Figure 1

Schematic depictions of the capillary wall ultrastructure in different blood capillary microvasculatures. Shown in red are the anatomic sites in the capillary walls of the respective blood capillary types that are the primary pathways for transvascular flow and transport across the capillary wall, and as such, constitute the ultrastructural determinants of the physiologic upper limit of pore size to transvascular flow. The green pillars that emanate from the luminal surface of the endothelial lining represent the individual mucopolysaccharide fibers of the endothelial glycocalyx layer (EGL), and the orange hatched region that encircles the abluminal surface of the endothelial cell lining represents the collagenous basement layer (interna and externa). As depicted in the schematics, the capillary walls of the different types of non-sinusoidal blood capillaries are proficient in all three layers (panels A, B and C), which is not the case for the capillary walls of the sinusoidal blood capillaries of myeloid (red) bone marrow and the liver (panels D and E). Also depicted in panels D and E are the 'bristle-coated pits' of myeloid bone marrow and hepatic sinusoidal blood capillary the reticuloendothelial cells, which constitute the anatomic sites at which the phago-endocytosis of non-endogenous macromolecules occurs.

A. Non-sinusoidal non-fenestrated blood capillaries

B. Non-sinusoidal fenestrated blood capillaries with diaphragmed fenestrae

C. Non-sinusoidal fenestrated blood capillaries with open 'non-diaphragmed' fenestrae

D. Sinusoidal reticuloendothelial non-fenestrated blood capillaries of myeloid (red) bone marrow

E. Sinusoidal reticuloendothelial fenestrated blood capillaries of the liver

(Please view Additional files 1, 2, 3, 4 and 5 for individual Figure 1 panels A, B, C, D and E with detailed panel descriptions)