Drosophila germ-band extension as a model for understanding the integration of cell intrinsic and extrinsic forces during animal morphogenesis

点击翻译按钮获取中文摘要

When fertilized eggs start their development they are faced with an enormous challenge. The egg must divide many times to produce huge numbers of cells. In turn, these cells must be directed to become differentiated from one another and simultaneously rearrange in stereotypical ways to shape the tissues and organs of the body. This programme of movements is called morphogenesis. One of the most important early transformations that takes place in all animal embryos is the elongation of the embryonic head-tail axis from a short, wide tissue to an elongated narrow body. If this process of convergence and extension should fail, it is highly detrimental or more usually lethal to embryos. In humans, neural tube defects such as spina bifida and anencephaly are examples of the consequences of abnormal convergence and extension.Two types of information are vital in trying to understand how morphogenesis happens. Firstly, identifying which genes are turned on to orchestrate it and, secondly, working out how the relevant genes control the generation of forces that drive embryonic reshaping. The last 30 years or so of developmental biology have seen a focus primarily on genetic explanations. This has begun to change with the recent revolution in our ability to image live embryonic development and even more recent progress in the automation of the tracking of cells and cell shapes within developing embryos. Our laboratories have been at the forefront d of these recent developments, and we are in an excellent position to be able to investigate the nature and changing balance of forces during development, and how these are orchestrated by the genes. The aim of this project is to use fruit-fly embryos as a model with which to develop methods to understand the forces that drive embryonic development. The fruit-fly embryo has a relatively simple single-layered 'epithelium' of cells on the outside of the embryo that converges and extends, and its genetics is very well understood. Forces in embryos are generated by contractions of cells in ways analogous to how muscles contract. During convergence and extension movements, cell contractions drive changes in cell shape and cause cell to rearrange. Importantly, forces generated by active cell behaviour will exert effects on neighbouring cells, that can transfer force onto their neighbours in turn, or respond by dissipating the force through changing shape or arrangement. Thus cells can experience a variety of extrinsic forces from cells near and far. Disentangling active cell forces from forces imposed extrinsically (from elsewhere) is one of the goals of this project.Convergence and extension does not happen in a homogenous tissue. Gene expression varies across the tissue, and is correlated with patterns of the strength of active cell rearrangement behaviour. We will investigate the patterns of behaviour variation in detail, and correlate these with the patterns of gene expression and of the contraction of the cell. We will apply new methods to distinguish intrinsic cell rearrangement from passive rearrangement induced by extrinsic forces.We will extend our current automated methods for tracking cells to track the full three-dimensional shapes of cells during convergence and extension. With such data we will be able to ask whether there are differences in the shapes and orientations of cells, whether they are tilted or wedge-shaped in ways that indicate the presence of local or distant forces. We will test hypotheses generated above about the nature of tissue forces using focused laser ablation, making punctures or cut lines to test if the tissue pulls apart in ways predicted by our hypotheses. The combination of developing new generic methods to disentangle embryonic forces in this simple model will be a major step in being able to tackle more complicated vertebrate models, such as the zebrafish and mouse and other models relevant to human birth defects and disease states.