[Cellular origins of regeneration in planarians. Current concepts of neoblasts].

The problem on the origin of cellular material for planarian regeneration is still disputable. Some old and modern authors, among them those of Et. Wolff's school (France), believe that the only construction material for regeneration are neoblasts which are considered as poorly differentiated cells of the embryonal type. Under light microscopy, they are seen as small highly basophilic cells with a large vesicular nucleus and a large nucleolus. Electron microscopic investigation demonstrates in the neoblasts a rather large nucleus with clumped chromatin scattered in the nucleoplasm and with small amount of cytoplasm rich in free ribosomes and poor in other organoids. A characteristic feature for activated neoblasts in the regenerating planaria is presence, in cytoplasm, of chromatoid bodies of nuclear origin. Another point of view suggested by some old authors and supported by several modern American investigators (E. D. Hay a. St. J. Coward) considers that the cells dedifferentiate within the wound area and convert into neoblasts.