Cell movement in intact and regenerating planarians. Quantitation using chromosomal, nuclear and cytoplasmic markers

One of the tenets of Wolff and Dubois' 'neoblast theory' of planarian regeneration (Wolff & Dubois, 1948) is that blastema is mainly formed by the accumulation of undifferentiated parenchymal cells (neoblasts) that can migrate, if needed, over long distances to the wound. That neoblasts migrate was claimed by these authors after partial X-irradiation, and total X-irradiation and grafting using planarian strains of different pigmentation. From this they suggested that migration of neoblasts is stimulated by the wound and directed towards it. To study the nature and extent of such 'migration' in intact and regenerating organisms, and in order to avoid the flaws of using pigmentation as a marker, we made grafts between sexual and asexual races of Dugesia(S)mediterranea that differ in a chromosomal marker, and between diploid and tetraploid biotypes of Dugesia(S)polychroa that differ in nuclear size. Also, fluorescent latex beads were used as cytoplasmic markers to follow 'migration' of differentiated cells. The hosts were irradiated or non-irradiated intact and regenerating organisms. The results show that: 1) movement of graft cells into host tissues occurs in intact organisms at a rate of approximately equal to 40 micron/day, and that this increases up to approximately equal to 75 micron/day in irradiated hosts; 2) movement of cells occurs evenly in all directions; 3) regeneration does not speed up rate of movement nor drives cells preferentially to the wound; 4) spreading of cells is mainly due to the movement of undifferentiated cells (neoblasts); and 5) higher rates of movement are correlated with higher mitotic indexes. From this, it is concluded that the so-called 'migration' of neoblasts is not a true cell migration but the result of the slow, even and progressive spreading of these cells mainly caused by random movements linked to cell proliferation. The implications of these results for blastema formation and the origin of blastema cells are discussed.