EM evidence of ER and mitochondrial disorganization in Cert^gt/gt embryos. Cells from the three primary layers of the developing heart, namely endocardium, myocardium, and epicardium, of E10.5 Cert^+/+ and Cert^gt/gt embryos were examined under an electron microscope. The panels on the left are from the Cert^+/+ (+/+) embryos, and the Cert^gt/gt (−/−) mutants are on the right. The black arrow shows a lipid-laden engorged rough ER (rer) in the endocardial layer of the mutant heart. The white arrows show lipid-laden structures found in the vicinity of swollen ER and mitochondria (m) found often in the mutant cells. The arrowheads show mitochondria undergoing degeneration with discontinuous outer and inner mitochondrial membranes in the myocardial layer (middle). In the epicardial cells (bottom), the arrowheads depict swollen and pale mitochondria with ruptured inner and outer mitochondrial membranes in the mutant heart. The mitochondria in the mutant are generally swollen, pale, and have fewer cristae. n, nucleus; mf, muscle fibers of the myocardial cells. Bars, 500 nm.