Scanning electron microscopy was used on the mucosa of the rat urinary bladder after digestion with strong alkali and microdissection. The underside of the epithelium (and the plane of the epithelium-tunica propria interface) is not smooth but is scored by grooves-10 μm wide and 3–4 μm deep—connected into a fine mesh. A net of blood capillaries located in the uppermost part of the tunica propria occupies these grooves. They measure 3–9 μm in diameter, are separated from the epithelium by a gap of 0.3 μm, often show fenestrations, and are accompanied by numerous and extensive pericytes and by some fibroblasts. We discuss these observations in the light of current knowledge of blood flow in the bladder, contraction and distension of the bladder wall and formation of mucosal folds, transport of solutes through the epithelium, and plasma extravasation from mucosal blood vessels in neurogenic inf
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