Mef2c factors are required for early but not late addition of cardiomyocytes to the ventricle

During heart formation, the grows and undergoes dramatic morphogenesis to achieve efficient embryonic function. Both in fish amniotes, much of growth occurring after initial tube formation arises from second field (SHF)-derived progenitor cell addition arterial pole, allowing chamber formation. In zebrafish, this process has been extensively studied during life, but it is unclear how larval cardiac occurs beyond 3 days post-fertilisation (dpf). By quantifying zebrafish myocardial using live imaging GFP-labelled myocardium we show that between 5 dpf. Using methods assess division, cellular development timing assay Kaede photoconversion, demonstrate proliferation, CM addition, hypertrophy contribute ventricle growth. Mechanistically, reduction Mef2c activity (mef2ca+/−;mef2cb−/−), downstream or parallel with Nkx2.5 upstream Ltbp3, prevents some differentiation, resulting a significantly smaller by After dpf, however, mef2ca+/−;mef2cb−/− mutants recovers normal pace, size gap their siblings diminishes into adulthood. Thus, as mice, there an early time window when SHF contribution particularly sensitive loss activity.