FIGURE 1. Method to detect cell fusion events.

a, b, Schematic representation of the transgenes expressed by the mouse lines used. c, When a cell expressing Cre recombinase (a) fuses with a cell bearing the LacZ reporter transgene (b), the floxed stop cassette is excised and the LacZ reporter is expressed in the fused cell. LacZ expression can be detected by the generation of a blue precipitate after X-gal staining. RTV, Integration retroviral sequence (LTR). d–g, Co-cultures of R26R BMSCs with Cre⁺ neurospheres stained for -gal and the nuclear dye DAPI (4,6-diamidino-2-phenylindole). d, e, Multinucleated -gal⁺ fused cell (4 DIV). f, g, Colony of -gal⁺ fused cells (15 DIV). These cells were mononucleated and mitotically active, as demonstrated by a cell in metaphase (inset in g). h–k, Co-culture of R26R BMSCs with BrdU-labelled Cre⁺ neurospheres. Bi-nucleated -gal⁺ fused cells (h) with one nucleus immunopositive for BrdU (j, k, arrowhead) and the second nucleus negative for BrdU (i, k, arrow). Scale bars: d, f, h, 20 m; g (inset), 5 m.