There 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 a n 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.
Free-living flatworms such as planarians are inexpensive to culture, maintain, and use for toxicologic testing in the laboratory. A considerable number of basic studies by ourselves and others indicate that, in simplified miniature, they possess many features of biochemical and physiologic organization similar to higher animals such as mammals. These include a well-developed brain with a varied behavioral repertoire including complex maneuvers of prey capture and learning, with a number of the same neurotransmitters used in mammalian brain. They are sensitive to a variety of the same toxicants. Undifferentiated totipotent stem cells, i.e., "neoblasts," which are capable of mitosis and differentiation into any of the various specialized cell types, permit regeneration of complete planarians from fragments. They also provide new cells to replace those lost in the normal cellular turnover of nonregenerating planarians. Both regeneration of surgical fragments and aberrant remodeling of whole planarians model important features of embyrogenesis and are potentially useful for assaying teratogens. Results are described from studies in which various representative teratogenic toxicants were tested in these two different planarian paradigms. The potential of planarian cephalic regeneration for behavioral teratogenesis investigations is also indicated.
Scimces, The liniversity of Chicago ONE FIGURE It is the purpose of this paper and a companion one (Best, '53) to demonstrate a method by means of which some of the properties of enzymes in living cells can be investigated and the membrane permeability of the cell to their substrates measured.Because of the limitations imposed by most of the methods hitherto available for the technical execution of the measurement of the permeability constant of cells for substrates that undergo metabolic conversion by those cells, few such values are available. A resume of the more important of these methods is given in the companion paper. The considerations on which these methods are based include the assumption that the test substance is, for all practical purposes, conserved Ciu~ing the course of the experiment, thereby precluding their use for the measurement of the permeability for test substances that are metabolized at an appreciable rate by the cell under investigation.If one wishes to measure the membrane permeability of a cell to a substance that undergoes metabolic conversion at a ' This work was done in partial fulfillment of the requireinents for the degree of
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.