The Role of the Second Heart Field in Zebrafish Heart Development

The vertebrate heart forms from two distinct progenitor populations called the First Heart Field (FHF) and Second Heart Field (SHF). FHF progenitors differentiate first to form the initial beating heart tube. SHF progenitors differentiate later than FHF progenitors and contribute to distinct parts of the developing vertebrate heart. Heart structures derived from the SHF are perturbed in congenital heart defects, which affect approximately 0.5% of newborns. Here I provide data pertaining to the identification, visualization, and regulation of the SHF in the vertebrate teleost zebrafish (Danio rerio). I developed a photoconversion assay to observe and quantify SHF cell addition to the developing heart. I discovered that SHF cell addition occurred continuously between 20 hours post fertilization (hpf) and 48hpf to both the atrium and the ventricle. I identified a transgenic, nppa:EGFP, as a novel marker of SHF progenitors, allowing us to visualize the SHF progenitors before differentiation and addition to the developing heart. FGF signaling was required for SHF progenitor establishment and SHF cell addition to the heart tube. mef2cb was shown to be required for zebrafish SHF development. mef2cb expression marked SHF cells fated to be added to the ventricle, with inhibition of mef2cb resulting in a strong reduction in SHF cell addition. I also identified hey2 as a novel negative regulator of the zebrafish SHF. hey2 expression was observed in candidate SHF progenitors and hey2 inhibition resulted in an increase of SHF progenitors and SHF cell addition. My studies demonstrated that the SHF is an integral part of zebrafish heart development and that zebrafish SHF development shows many similarities this process in mammals. This places the zebrafish as a new and strong model system for the study of SHF development and its role in congenital heart disease.Ph.D