Mitosis in the intact and regenerating planarian Dugesia mediterranea n.sp. I. Mitotic studies during growth, feeding and starvation

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Abstract
Planarian neoblasts, the only cell type in this organism endowed with mitotic power, decrease in density as total body volume, total cell number, and length increase. Studies on mitosis show that the neoblast mitotic rate is a function of body length, being lower as the organism increases in length. The cephalo‐caudal distribution of mitotic figures in animals of different sizes shows a change towards a more even distribution of mitosis jointly with a mitotic increase in the caudal regions as length increases. However, in the sagittal and dorso‐ventral axes, the distribution of mitosis is always nonuniform, probably due to the uneven distribution of parenchymal tissue along them.The pharynx of animals in all size groups examined shows no mitotic figures except some near its base. This may suggest for this organ a proximal type of growth with some cell migration.Feeding elicits a very rapid increase in mitosis which lasts for almost a week, whereas long periods of starvation do not cause a significant increase or decrease in the basal mitotic rate. All these results stress the need of looking at the planarian neoblast mainly as a stem or replacement cell of most differentiated cell types and not as a mere “regeneration cell.” The implications of the results obtained for the mechanisms of growth and shrinkage are discussed.