Table 4 Classification of sinusoidal non-reticuloendothelial blood capillary microvasculature
SINUSOIDAL CAPILLARY TYPE | Primary anatomic sites of transvascular flow | Ultrastructural determinants of transvascular transport | Physiologic upper limit of pore size |
|---|---|---|---|
NON-RETICULO-ENDOTHELIAL | •Endothelial cells w/low levels of phago-endocytosis[20–22] •Basement membrane discontinuous[23–25] •Thin anionic glycocalyx over endothelial cell surfaces of both capillary types[26] | ||
Splenic red pulp arterial blood capillary (Terminal) | Terminal capillary ending •Terminal capillary ending openings ~5 microns (μm) in diameter [27, 28] •Basement membrane sparse and intermittent •Macrophages in the terminal arterial pericapillary sheath and within the splenic red pulp reticular meshwork [29, 30, 212] | •Terminal arterial capillary network of the splenic red pulp reticulum constitutes the primary mode of splenic filtration •Macromolecules as large as 5 μm pass into splenic red pulp reticulum through terminal capillary ending openings •Exogenous macromolecules phagocytosed by macrophages in the terminal arterial pericapillary sheath and in the red pulp reticulum | ~5 μm |
Splenic red pulp venous blood capillary (Sinus) | Interendothelial slits •Cuboidal endothelial cells •Interendothelial slits between apical and basal adherens junctions •Basement membrane ringed and belts of basement membrane rings 2-3 μm apart [25] •Slits closed except during active blood cell migration [25, 31] and macrophage phagocytosis [32] | •Few direct connections exist between splenic arterioles and venous capillaries and constitutes the minor pathway in splenic filtration •Exogenous macromolecules in sinus lumen phagocytosed at level of the interendothelial slits by finger-like pseudopodia of splenic pulp reticulum macrophages | __ |