Fig. 8. PS is transiently exposed by differentiating cardiomyocytes of E12 mouse embryos. Adjacent transverse sections were stained for -smooth muscle actin to detect cardiac muscle cells (A) and annexin V to label surface-exposed PS (B). Boxed areas in A1 and B1 are depicted at a higher magnification in A2 and B2 (upper box), and A3 and B3 (lower box). A strong annexin V-biotin labeling was observed in the smooth-walled atrial myocardium (A2,B2, arrow), the subepicardial ventricular myocardium (A2,A3,B2,B3, in-between arrowheads) and the epicardium (A2,A3,B2,B3, open arrowhead). Also, at this stage of development, some pyknotic cardiomyocytes were observed in the trabecular myocardium (A3,B3, open arrows). Bars, 25 µm (A2,B2); 50 µm (A3,B3); 500 µm (A1,B1). Abbreviations: n, neural tube; l, limb; p, pericardial cavity.