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With advancing age there is a progressive decline in the function of the heart. In humans reductions in stroke volume and cardiac output occurs often resulting in cardiac disease and subsequent death. The pathology found in the heart due to advancing age is attributed to a reduction in cardiomyocytes which causes cardiac dysfunction and heart disease, leading to heart failure. Zebrafish are a valuable tool in studying ageing and heart disease. As zebrafish age they gradually senesce. This is similar to humans and other mammals. However the response of the zebrafish heart to ageing has not been explored. The zebrafish heart changes due to ageing, with increased fibrosis and ventricular wall thickness. I have established new assays to measure proliferation and apoptosis in zebrafish cardiomyocytes using multiplexing of thymidine analogues and cleaved caspase 3, respectively. Using these developed assays it was discovered that these changes may be caused by an observed increase in cardiomyocyte apoptosis. This was coupled with no change in cardiomyocyte proliferation. These changes may be mediated by an increase in natriuretic peptide expression. In response to exercise, cardiomyocyte proliferation increases signalled by increased gata4, nkx2.5, tbx5, and mef2c expression and a reduction of natriuretic peptide expression. In the long term these genetic and cellular changes in the heart in response to exercise may slow some of the pathological changes observed in the heart.

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Some anthropogenic noise is now considered pollution. Evidence is building that noise from human activities such as transportation, construction and exploration can impact behaviour and physiology in a broad range of taxa . However, relatively little research has considered effects on development or directly assessed fitness consequences, particularly with respect to repeated or chronic noise. All fishes and many invertebrates use underwater sound for processes such as orientation and communication, and are thus vulnerable to anthropogenic noise pollution. Fishes and invertebrates detect the particle motion component of sound; this component has. been neglected, but must be understood alongside acoustic pressure if the potential impacts of noise are to be fully understood. As the first part of my thesis (chapter two), I provide a computer program which allows users to determine under what circumstances they should measure particle motion; I explain how these measurements can be made and provide a program for analysing this type of data. The main part of my thesis comprises experiments investigating the impact of repeated exposure to traffic noise, the most common anthropogenic source of noise in the marine environment, on fishes and invertebrates during development. In all three chapters involving experiments on fish in tanks and in the field, I found that a variety of behaviours were impacted by traffic noise playback. I also found that predictable noise can lead to different impacts on development from unpredictable noise, but that some species of fish may be able to habituate to traffic noise, while others suffer lower survival. Further, I found that the development and survival of sea hares (Stylocheilus striatus) can be negatively impacted by traffic-noise playback. Fishes 'and invertebrates provide a vital food source to millions of people and form crucial links in many food webs; studying their behaviour, development and fitness can give us an insight into population and community level impacts of noise that are relevant to species survival and evolution. Developing some of the novel ideas and techniques discussed in this thesis will enable us to advance this vital area of research.

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Retinoic acid (RA) is thought to signal through retinoic acid receptors (RARs), i.e. RARα, β, and γ to play important roles in embryonic development and tissue regeneration. In this thesis, the zebrafish (Danio rario) was used as a vertebrate model organism to examine the role of RARγ. Treatment of zebrafish embryos with a RARγ specific agonist reduced the axial length of developing embryos, associated with reduced somite number and loss of hoxb13a expression. There were no clear alterations in hoxc11a or myoD expression. Treatment with the RARγ agonist disrupted the formation of anterior structures of the head, the cranial bones and the anterior lateral line ganglia, associated with a loss of sox9 immunopositive cells in the same regions. Pectoral fin outgrowth was blocked by treatment with the RARγ agonist; however, this was not associated with loss of tbx5a immunopositive lateral plate cells and was reversed by wash out of the RARγ agonist or co-treatment with a RARγ antagonist. Regeneration of the transected caudal fin was also blocked by RARγ agonist treatment and restored by agonist washout or antagonist co-treatment; this phenotype was associated with a localised reduction in canonical Wnt signalling. Conversely, elevated canonical Wnt signalling after RARγ treatment was seen in other tissues, including ectopically in the notochord. Furthermore, some phenotypes seen in the RARγ treated embryos were present in mutant zebrafish embryos in which canonical Wnt signalling was constitutively increased. These data suggest that RARγ plays an essential role in maintaining neural crest and mesodermal stem/progenitor cells during normal embryonic development and tissue regeneration when the receptor is in its non-ligated state. In addition, this work has provided evidence that the activation status of RARγ may regulate hoxb13a gene expression and canonical Wnt signalling. Further research is required to confirm such novel regulatory roles.

Cyclical activity was demonstrated in both the pituitary and thyroid glands during the 3 - 4 week gestation cycle of the viviparous cyprinodont fish Poecilia reticulata. These cyclical variations were not seen in unfertilized, non-cycling fish and thus it is thought that both glands are involved in the maintenance or control of gestation. In the pituitary gland, cyclical changes occurred in the histology of the gonadotropic, STH and TSH cells and in the amounts of neurosecretion in the neurohypophysis during the gestation cycle. The histological changes in the gonadotropic cells during gestation were interpreted in terms of secretory activity. This was achieved by experimentally raising or lowering the levels of circulating gonadal steroids, which exert a negative-feedback on the gonadotropic cells, and observing histologically the results of these experiments. Further evidence was provided by observing the development of the pituitary and thyroid glands and gonads from birth to sexual maturity. The results of these experiments were used to define the characters typical of inactive and active gonadotropic cells. The activity of the thyroid gland was measured using the technique of interference microscopy. A peak of thyroid activity was demonstrated during the first eight days of the gestation cycle. The lowest thyroid activity occurred during the next eight days, and during the last eight days of the cycle there was an increase in thyroid activity. The levels of thyroxine were found to affect the gonadotropic cells; high levels of thyroxine decreased and low levels increased gonadotropic activity. Thus, in addition to the more well known effect on TSH activity, it is likely that thyroxine also exerts a feedback effect on the gonadotropic cells. The role of the pituitary and thyroid glands in the control of gestation and gonadal maturation, and the complex nature of the interaction of hormonal feedback mechanisms on these glands and on the gonads, is discussed. The hypothalamic mechanisms controlling pituitary-gonadotropic activity are also discussed. It is thought that several different hormones may be involved in the control of reproduction in the guppy.

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No description available.