The innervation of mesenteric arteries and vas deferens of guinea-pig and vas deferens of mouse was examined by freeze-etching. Axons in bundles at large distances from the smooth muscle cells, were invested by Schwann cells and contained mainly neurotubules, while axons close to the smooth muscle cells had varicosities up to 1.6 µm in diameter and 2.0 µm long containing mainly small (approximately 50 nm) and large (approximately 100 nm) synaptic vesicles. Vascular axons differed from those in the vas deferens in that the former were at the medial adventitial border with an observed closest neuromuscular distance of approximately 200 nm and the latter were between smooth muscle cells at distances of 20-50 nm. Depressions of the axonal surface were seen and particles up to 15 nm were found on the axonal membrane.

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