A quantitative study of the number and distribution of neoblasts in Dugesia lugubris (Planaria) with reference to size and ploidy.

The role of the planarian neoblast as a totipotential stem cell has been discussed in the literature for well over a century (cf. Brondsted's excellent review, 1955). It remained a matter of strong debate until 1949 when Dubois demonstrated conclusively the migration of neoblasts (through regions depleted of their neoblasts by radiation) to the surface of a wound. She showed that the onset of regeneration was delayed until the neoblasts reached the wound area, and that once they had arrived regeneration took place at the normal rate with the neoblasts actively dividing in and just posterior to the blastema. Since then many authors (the Brondsteds, Stephan-Dubois, Pedersen, Lender, the Benazzis) have studied the histochemistry, the distribution, and the factors which influence differentiation of the planarian neoblast in several species. Only Br0ndsted & Brondsted (1961) have reported on the total number of neoblasts in a planarian. This paper reports observations on the number of neoblasts in diploid and triploid planarians, the relation of this number to the size of the animal and the distribution of neoblasts with respect to the animal's long axis.