Migration and cell mass formation of Ha-Ras transformed mouse mammary epithelial cells injected into the yolk sac of zebrafish embryos. EpRas (parental) and EpRas cells stimulated with TGF-β (EpRasTGF) were labelled and ectopically injected into the yolk sac of 2 dpf zebrafish embryos. In A, E and F transgenic zebrafish embryos expressing GFP under an endothelial promotor (Tg(fli1:eGFP) were used. An example of newly injected EpRas cells at 1 hour post injection is given in (A). In (B) an ectopic tumour cell mass formed in the yolk sac by EpRasTGF cells is shown. Examples of cell masses formed by EpRasTGF cells at distance from the place of injection are shown for the tail region (C) and blood islands with surrounding ventral fin (D). Pictures in B-D were taken at 3 dpi. While EpRas cells remained in the yolk and never invaded the embryo (E), EpRasTGF cells invaded, migrated and formed distant micrometastases, which are indicated with arrows (F). Red fluorescence of cells is still visible after 7 dpi (E, F). Images G to J show tumour cell masses (cm) and migrated cells in blood islands (blood isl.), the liver, heart, intestine and the caudal fin of 6 dpi larvae. Scales shown are for A: 200 μm; D-H: 600 μm, for B, C, I and J: 100 μm. 3D reconstructions of EpRAS and EpRasTGF cells in zebrafish larvae are shown in two supplemental movies (see Additional file 2 and Additional file 3).