The effect of NvStbm knockdown on endoderm specification and primary invagination in Nematostella embryos. (A) Control-MO injected gastrula at 12 to 14 hpf. (B) NvStbm-MO injected embryo at 12-14 hpf. Embryos are stained with phalloidin (green) and propidium iodide (red). (C, D, E, F) Embryos co-injected with Nvß-catenin::RFP RNA and with either Cont-MO or NvStbm-MO. (C) In embryos injected with the Cont-MO, initially Nvß-catenin::RFP is expressed in all blastomeres as previously shown. (D) By the blastula stage Nvß-catenin::RFP is stabilized in animal-half derived blastomeres and enters the nuclei in these cells. Similar expression dynamics are seen in NvStbm-MO and Nvß-catenin::RFP RNA injected embryos at the early cleavage stage (E) and blastula stage (F), indicating that knockdown of NvStbm does not affect Wnt/ß-catenin signaling. (G-L) Analysis of gene expression in Control- and NvStbm-MO injected embryos. Similar to Cont-MO injected embryos that express NvSnail (G) and NvFz10 (I) in the endoderm, NvStbm-MO injected embryos also express NvSnail (H) and NvFz10 (J) but show no signs of archenteron invagination. NvFz5 expression in Cont-MO (K) and NvStbm-MO (L) injected embryos. (M) Quantification of the gastrulation phenotype in embryos using confocal microscopy shows that in contrast to Uninjected- (94.6%, n = 37) and Control-MO injected (96.8%, n = 31) embryos which gastrulated normally, very few NvStbm-MO injected embryos displayed an archenteron (9%, n = 66).