Flatworm asexual multiplication implicates stem cells and regeneration

The phenomenon of asexual multiplication is rare in the animal kingdom, but it occurs in all main flatworm taxa. In the present paper, we review data regarding the presence of different forms of asexual multiplication in flatworms and argue that the presence of a population of totipotent or pluripotent stem cells, "neoblasts", is a primitive feature of decisive importance for the developing potential of flatworms. Next we present information on the role of stem cells in fission, head regeneration, and pharynx regeneration of planarians. Furthermore, the tracing of neoblasts in lower flatworms and cestodes is presented, and the results indicating heterogeneity of the neoblast pool are dis - cussed. Finally, the mode by which the neoblasts are stimulated to divide, migrate, and differentiate and the nature of the interactions are discussed. We focus on (i) biogenic amines and neuropeptides, (ii) the role of neuropeptides in the early stage of regeneration, (iii) the evidence for the influences of growth factors and nitric oxide, and ( iv) the influ- ence of weak electromagnetic fields. We discuss the pattern in which a gradient system of morphogens and (or) a hier- archical system of inductions is expressed in development.