Mitosis in the intact and regenerating planarian Dugesia mediterranea n.sp. II. Mitotic studies during regeneration, and a possible mechanism of blastema formation

Abstract In planarian regeneration mitosis occurs in both anterior and posterior blastemas. The mitotic increase is visible as early as one hour after amputation, reaches a rapid first maximum at 5–12 hours, a relative minimum at 24–30 hours, and a higher and longer second maximum at two to four days depending on body size, level of amputation, and on whether blastema is anterior or posterior. The spatial dynamics of mitosis show a rapid and high mitotic increase in the distal regions but not in the proximal regions. Later, the proliferative zone shifts in sucessive mitotic waves to more proximal regions, paralleling the formation of the head ganglia and nerve cords. These results do not agree with theories of blastema formation through neoblast migration or through cell dedifferentiation, in as much as the local proliferation of neoblasts encountered is sufficiently high and early to account for the number of blastema cells found during period of regeneration. Therefore, we suggest that blastema formation during planarian regeneration occurs mainly through local neoblast proliferation. These results and the data obtained on regeneration rates enable us to suggest that nervous tissue may be one of the factors responsible for the differing mitotic increases found. Since neoblasts are the only planarian cells capable of mitosis, we suggest that neoblast proliferation could be under nervous tissue control through some stimulatory substance (s) released from the nerve terminals. The implications of this hypothesis for the maintenance of axial polarity are also discussed.