Surface densities of diaphragmed fenestrae and transendothelial channels in different murine capillary beds

Abstract

Fenestrated capillaries are provided with two types of regular discontinuities: (1) fenestrae with negatively charged diaphragms, and (2) transendothelial channels fitted with two diaphragms, of which the luminal one is uncharged. These structures are expected to affect macromolecular exchanges on the basis of size and charge. We have detected variations in the surface density of fenestrations and transendothelial channels (normalized to 1 μm of capillary endothelial profiles in sections) in selected areas of different murine organs, i.e., kidney cortex, duodenal mucosa, and exocrine pancreas. The survey was limited to endothelial segments ≤400 nm thick, and covered a total length of endothelial profiles of 1180 μm, 730 μm, and 1189 μm in the exocrine pancreas, intestinal mucosa, and kidney cortex, respectively. At least 1000 transendothelial openings were recorded by scoring for easily recognizable fenestrations, transendothelial channels, and unknowns, the latter representing either of the above in grazing sections. The linear density of all transendothelial openings taken together was found to vary among the different capillary beds (kidney cortex > intestinal mucosa > exocrine pancreas). This same sequence was observed for the linear densities of fenestrations, transendothelial channels, and unknowns considered individually. The values of obtained were as follows; kidney cortex, 1.35 fenestrations per μm 0.25 transendothelial channels per μm, 0.52 unknowns per μm; intestinal mucosa, 0.92 fenestrations per μm, 0.10 transendothelial channels per μm, 0.38 unknowns per μm; exocrine pancreas, 0.58 fenestrations per μm, 0.04 transendothelial channels per μm, 0.27 unknowns per μm. The differences in distribution of transendothelial openings among fenestrated capillary beds probably reflect capillary permeability modulations connected with the functions of these various organs.