Endocardial specification, but not cellular organization, is normal in the absence of FNs. Immunohistochemistry with antibody MEC 13.3 to mark PECAM-1–positive cells in wild-type E 8.0 (A), FN.null-/- E 8.0 (B), FN.null-/- more severe class at E 8.5 (C), wild-type E 8.5 (D), and FN.null-/- less severe class at E 8.5 (E and F) mouse embryos. Note that no lumen is visible in FN.null-/- endocardium. M, myocardium; En, endocardium; DA, dorsal aorta.