The expression of peripherin, an intermediate filament protein, had been shown by biochemical methods to be localized in the neurons of the PNS. Using immunohistochemical methods, we analyzed this expression more extensively during the development of the rat and compared it with that of the low-molecular-mass neurofilament protein (NF-L), which is expressed in every neuron of the CNS and PNS. The immunoreactivity of NF-L is first apparent at the 25-somite stage (about 11 d) in the ventral horn of the spinal medulla and in the posterior part of the rhombencephalon. The immunoreactivity of peripherin appears subsequently, first colocalized with that of NF-L. Both immunoreactivities then spread out along rostral and caudal directions, but whereas the immunoreactivity of NF-L finally becomes noticeable in every part of the nervous system, that of peripherin remains localized to (1) the motoneurons of the ventral horn of the spinal medulla; (2) the autonomic ganglionic and preganglionic neurons; and (3) the sensory neurons. These results demonstrate that, in the neurons that originate from migrating neural crest cells, the immunoreactivities of peripherin and of NF-L become apparent only when they have reached their destination. The results also show that peripherin is expressed more widely than has been previously observed and that this protein occurs in neuronal populations from different lineages (neural tube, neural crest, placodes) with different functions (motoneurons, sensory and autonomic neurons). The common point of these neurons is that they all have axons lying, at least partly, at the outside of the axis constituted by the encephalon and the spinal medulla; this suggests that peripherin might play a role in the recognition of the axonal pathway through the intermediary of membrane proteins.

Solid fragments of olfactory bulb from new-born normal (B6CBA and C57BL6) mice were implanted into new-born shiverer (shi/shi) brains. The shiverer mouse being characterized by the absence of myelin basic protein (MBP), myelination due to implanted oligodendrocytes can be detected in the shiverer brain using an antiserum anti-MBP. Observation of sagittal sections of the host brains revealed very extensive areas of normal myelination from the level of the graft (rostral thalamus) up to the caudal brain (diencephalon, cerebellum, pons). Thus, oligodendrocytes contained in the implant migrate out of the graft over long distances in the host brain, before they differentiate and synthesize myelin. These results raise the question of the behaviour of oligodendrocytes in normal development.

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