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Abstract:
In contrast to mammals, all teleost fish examined thus far exhibit an enormous potential to regenerate not only neuronal processes (axonal regeneration), but even whole neurons (neuronal regeneration) after injuries in the central nervous system. By application of lesions to one subdivision of the cerebellum, the corpus cerebelli, the role of apoptosis in neuronal regeneration was examined in the gymnotiform fish, Apteronotus leptorhynchus. Apoptotic cells were identified by examination of cryosections with the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL) reaction, an in situ technique employed for detection of nuclear DNA fragmentation. Additional evidence for the apoptotic nature of dying cells was obtained through analysis of morphologies displayed at both the light microscopic and the ultrastructural level. The first TUNEL-positive cells at the site of the lesion appeared as rapidly as 5 min following mechanical damage of the tissue. Thirty minutes after stab wound lesion, their number reached maximum levels. Starting with 2 days of postlesioning survival time, a gradual decline in the number of TUNEL-positive cells was evident, until this process reached background levels 20 days after the lesion. We hypothesize that apoptosis is used in A. leptorhynchus as an efficient mechanism for the removal of cells damaged through injury in the central nervous system. Since apoptosis is not accompanied by the side-effects known from necrosis (which is predominant after injuries in the mammalian central nervous system), this "clean" type of cell death may, at least partially, explain the tremendous regenerative capability of teleosts.
