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.