Immunoelectron microscopic study of substance P-containing fibers in feline cerebral arteries

The ultrastructure of substance P-containing fibers in feline cerebral arteries was examined by combining substance P immunohistochemistry and electron microscopy. At the light and electron microscopic level, positive fibers were observed in the adventitia and at the border between the adventitia and media, but not within the media or the endothelium. The substance P-containing fibers were unmyelinated with diameters consistent with C-fiber caliber. Positive axons were in close contact with Schwann cell processes. Positive axons contained 24 nm microtubules, 10 nm neurofilaments, clear vesicles and scattered mitochondria. The number of mitochondria and organelles resembling vesicles appeared to increase in presumptive axon terminals. No synaptic membrane specializations were observed.