Morphology Favors an Endothelial Cell Pathway for Longitudinal Conduction within Arterioles

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We examined the morphological parameters of arteriolar endothelial and smooth muscle cell dimensions and gap junctional surface areas to obtain an indication of the coupling capacity of each cell type. Silver nitrate staining was utilized to define cell borders of endothelial and smooth muscle cells in arterioles of several vascular beds from two species. From video images of silver-stained arterioles, the mean endothelial cell length of hamster cheek pouch arterioles (diameter 20 to 110 μm) was found to be 141 ± 2 μm. Mean endothelial cell width was 7 ± 0.2 μm in the same arterioles. Mean smooth muscle cell length in hamster cheek pouch arterioles of diameter 80 to 150 μm was 66 ± 3 μm, with an average cell width of 8 ± 0.2 μm. Dimensions of both endothelial and smooth muscle cells varied moderately with arteriole size and tissue type, but no general trends were seen. Based on the measured dimensions and the specific orientation of cell types within the arteriole, it was calculated that in hamster cheek pouch arterioles (60 μm diameter), 6 or 7 endothelial cell lengths would constitute a 1-mm segment of vessel, whereas approximately 140 smooth muscle cell widths would be required to span the same length. Estimates of connexin43 gap junctional plaque surface areas in each cell type suggest that endothelial cell junctional surface area is approximately eight times that of smooth muscle cells. Thus, combined measurement of cell dimensions and orientation with estimates of junctional plaque density leads to the conclusion that the endothelial cell layer forms a more permissive pathway for longitudinal conduction of signals through the blood vessel.