Axonal guidance during embryogenesis and regeneration in the spinal cord of the newt: The blueprint hypothesis of neuronal pathway patterning

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Abstract
In our previous studies on spinal cord regeneration in the adult lizard and the newt, we observed that the radial processes of the regenerating ependyma form between them channels which are subsequently invaded by growing neurites. In the present study we compare embryogenesis of the newt spinal cord with regeneration in the adult. Except for minor differences, we observed that the germinal neuroepithelium of the embryo and larva patterns the longitudinal neural tracts in a similar manner. With these facts in mind we propose the blueprint hypothesis which asserts that inherent in the primitive germinal neuroepithelium and its derivative primitive glia is the pattern of the primary neuronal pathways which is expressed in neurogenesis as formed channels or spaces between the processes of the epithelial cells, the surfaces of which contain trace pathways which the growing neurites follow toward their destination. The trace pathways are envisioned as mechanical‐chemical itineraries which the neurites follow according to their individual affinities. The hypothesis is compared to extant theories and the limitations in central nervous regeneration of vertebrates is compared.