Axonal regeneration in the central nervous system (CNS) was investigated in the fine structural and histochemical aspects using carp spinal cord, which was completely transected at the level of the dorsal fin. Fusion of the transection region and the regeneration of axons already began to be recognized 26 days after operation by electron microscopy. At 115 days after operation, the rostral and caudal parts of the transected spinal cord were completely connected by the regenerating nervous tissue, which contained numerous axons among the ependymal and glial processes.

Horseradish peroxidase (HRP), which was injected in the spinal cord at the portion caudal to the transection site was detected in the cytoplasm of large neurons located in the reticular formation of midbrain. This demonstrates that these long axons were regenerated passing through the ablation gap 151–204 days after operation. These findings indicate that regenerating axons in the carp spinal cord can pass through the glial scar formed in the transected portion, which is considered to be the main obstacle for the prolongation of axons in the mammalian CNS.

Many regenerating axons, both unmyelinated and myelinated, were observed being in contact directly with the cell membrane of the ependymal as well as astroglial cells. This indicates that neither ependymal nor glial cells play a role as an obstacle for elongation of axons in the carp spinal cord. Numerous GFAP (glial fibrillary acidic protein) positive intermediate filaments were observed in the cell bodies and cytoplasmic processes of both ependymal cells and astroglia. Their morphological similarity and similar property for axonal elongation indicate that the cells of both types in the carp spinal cord are intimately related with each other in origin, morphology and function.