Ribosomes in myelinated axons of the rabbit spinal ganglion neurons

To clarify whether, in the mature pseudounipolar neurons of mammalian spinal ganglia, a small number of ribosomes are dispersed in many or possibly all axons beyond their initial segments or if a large number of ribosomes is confined to a few axons only, three series of consecutive sections (of 449, 702 and 865 sections respectively) were cut from the sensory portion of rabbit spinal nerves very close to the ganglion. In these series of sections, portions (from 4.5 to 69.2 microns long) of 198 myelinated axons were examined. Clusters of ribosomes were detected only in three of the 198 axons examined, where they were present in 73-99% of the sections. In the nerves we studied ribosomes are not sparsely dispersed in many or possibly all axons, but are confined to a few axons only, where they are present in a high percentage of sections. Both size and density of axonal ribosomes were identical to those of adjacent Schwann cell ribosomes and clearly different from those of glycogen particles. The great majority of axonal ribosomes were localized within the subaxolemmal band of axoplasm whereas their frequency was lowest within the axonal core. The possible origin of the ribosomes found within the three axons is briefly discussed.