Investigating the roles of the extracellular matrix in zebrafish heart morphogenesis

Tissue morphogenesis requires co-ordination of changes in cell shape, gene expression and the integration of both intrinsic and extrinsic patterning cues. One such example of a complex tissue morphogenesis is the asymmetric looping of the linear heart tube during embryogenesis. Heart looping is required for the correct alignment of the chambers to facilitate septation, chamber ballooning and subsequent connection to the rest of the cardiovascular system. The necessity for robust heart morphogenesis is evident in the prevalence of congenital heart diseases which occur in around 1% of live births, and are the leading cause of birth defect-related deaths worldwide. Most organ systems, including the heart are composed of distinct tissue types with specialised functions: during cardiac development, the heart tube comprises an outer layer of contractile myocardium surrounding a specialised endothelial cell layer known as the endocardium. During heart looping both the myocardial and endocardial tissue layers undergo distinct morphogenetic changes, and each play important roles in providing temporal and spatial signals to the other, together driving robust early cardiac morphogenesis. Separating the myocardium and endocardium is a specialised extracellular matrix (ECM) termed the cardiac jelly. Numerous studies have highlighted the importance of the cardiac jelly during later stages of heart development, however little work since its initial characterisation has investigated the mechanisms by which this ECM promotes asymmetric heart morphogenesis. In this thesis I highlight the pivotal and previously underappreciated role of the embryonic cardiac jelly in promoting distinct aspects of cardiac morphogenesis in zebrafish. Common to all ECMs are the large, heterotrimeric Laminin complexes, secreted early during ECM deposition. I first identify and characterise the expression of multiple different Laminin subunits in the heart at the onset of heart looping and go on to show a broad requirement for Laminins in promoting cardiac morphogenesis. Subsequently, I identify the specific Laminin isoforms which are required during heart development and uncover two distinct roles for Laminins: promoting asymmetric morphogenesis and restricting cardiac size. Secondly, I uncover an asymmetry in the embryonic cardiac jelly, prior to the initiation of heart looping morphogenesis (which correlates with the expression of an ECM-modifying gene). I show that the cardiac ECM is rich in the Proteoglycan Hyaluronic Acid and that chamber-specific regionalisation of the ECM is required to promote heart morphogenesis. Together, this work demonstrates that the constituents of the embryonic cardiac jelly and their regionalised expression are critical in promoting the robust, asymmetric morphogenesis of the linear heart during embryonic development