescence immunohistochemistry. Sections were incubated with anti-collagen type IV and DAPI to stain for vessels and cell nuclei. The length density (mm/mm³) of vessels was taken as a measure for vascular density. Layer and area specific regions of interest were determined on the basis of the DAPI stain and if necesary with the help of consecutive Nissl and myelin stains. Results: The procedure yielded high quality vessel-specific images with excellent reproducibility within and between animals. In V1, the vascular density was highest in layer IVc-β (871.9 +/- 47.1 mm/mm³, mean +/- sd of 4 animals) and lowest in layer I (587.17 +/- 31.7 mm/mm³). In all extrastriate visual areas analyzed, the vascular density was generally lower, and the difference between layer IV and the remaining layers was less prominent when compared to V1. The vascular length density in V2 was 674.0 +/- 31.8 mm/mm³ in layer IV and 585.1 +/- 40.0 mm/mm³ in layer I. As a further example, in V5 672.7 +/- 41.6 mm/mm³ was measured in layer IV and 584.0 +/- 52.0 mm/mm³ in layer I. Conclusion: In summary, V1 was clearly different from all extrastriate areas analyzed with respect to the laminar vessel distribution and the overall vascular density. Differences between extrastriate areas were negligible. The influence of differences in vascularization on the neuroimaging signals remains largely unknown. However, this study suggests that caution is advised particularly when response patterns are compared between V1 and extrastriate areas