Ultrahigh-Frequency Echocardiography of Autonomic Devoid Phox2B Homozygous Embryos Does Not Reveal a Significant Cardiac Phenotype before Embryo Death

In vivo micro-imaging of mice is useful in studying the genetic basis of cardiac development in mutant embryos. We examined Phox2b^–/– mutant mice, which lack autonomic innervation to the heart and die in utero, and investigated whether this lack of innervation causes cardiac dysfunction during embryogenesis. A VisualSonics Vevo 2100 ultrahigh-frequency linear array ultrasound machine with 30- and 40-MHz probes was used to analyze embryo size, gross characteristics, ventricular contractility and rhythm. Phox2b^–/– mutant embryos underwent cessation of heartbeat and death at a greater rate than wild-type controls. We did not observe a hydrops phenotype or congenital heart defects in Phox2b^–/– mutants. Analysis of heart rhythm revealed no significant correlation with genotype. Absent these signs of a progressive pathology, we suggest that Phox2b^–/– mutant embryos likely die of sudden death secondary to acute arrhythmia. These data provide insight into the role of cardiac autonomic innervation during development.