Fig. 5. Defect in posterior axis extension in Sfrp1^-/-;Sfrp2^-/- embryos at E8.5. (A-C) Brachyury (T) expression in control (A,B) and Sfrp1^-/-;Sfrp2^-/- (C) embryos. PS, primitive streak; N, node. Defective posterior axis extension initially appeared as an increased thickness of the mesoderm layer (bar) in the posterior region of the Sfrp1^-/-;Sfrp2^-/- embryos when somite formation began. Scale bar: 250 µm. (D-I) Tbx6 expression in control (D,E,G,H) and Sfrp1^-/-;Sfrp2^-/- (F,I) embryos. D-F, lateral view; G-I, posterior view. An unusually high intensity of staining was detected on both sides of the primitive streak of Sfrp1^-/-;Sfrp2^-/- embryos. Furthermore, the PSM region (asterisk) was markedly reduced in these embryos. Scale bars: 250 µm in D-I. (J-O) Dll1 expression in control (J,K,L) and Sfrp1^-/-;Sfrp2^-/- (M,N,O) embryos at the 11 somite stage. K,L,N and O show cross sections of the tail bud region indicated in J and M. Cross sections were generated following in situ hybridization. Scale bars: 500 µm in J,M; 50 µm in K-L,N-O. (P-S) DiI cell-labeling assay of mesoderm cells in control (P,Q) and Sfrp1^-/-;Sfrp2^-/- (R,S) embryos at several somite stages. DiI was injected into mesoderm cells around the primitive streak (t=0; P,R), followed by a 2-hour culture of the embryos (t=2h; Q,S). TB, tail bud. Scale bar: 250 µm.