Fig. 3. Cardiac defects in truncated trkC transgenic animals. A, Section of a newborn trkC TK− transgenic heart demonstrating an atrial septal defect (asd), markedly dilated atria with decreased trabeculations, and large septum primum. Magnification, 50×. B, Higher magnification ofA with asd, septum primum (sp), and septum secundum (ss) indicated by arrows. Magnification, 100×. C–F, Sections through the pulmonic (C, D) and aortic (E, F) valves of a normal (C, E) and a transgenic (D, F) mouse heart. Magnification, 100×. Note the thickening of the pulmonic (pv) and aortic (av) valve leaflets (arrows) of the mutant animal compared with the control littermate. rv, Right ventricle; lv, left ventricle; ra, right atrium; la, left atrium; pa, pulmonic artery; ao, aorta; tv, tricuspid valve; mv, mitral valve.