Developmental plasticity in the cardiovascular system of fish, with special reference to the zebrafish

During development the circulatory system of vertebrates typically starts operating earlier than any other organ. In these early stages, however, blood flow is not yet linked to metabolic requirements of tissues, as is well established for adults. While the autonomic nervous system becomes functional only quite late during development, in the early stages control of blood flow appears to be possible by blood-borne and/or local hormones. This study presents methods based on video-imaging techniques and fluorescence microscopy to visualize cardiac activity, as well as the vascular bed of developing lower vertebrates, and tests the idea that environmental factors, such as hypoxia, may modify cardiac activity, or even the early formation of blood vessels in embryos and larvae. In zebrafish larvae, adaptations of cardiovascular activity to chronic hypoxia become visible shortly after hatching, and the formation of some blood vessels is enhanced under chronic hypoxia. Exposure of early larval stages of zebrafish to a constant water current induces physiological adaptations, resulting in enhanced swimming efficiency and increased tolerance towards hypoxia. Furthermore, application of hormones such as NO can modify cardiac activity as well as peripheral resistance, and they can stimulate blood vessel formation. In consequence, even during early development of fish or amphibian larvae, the performance of cardiac muscle and of skeletal muscle can be modified by environmental influences and peripheral resistance can be adjusted. Even blood vessel formation can be stimulated by hypoxia, for example, or by the presence of specific hormones. Thus, at approximately the time of hatching the physiological performance of vertebrate larvae is already determined by the combined action of environmental influences and of genetic information.