Fig. 2. Cardiac defect in SM22-Cre;R26R;Bmpr1a^flox/flox mouse embryos. (A) Blue staining (Cre activity) in the atrial (am) and ventricular (vm) myocytes but not in endocardial cells (ec) by whole-mount lacZ staining in E10.5 SM22-Cre;R26R;Bmpr1a^flox/flox embryos. (B) An enlargement of A. (C,D,F,G,H,I) Consecutive transverse sections of wild-type (WT) (C,F,H) and SM22-Cre;R26R;Bmpr1a^flox/flox (D,G,I) hearts taken at the same level from viable E11 embryos. H&E staining shows thinning of the ventricular wall (arrowheads in high-magnification insets) in the flox/flox mutant heart (D) versus WT (C). Apoptosis was infrequent in ventricular sections of E11 SM22-Cre;R26R;Bmpr1a^flox/flox mutant (G) and WT (F) by TUNEL immunostaining. However, fewer PCNA-positive cells (brown) were observed in the mutant ventricles (I, high-magnification inset, arrows) compared with the WT (H, inset, arrows). (E) A numerical assessment of hematoxylin-stained nuclei per ventricular section of E10.5-11 WT and SM22-Cre;R26R;Bmpr1a^flox/flox mutants. Bars indicate mean±s.e.m. (n=3). ^*P<0.05. (J) A numerical assessment of percentage of PCNA-positive cells over the total number of ventricular cells in heart sections of WT and flox/flox (f/f) mutant embryos at E9.5 and E10.5-11. Bars indicate mean±s.e.m. (n=3-4). ^*P<0.05. (K,L) p57KIP2 immunostaining in E10.5 WT (K) and flox/flox mutant (L) heart sections. RA, right atrium; RV and LV, right and left ventricle, respectively; EC, endocardial cushions; IVS, interventricular septum. Panels depicting WT and their corresponding mutants have the same magnification. Scale bars: 100 µm.