The Paracellular Pathway in Capillary Endothelia
The main function of the circulation is exchange of gases and solutes between blood and tissue occurring across the walls of microvessels. In the days before electron microscopy the water-filled clefts between the endothelial cells were considered the obvious pathway for exchange of hydrophilic solutes.1 The introduction of electron microscopy in studies of capillary wall structure somewhat confused this simple picture. The electron micrographs indicated that the clefts between the endothelial cells are closed by cell contacts;2,3 and thus they could not serve as hydrophilic diffusion pathway. Consequently, an extensive search for transcellular pathways for hydrophilic solutes was initiated. Starling’s original concept has now been revived. Recent data, both physiological and ultrastructural, strongly indicate that solutes — at least up to the size of small proteins — may permeate the microvascular endothelium via the paracellular pathway.4,5 It remains unclear how macromolecules and particles (diameter larger than 5 nm) permeate the endothelium.
KeywordsTight Junction Contact Line Capillary Wall Capillary Endothelium Paracellular Pathway
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