Zebrafish Cardiac Development: The Effects of Retinoic Acid on Heart Tube Morphology and Function
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The formation of a heart occurs through a series of dynamic developmental events where individual cells differentiate, migrate, fuse, and loop to form a muscular tube which then has the ability to pump blood throughout the body. The zebrafish has been used as a model organism for vertebrate cardiac development due to the transparency of its embryonic tissues. The zebrafish heart is comprised of two chambers, an atrium and a ventricle. During heart cell determination, it is believed that retinoic acid acts as a chemical signal which helps to restrict the fate of pre-ventricle and pre-atrium cells (Hochgreb et al., 2003; reviewed in Simoes-Costa et al., 2005). Research has found that both excess and inhibition of retinoic acid signaling during development causes abnormal heart formation. This thesis investigates the effects of excess retinoic acid on heart morphology and function by recording real-time videos and time-lapse images of the heart tube. Embryos were treated with varying concentrations of retinoic acid. The percentage of embryos which developed abnormal hearts increased with an increase in retinoic acid concentration. In addition, the stage and length of exposure had a significant effect on the percentage of abnormal hearts which formed. Three general heart morphologies were observed as a result of excess retinoic acid exposure: Wide Atrium, Linear Heart, and Small Heart. These abnormal heart tube morphologies exhibited variation in normal heart looping. Treated embryos exhibited decreased circulation, retrograde blood flow, and/or no circulation. This thesis proposes a model to explain how excess retinoic acid signaling affects the determination of pre-cardiac cells which then leads to the formation of abnormal heart tube morphologies. It is proposed that insufficient mechanical stimuli due to poor circulation induces abnormal heart looping (Miyasaka et al., 2011).