Marchi-positive myelinoid bodies at the transition between the central and the peripheral nervous system in some vertebrates.

The CNS-PNS (central nervous system-peripheral nervous system) transitional region of cranial and spinal nerve roots in some vertebrate species was analysed with respect to the occurrence and the distribution of myelinoid Marchi-positive bodies. Both cranial and spinal nerve roots contained more Marchi-positive bodies in their CNS than in their PNS segments. An accumulation of Marchi-positive bodies was usually noted just central to the CNS-PNS borderline. Comparisons between calibre spectra and Marchi index in the cat revealed a particularly high number of Marchi-positive bodies in nerve roots with a high content of myelinated fibres with diameters greater than or equal to 5 microns. Marchi-positive bodies were absent in CNS tissue lacking myelinated nerve fibres. CNS borderline internodes measuring between 200 and 300 microns in length were noted in fibres as thick as 15 microns in feline S1 ventral and dorsal roots. The general picture was similar in all analysed species. Noteworthy however, was the small difference in number of Marchi-positive bodies between CNS and PNS tissue in Xenopus. The chicken contained many myelinoid bodies of similar size and texture as the Marchi-positive bodies but without the Marchi-positive staining properties. The results show that normally occurring Marchi-positive bodies in the CNS are more numerous along paranodal segments than along mid-internodal segments of myelinated nerve fibres and thus support the hypothesis that Marchi-positive bodies are preferentially derived from paranodal myelin.