Electron microscopic studies of planarian regeneration. IV. Cell division of neoblasts in Dugesia dorotocephala

Abstract

AbstractThere has been disagreement regarding the source of new cells for planarian regeneration. Electron microscopy was used to study this problem in surgically decapitated asexual Dugesia dorotocephala. Comparison of the fine structural morphology of cells found in prophase, metaphase, anaphase, and telophase with those of various cell types in interphase shows that the mitotic cells and resulting daughter cells are all neoblasts. This mitosis, which appears quite classical, was observed in the parenchyma of preexisting tissue but not in the regeneration blastema. This accords with previous observations indicating that blastema cells are already differentiated out of the neoblast stage, and the present observation that mitotic activity appears restricted to neoblasts. However, persistence of neoblast characteristics, e.g., numerous free, or poly‐, ribosomes and diminished, but still prominent, nuclear satellite material, indicate recent differentiation of blastema cells from neoblasts. Previous studies have shown a basal mitotic rate in the parenchymal neoblasts of uninjured asexual planarians. This mitotic incidence remains the same for the first day, then increases to a maximum on the second and third days following decapitation. Despite an exhaustive search involving construction of electronmicrographic montages of all or most of the wound area and adjacent tissue at various times ranging from 1 to 72 hours after decapitation, no evidence of cellular dedifferentiation was observed. Joint consideration of present and previous results indicate that new specialized cells, whether required for normal turnover replacement, asexual reproduction, or reconstitutive regeneration, are all differentiated from parenchymal neoblasts which maintain their numbers by mitotic production of more neoblasts.