Fine structure studies on the planarian, Dugesia: I. Nature of the “neoblast” and other cell types in noninjured worms1

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An electron microscopic descriptive and quantitative account is presented of all cell types present in the 1 mm zone immediately postcephalic in Dugesia dorotocephala. Since this is the usual site of amputation in regeneration studies, the data form a baseline for future comparisons with regeneration cell populations. The data permit the conclusion that light microscope identifications of “neoblasts” are actually descriptions of a heterogeneous population of undifferentiated-appearing cells, which we call beta cells, and gland and muscle cell bodies. The extensive granular endoplasmic reticulum of these cell bodies would give more intense basophilia than the diffuse ribosomes of beta cells when viewed with the light microscope, and thus gland cells would be more likely visualized than beta cells in earlier studies concluding that “neoblasts” are reserve cells for regeneration. Beta cells are clearly described here for the first time; their unique nuclei, prominent chromatoid bodies, and possible role as a progenitor cell in physiological cell renewal are emphasized.

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      Many of these genes have been show by RNAi to have an important function in neoblast maintenance or differentiation, including orthologs of vasa and piwi (Reddien et al., 2005; Wagner et al., 2012). Furthermore, neoblasts have perinuclear ribonucleoprotein granules denominated “chromatoid bodies”, which are morphologically, molecularly and functionally similar to the germ granules present in the germ cells of many animals (Morita et al., 1969; Hay and Coward, 1975; Auladell et al., 1993; Yoshida-Kashikawa et al., 2007; Shibata et al., 2016). In cestodes, undifferentiated proliferating cells (equivalent to the planarian neoblasts) are usually denominated “germinative cells”.

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    1

    Supported by U.S. Public Health Service Grant HD-00143 and National Science Foundation Grant GB-19630.

    2

    USPHS Special Postdoctoral fellow (GM-25351-01).

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