The distribution of ribosomes and polysomes in uninjured myelinated axons of rat sciatic nerve was analyzed. Ribosomes were identified by immunocytochemistry at the light and electron microscopic levels. A polyclonal antibody developed against ribosomes recognized both rRNA and ribosomal proteins. The distribution of the immunoreaction product was similar to that obtained with human anti-ribosomal P protein. The immunoreaction product distributions were of two types in axons: 1) periodic localization in the cortical region of axoplasm that appeared as a compact structural aggregate, consistent with that described as a periaxoplasmic ribosomal plaques (PARP) domain (Koenig et al. [2000] J. Neurosci. 20:8390-8400), and 2) scattered small immuno-reactive clusters of varying sizes (RNP) within the central core of the axon. The latter observation suggested the possibility that RNP-like particles could be associated with the axonal transport system and in transit. Immunoreaction product was also associated with a novel structural inclusion, possibly multi-vesicular in makeup that was located in the axon and at the myelin-axon interface, and visible at the light and EM levels. The potential significance of this structural peculiarity is considered.
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