Fig. 7. MEK5DD transgenic hearts show progressive dilation and thinning of ventricular walls with age. (A) Hearts were removed from wild-type and MEK5DD transgenic mice at 3, 6 and 12 weeks of age. Hearts were fixed in 10% PBS-buffered formalin and photographed. (B) Hearts from 12-week-old MEK5DD-transgenic and wild-type mice were fixed and sectioned longitudinally or at the midsagittal level parallel to the base and stained with hematoxylin–eosin. ra, right atrium; la, left atrium; rv, right ventricle; lv, left ventricle.