. The Biological bulletin. Biology; Zoology; Marine biology. 310 S. PIRAINO ET Figure 17. Striated muscle cells from BrdU-incubated, whole medusa of Turritopsis nutricula undergoing transformation into polyps after rearing at T = 27°C. (a-b) same frame: (a) two DNA-replicating, BrdU- labeled nuclei (arrows), with mAb 93-stained myofibrils running around them; (b) DAPI staining showing the two BrdU-labeled cells (arrows) surrounded by nonreplicating, striated muscle cells. Scale bar: 50 polyp colony growth are not encountered after "primary" planulae settlement (Richmond, 1
. The Biological bulletin. Biology; Zoology; Marine biology. 310 S. PIRAINO ET Figure 17. Striated muscle cells from BrdU-incubated, whole medusa of Turritopsis nutricula undergoing transformation into polyps after rearing at T = 27°C. (a-b) same frame: (a) two DNA-replicating, BrdU- labeled nuclei (arrows), with mAb 93-stained myofibrils running around them; (b) DAPI staining showing the two BrdU-labeled cells (arrows) surrounded by nonreplicating, striated muscle cells. Scale bar: 50 polyp colony growth are not encountered after "primary" planulae settlement (Richmond, 1985). However, that ontogenetic reversal occurs at the beginning of devel- opment, before colony growth and sexual reproduction, and does not involve a return to clonal or larval stages from aclonal ones. Similarly, scyphozoan ephyrae are reportedly able to transform into polyps under starvation conditions (Hadzi, 1912). As already mentioned, this "metamorphic" potential is present in the first stages of medusan development of other hydroidomedusan spe- cies, but is lost before the medusa detaches from the polyp (Miiller, 1913; Hauenschild, 1956; Frey, 1968; Kakinuma, 1969; Schmid, 1972). In contrast, in T. nu- tricula each stage of medusa development can shift to hydroid structures as a response to adverse conditions, including senescence. This is an exceptional situation, like that of a hypothetical insect imago able to reverse to a larval stage after sexual reproduction. Stem cells (interstitial cells) and transdifferentiation The change of medusae into polyps requires differen- tiation of new cell types and major reorganization of tis- sues. New cell types could arise either through a differ- entiation of uncommitted stem cells of the medusa or by cellular trandifferentiation of differentiated medusa cells. The presence of a population of continuously prolif- erating cells (stem cells, interstitial or I-cells) was long thought to play a key role in asexual reprod
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