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Fig. 7. Summary schematic of smedinx-11 function in intact and
regenerating planarians. (A) In intact animals, smedinx-11
is required to maintain proliferative neoblasts. Representative phenotype
progression after smedinx-11(RNAi) shows that smedinx-11 is
required for the maintenance of neoblasts with the capability to proliferate
(red dots). Our data suggest that intact animals possess an anterior-posterior
endogenous gradient of mitotic neoblasts that is gradually reversed as the
neoblasts lose their proliferative capabilities after
smedinx-11(RNAi). For simplicity, smedinx-11 postmitotic
expression is not represented. (B) smedinx-11 is specifically
required for blastema formation. Animals amputated 1 week after
smedinx-11(RNAi) failed to regenerate even though proliferative
neoblasts were present, suggesting a loss of normal neoblast behavior even
prior to their disappearance. Although mitotic neoblasts disappear by about 2
weeks after smedinx-11(RNAi), the RNAi-treated animals survived
longer than 1 month in both intact and trunk fragments, but were unable to
mount a regeneration response, demonstrating the distinct requirement for
smedinx-11 in blastema formation. (C) Summary of the key
features of different cell populations (X1, X2 and Xins) obtained in flow
cytometry experiments. Asterisk denotes the fact that approximately 10-12% of
X2 cells can be positive for smedwi-1 and smedwi-2 gene
expression while none will be positive for smedcyclinB
(Reddien et al., 2005b).
(D) Schematic model of the dynamics of different cells (X1, X2 and
Xins) obtained in flow cytometry experiments. Proliferative neoblasts (X1) are
shown in red, X2 cells in green and blue, and differentiated cells (Xins) in
gray. The graded bar represents the level of smedinx-11 expression;
the percentages of smedinx-11-positive cells are indicated.
Neoblast-related cells are included in X1 and X2 subpopulations that express
smedinx-11. The proposal that X2 cells represent the progeny of the
proliferative neoblasts is based on: (i) irradiation-sensitive
smedinx-11 expression is observed in both proliferative and
postmitotic areas; (ii) the time-dependent manner in which neoblasts disappear
after smedinx-11(RNAi) (i.e. first X1 cells disappear followed by
X2); (iii) smedwi-1 gene expression is restricted to proliferative
cells, but its protein signal is broadly observed in both proliferative and
postmitotic irradiation-sensitive cells, consistent with the switching off of
the normal smedwi-1 expression by cells as they exit X1 but retaining
the protein in postmitotic stages prior to protein turnover; and (iv)
smedinx-11 expression overlaps with that of smedwi-1 at both
the gene and protein level. Therefore, it is possible that X2 cells might
represent a transitional stage between X1 and Xins cells, which is consistent
with results obtained in vitro for gap junction genes using mammalian neuronal
progenitor cells.
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