Growth of the zebrafish heart

Outline of this thesis In order to find new genetic factors, which are involved in cardiac growth we performed a forward genetic screen and we report the outcome of this screen in chapter 2. We screened at 5 days post fertilization (dpf) for mutants with either an enlarged heart or reduced heart size. In this chapter we describe one mutant with a smaller heart at 5 dpf. Further analysis revealed that cardiomyocytes in these mutants are smaller and the heart contains fewer cells. In chapter 3 we describe cardiac growth in the zebrafish embryo between 24 and 48 hpf. We show that the heart tube initially extends at the inflow region of the heart through a process of continuous differentiation. The completion of this process requires Isl1 and Isl1 mutant embryos have an abnormal heartbeat with frequent stops. Furthermore, the OFT of the heart is added through discontinuous differentiation. The OFT is formed between 34 and 48 hpf and requires Fgf signaling. In chapter 4 we describe the process of continuous differentiation in more detail and describe the role of Bmp signaling in this process. We show that we can rescue the cardiac phenotype of laf embryos by re-expressing alk2/8 at 14 somites stage. Furthermore we show that cardiac specification is independent from dorso-ventral patterning during gastrulation and Bmp signaling is continuously required during cardiac determination. Surprisingly, before cardiomyocytes are fully differentiated, the Bmp signal is turned off in these cells. In chapter 5 we describe the cardiac expression pattern for islet1. Islet1 is expressed at the inflow region of the zebrafish heart and is expressed in the adult heart at the site of the sino-atrial node (SAN). In chapter 6 we describe that the SAN is formed in the proper place from the distal most cardiomyocytes at the venous pole of the heart, regardless of the size of the venous pole. Furthermore, in hearts were the venous pole of the heart is either truncated or contains a surplus of cardiomyocytes the function of the cardiac conduction system is normal and the embryos have a proper heart beat. In chapter 7 we describe the results of a forward genetic screen for cardiomyopathy. Few genes are described that cause cardiomyopathy and known players in cardiomyopathies in mouse often present the phenotype only after birth. In order to identify new genetic factors that are required for cardiomyopathy we participated in a forward genetic screen in adult zebrafish. We describe an adult mutant which presents with cardiac dilation of the atrium at 12 weeks post fertilization. This thesis is concluded in chapter 8 with a general discussion on cardiac specification and differentiation