Quantifying cell behaviour in morphogenesis

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During animal development, the cells of the embryo undergo enormous movements and reorganisations to shape themselves into the tissues and organs of the body. The complexity of the process is such that there are many stages at which errors might occur, the results of which can be seen in the form of birth defects. In many cases, such as neural tube defects, the consequences of there errors can be severe. Research into understanding how these errors arise depends upon us being able to view and measure the movements of cells during the time at which they are rearranging to ask how their behaviours diverge from normal development. Advances in microscopy now allow us to see the outlines of cells in three dimensions and to follow the development of living embryos as these movements take place. In our work we use a small fish, the zebrafish, for which there are many mutant strains with defects analogous to human developmental disorders, particularly for the development of the central nervous system. By studying this model animal it is hoped that understanding can be found that can be translated into insight into the human condition.The work that we propose here is to develop new, more advanced computational methods that will allow us to follow and measure the movements and reorganisations of cells visualised in great detail. Cells and tissues have complex and varied three dimensional shapes and individual 3D images contain many hundreds of cells. Only a very small proportion of these can be analysed manually, but many of the developmental errors that we detect are subtle variations from the normal path, so many precise estimates are needed. The development of a complex organ such as the brain involves many different movements. One example case is the formation of the two eyes. Early during development there exists just one flat sheet of cells that will split and reshape to form two eyeballs. The folding and reshaping that this entails is only now being understood by using these new imaging methods to follow the movements of many hundreds of cells in time-lapse movies of living embryos. Comparing movements seen in normal embryos with mutant embryos, in which the eyes fails to develop correctly, has allowed us to identify the distinct mechanisms that lead to the formation of the eye. In this way we can now begin to ask detailed questions about how errors arise in these mechanisms to cause birth defects. To progress further with these studies we now need to carefully and comprehensively measure the movements of cells of the eye while manipulating the activity of the genes involved in generating these defects. The methods that we propose here will permit us to study this and similar problems in ways that have never before been possible.The methods that we propose to develop here address three related problems. The first is to enable us to measure in three dimensions the shapes and movements of all of the cells within the 3D movies we collect of developing tissues. The second is to develop the mathematical methods needed to measure how reorganisations within this structure changes over time by characterising shape changes and rearrangements of the cells of whole tissues, such as the brain. The third avenue of research is to develop computer models of developing tissues that mimic the behaviour of the actual embryo. The movements of many hundreds of cells is very complicated to understand, but by building numerical simulations we are able to ask what features are important in achieving the movements seen in real animals. All characteristics of the model can be compared to experimental observations and the models then used to test hypotheses about how forces are applied within the embryo to actually cause the brain to form, and why they sometimes go wrong.