title: Irx4 identifies a chamber-specific cell population that contributes to ventricular myocardium development.
abstract: BACKGROUND
  The ventricular myocardium is the most prominent layer of the heart, and the most important for mediating cardiac physiology. Although the ventricular myocardium is critical for heart function, the cellular hierarchy responsible for ventricle-specific myocardium development remains unresolved.
  
  
  RESULTS
  To determine the pattern and time course of ventricular myocardium development, we investigated IRX4 protein expression, which has not been previously reported. We identified IRX4+ cells in the cardiac crescent, and these cells were positive for markers of the first or second heart fields. From the onset of chamber formation, IRX4+ cells were restricted to the ventricular myocardium. This expression pattern persisted into adulthood. Of interest, we observed that IRX4 exhibits developmentally regulated dynamic intracellular localization. Throughout prenatal cardiogenesis, and up to postnatal day 4, IRX4 was detected in the cytoplasm of ventricular myocytes. However, between postnatal days 5–6, IRX4 translocated to the nucleus of ventricular myocytes.
  
  
  CONCLUSIONS
  Given the ventricle-specific expression of Irx4 in later stages of heart development, we hypothesize that IRX4+ cells in the cardiac crescent represent the earliest cell population in the cellular hierarchy underlying ventricular myocardium development.
authors.forename: Daryl O
authors.initials: DO
authors.lastname: Nelson
authors.forename: Dexter X
authors.initials: DX
authors.lastname: Jin
authors.forename: Karen M
authors.initials: KM
authors.lastname: Downs
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Kamp
authors.forename: Gary E
authors.initials: GE
authors.lastname: Lyons
mesh_terms: D000818:Animals; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D006352:Heart Ventricles / Q000166:cytology / Q000196:embryology; D018398:Homeodomain Proteins / Q000096:biosynthesis*; D051379:Mice; D008807:Mice, Inbred BALB C; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism; D009928:Organ Specificity; D038081:Organogenesis / Q000502:physiology*
chemical_list: D018398:Homeodomain Proteins; C485534:Irx4 protein, mouse