Fig. 4.

Tbx6;CreER^Tg5769 is active on the first day of somitogenesis (E7.5→E8.5). (a–c) Live Tbx6;CreER^Tg5769;R26R^YFP/+ embryos exhibit fluorescent reporter activity along the A/P axis of the embryo including the presomitic mesoderm (psm), somites (som) and primitive streak (ps). (c) White dotted lines mark sections (I, II and III) merged for GFP and DAPI staining (d–f). (g–i) GFP only. (d and g) Section I has few +YFP cells (white arrowheads). Reporter activity increases posteriorly along the A/P axis of the embryo from moderate (e and h) to high in the presumptive somite (s0) of the psm (f and i). Cre activity was mosaic in the lateral plate mesoderm (white asterisks), labeling a few cells (e,h and f,i). (d–f) Cre activity was absent in the neural tube (nt), notochord (nc), intermediate mesoderm (white dotted circles), head and cardiac mesoderm (b,c). Scale bar=50 μm.