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Electron microscopic and immunohistochemical evidence that unmyelinated ventral root axons make U‐turns or enter the spinal pia mater

The occurrence of unmyelinated and small myelinated axons and of nerve fibres with a substance P‐like immunoreactivity was studied in juxtamedullary L7 ventral root fascicles and surrounding pia mater of kittens and cats. Electron microscopic analysis of thin transverse serial sections from this region in adult cats showed that the number of unmyelinated axon profiles decreases rapidly as the CNS is approached, reaching zero near or at the CNS/PNS border. No unmyelinated axons were found to enter the CNS through this root. At least to some extent, the disappearance of unmyelinated ventral root axons from the juxtamedullary root fascicles was due to presence of intrafascicular axonal hairpin loops and to a shift of axons from ventral root fascicles to the pia mater. It was also found that, as in the L7 ventral root, the content of unmyelinated axons in the pia mater is lower in kittens than in cats. Fluorescence microscopic examination of specimens incubated with substance P antiserum showed that some looping axons and ventral root‐pia mater axons were substance P immunoreactive. These observations suggest the hypothesis that sensory unmyelinated axons might grow through the L7 ventral root and enter the pia mater during postnatal development. Moreover they show that the occurrence of sensory unmyelinated axons in ventral roots does not necessarily contradict the law of Magendie.