Identification of the small pore in muscle capillaries.

Electron microscope studies have been carried out in order to identify the structural counterpart of the small pore of muscle capillaries. Experiments with horseradish peroxidase (40,000 MW) and microperoxidase (1,900 MW) indicate that the clefts between capillary endothelial cells, rather than endothelial vesicles or transendothelial channels, are the counterpart of the pore. The structural properties of the clefts that enable them to serve as the small pore are presumably related to the nature of the attachment between plasma membranes of neighboring endothelial cells.