Distribution of neoblasts and mitoses during the asexual reproduction of the planarian Dugesia tigrina (Girard).
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Abstract:
The neoblast distribution in the prepharyngeal portion of the body remains unchanged during the asexual reproduction of planaria. During the first few days after fission, the number of neoblasts decreases in the portion of the body immediately adjoining the site of daughter zooid detachment and considerably increases in the regenerative bud. Starting with the fourth day after division, there is an increase in the number of neoblasts in the region of future fission, which can be regarded as one preparatory mechanism for the next fission. The mitosis distribution pattern is just the reverse: the regions of the body with the highest neoblast density are characterized by a low mitotic index or no dividing cells at all. The neoblast and mitosis distributions in the daughter zooid during its asexual reproduction cycle duplicate those observed in the maternal zooid.Keywords:
Planarian
Zooid
Planaria
Asexual reproduction
Mitotic index
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Abstract In two asexual planarian strains ( Dugesia gonocephala and Dugesia tigrina ), reproduction by transverse fission and regeneration is observed under normal conditions and upon decapitation and reserpine treatment. The pattern of asexual reproduction processes in planarians, as it results from our quantitative observations compared with the literature data, indicates that autotomy and regeneration processes have an extremely wide range of possible expressions. In such a heterogeneous situation it is still possible to suppose the existence of a negative cephalic control of fissiparity, similar to the ones involved in general tissue organization and in the regeneration of planarians.
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Turbellaria
Asexual reproduction
Planaria
Autotomy
Flatworm
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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.
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Blastema
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Regeneration of lost tissues depends on the precise interpretation of molecular signals that control and coordinate the onset of proliferation, cellular differentiation and cell death. However, the nature of those molecular signals and the mechanisms that integrate the cellular responses remain largely unknown. The planarian flatworm is a unique model in which regeneration and tissue renewal can be comprehensively studied in vivo. The presence of a population of adult pluripotent stem cells combined with the ability to decode signaling after wounding enable planarians to regenerate a complete, correctly proportioned animal within a few days after any kind of amputation, and to adapt their size to nutritional changes without compromising functionality. Here, we demonstrate that the stress-activated c-jun–NH2–kinase (JNK) links wound-induced apoptosis to the stem cell response during planarian regeneration. We show that JNK modulates the expression of wound-related genes, triggers apoptosis and attenuates the onset of mitosis in stem cells specifically after tissue loss. Furthermore, in pre-existing body regions, JNK activity is required to establish a positive balance between cell death and stem cell proliferation to enable tissue renewal, remodeling and the maintenance of proportionality. During homeostatic degrowth, JNK RNAi blocks apoptosis, resulting in impaired organ remodeling and rescaling. Our findings indicate that JNK-dependent apoptotic cell death is crucial to coordinate tissue renewal and remodeling required to regenerate and to maintain a correctly proportioned animal. Hence, JNK might act as a hub, translating wound signals into apoptotic cell death, controlled stem cell proliferation and differentiation, all of which are required to coordinate regeneration and tissue renewal.
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Hatchling
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ABSTRACT Mitotic activity during regeneration in the planarian Dugesia (G) tigrina shows a biphasic pattern, with a first maximum at 4–12 h, a second and higher maximum at 2–4 days, and a relative minimum in between. The first peak is mainly due to pre-existing G2 cells entering mitosis shortly after cutting, whereas the second maximum is due to cells that divide after going through the S period from the onset of regeneration. From a spatial point of view, the highest mitotic values are found in stump (postblastema) regions near the wound (0–300 μm), though regions far from it also show increased mitotic values but always lower overall values. As regeneration continues the postblastema maximum shifts slightly to more proximal regions. In contrast, no mitosis has been found within the blastema, even though the number of blastema cells increases steadily during regeneration. These results suggest that blastema in planarians forms through an early accumulation of undifferentiated cells at the wound boundary, and grows by the continuous local migration of new undifferentiated cells from the stump to the base of blastema. The results obtained demonstrate that blastema formation in planarians occurs through mechanisms somewhat different to those shown to occur in the classical epimorphic models of regeneration (Annelida, Insecta, Amphibia), and suggest that planarian regeneration could represent an intermediate stage between morphallactic and epimorphic modalities of regeneration.
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Regeneration, relying mainly on resident adult stem cells, is widespread. However, the mechanism by which stem cells initiate proliferation during this process in vivo is unclear. Using planarian as a model, we screened 46 transcripts showing potential function in the regulation of local stem cell proliferation following 48 h regeneration. By analyzing the regeneration defects and the mitotic activity of animals under administration of RNA interference (RNAi), we identified factor for initiating regeneration 1 (Fir1) required for local proliferation. Our findings reveal that Fir1, enriched in neoblasts, promotes planarian regeneration in any tissue-missing context. Further, we demonstrate that DIS3 like 3′-5′ exoribonuclease 2 (Dis3l2) is required for Fir1 phenotype. Besides, RNAi knockdown of Fir1 causes a decrease of neoblast wound response genes following amputation. These findings suggest that Fir1 recognizes regenerative signals and promotes DIS3L2 proteins to trigger neoblast proliferation following amputation and provide a mechanism critical for stem cell response to injury.
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Regenerative process
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Summary In a series of bioassays, four planarian species—Dugesia dorotocephala (Woodworth), Dugesia tigrina (Girard), Cura foremanii (Girard), Dendrocelopsis vaginatus (Hyman)—were evaluated against six water quality parameters (hardness, temperature, salinity, pH, dissolved organics, and dissolved oxygen) for effects on species survival and asexual reproduction. No significant (P<0.05) effects were determined on survival and asexual reproduction of three species exposed to various levels of hardness. Dugesia tigrina was determined to have a significant increase in asexual reproduction with an increase in hardness. Dissolved oxygen was determined to be a critical parameter affecting both survival and reproduction of the species evaluated. Dugesia dorotocephala and D. tigrina were determined to be the most adaptable and tolerant of the species evaluated. Key words: Planariawater qualitysurvivalreproduction
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Turbellaria
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Asexual reproduction
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The neoblast distribution in the prepharyngeal portion of the body remains unchanged during the asexual reproduction of planaria. During the first few days after fission, the number of neoblasts decreases in the portion of the body immediately adjoining the site of daughter zooid detachment and considerably increases in the regenerative bud. Starting with the fourth day after division, there is an increase in the number of neoblasts in the region of future fission, which can be regarded as one preparatory mechanism for the next fission. The mitosis distribution pattern is just the reverse: the regions of the body with the highest neoblast density are characterized by a low mitotic index or no dividing cells at all. The neoblast and mitosis distributions in the daughter zooid during its asexual reproduction cycle duplicate those observed in the maternal zooid.
Planarian
Zooid
Planaria
Asexual reproduction
Mitotic index
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Citations (5)