Regeneration of the caudal neurosecretory system in the cichlid teleost Tilapia mossambica

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Abstract
After total extirpation of the caudal neurosecretory system in the teleost Tilapia mossambica by ligature and removal of the caudal peduncle, a new system regenerates rapidly. The new neurosecretory cells differentiate from ependyma. A series of transitional stages between slightly modified ependymal cells and fully developed caudal neurosecretory neurons can be readily constructed. Immediately after the operation, the distal part of the remaining spinal cord undergoes degeneration, during which no traces of the caudal system can be found. Eleven days after the operation the first signs of the new system appear, and after three weeks a potentially functional, although much reduced, system has developed, the elements of which are detectable with the electron microscope. After 5–6 months, an extensive system has regenerated; however, the neurohemal area equivalent to the normal urophysis is contained within the spinal cord. There is strong cytologic evidence for a considerable release of elementary neurosecretory granules into the cerebrospinal fluid in the regenerating and regenerated system, possibly by several different mechanisms. Prominent tubular reticula occur in the distal (preterminal) parts of the neurosecretory axons, and the possibility of local axonal production of vesicles and granules is strongly supported. Signs of secretion into the central canal and of distal production of neurosecretory material are also present in normal Tilapia, but both phenomena are much more strikingly demonstrated in regenerating systems.