Abstract
During heart formation, the heart grows and undergoes dramatic morphogenesis to achieve efficient embryonic function. Both in fish and amniotes, much of the growth occurring after initial heart tube formation arises from second heart field (SHF)-derived progenitor cell addition to the arterial pole, allowing chamber formation. In zebrafish, this process has been extensively studied during embryonic life, but it is unclear how larval cardiac growth occurs beyond 3 days post-fertilisation (dpf). By quantifying zebrafish myocardial growth using live imaging of GFP-labelled myocardium we show that the heart grows extensively between 3 and 5 dpf. Using methods to assess cell division, cellular development timing assay and Kaede photoconversion, we demonstrate that proliferation, CM addition, and hypertrophy contribute to ventricle growth. Mechanistically, we show that reduction in Mef2c activity (mef2ca+/−;mef2cb−/−), downstream or in parallel with Nkx2.5 and upstream of Ltbp3, prevents some CM addition and differentiation, resulting in a significantly smaller ventricle by 3 dpf. After 3 dpf, however, CM addition in mef2ca+/−;mef2cb−/− mutants recovers to a normal pace, and the heart size gap between mutants and their siblings diminishes into adulthood. Thus, as in mice, there is an early time window when SHF contribution to the myocardium is particularly sensitive to loss of Mef2c activity.
Highlights
The heart is the first functional organ to form in amniotes (Buckingham et al, 2005)
We show that normal ventricular growth prior to 3 dpf requires a threshold level of Mef2c, and this threshold diminishes after 3 dpf
Adult mef2ca+/-;mef2cb-/- fish with significantly smaller heart at 3 dpf survive to adulthood but are smaller than their siblings
Summary
The heart is the first functional organ to form in amniotes (Buckingham et al, 2005). Work in chick and mouse using dyes, transgenic lines, retrospective lineage analysis and genetic tracing with Cre recombinase has demonstrated that addition of cells to the arterial and venous poles of the heart tube occurs during heart growth (Cai et al, 2003; Kelly et al, 2001; Meilhac et al, 2004; Meilhac et al, 2003; Mjaatvedt et al, 2001 ; Verzi et al, 2005; Waldo et al., 2001; Zaffran et al, 2004) The progenitors of these late-added cells lie anterior to the heart in the pharyngeal region, referred to as the anterior or second heart field (SHF) to distinguish it from the first heart field, that forms the early or primitive heart tube. It is not clear when the addition of cells to the heart from an external source stops, and how much each growth mode contributes at late developmental stages
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