The histology of tail regeneration in the ammocoetes

Abstract

Ammocoetes of both Petromyzon marinus and Entosphenus lamottenii are able to regenerate morphologically complete tails, although at a slow rate. This ability persists in the lampreys throughout the larval life. Regeneration occurs via dedifferentiation of stump tissues and formation of the blastema. The tissues in the posterior region of the stump lose their specialized features and release dedifferentiated mononucleate cells, which migrate distally to form the blastema under the epidermal cap covering the amputation surface. Great mitotic activity appears in the tissues of the posterior part of the stump during the process of dedifferentiation following amputation of the tail. The blastema is formed by cellular contributions from the muscles, notochord, cartilaginous elements, and the general connective tissue of the stump. All these tissues arise de novo in the regenerated tail from redifferentiation of the blastema cells, and follow the ontogenetic mode of development not only in histogenesis but also in the sequence in which they appear. No precocious developments occur in the regenerating tails of the lamprey larvae. For the redifferentiation of the notochord in the blastema an actual histological connection with the stump notochord is not necessary. The regeneration of the spinal cord, after tail amputation, is also largely a new development involving a great degree of dedifferentiation followed by redifferentiation of the cells derived from the corresponding organ in the stump. The regenerated spinal cord is anatomically a complete organ. At least three to four pairs of spinal ganglia are regenerated.