During vertebrate development, cellular rearrangements and inductive interactions of gastrulation lead to the formation of an embryo with three germ layers and a defined body plan. A key morphogenetic movement of gastrulation is convergent extension, during which the embryo narrows along its medio-lateral axis, while extending its anterior- posterior dimension. Convergent extension involves multiple cellular morphogenetic behaviors including mediolateral intercalation and directional migration of cells, cellular divisions and cell shape changes. It is not understood what are the relative contributions of these different cellular behaviors to the convergent extension process, and how they are coordinated. Furthermore, virtually nothing is known about the biochemical nature of the signals that control convergent extension, how those signals are translated into morphogenetic cellular behaviors, and how convergent extension influences other morphogenetic and inductive events that shape the vertebrate embryo. The research proposed here aims to study the mechanisms of convergent extension in zebrafish (Danio rerio), by the unique and powerful combination of genetic analysis with embryological and molecular methods. The optical clarity of zebrafish embryos allows direct examination of convergent extension movements at single cell resolution in vivo. The embryo's large size facilitates embryological manipulations like transplantations and microinjections. Most importantly, genetic screens have identified mutations in seven loci knypek (kny), trilobite (tri), ogon (ogo), volcano (vol), one-eyed pinhead (oep), no tail (ntl) and spadetail (spt) the functions of which are required for normal convergent extension. These convergent extension mutants provide a framework for genetic analysis of this morphogenetic process that is not available for any other vertebrate. Embryological analysis of the convergent extension mutants will identify cellular behaviors required for normal convergent extension, and the time and place of action of these genes during development. Studies of expression of cell type-specific genes in the mutants will determine the relationships between defects in cellular movements and formation of specific cell types. Phenotypes of double mutants will uncover functional redundancies between the convergent extension genes and establish a hierarchy in which they work in normal development. The biochemical nature of the molecules encoded by these genes will be determined by cloning them. Elucidation of the cellular basis of convergent extension defects in zebrafish mutants and revealing the molecular nature of genes identified by these mutations will be an important step toward understanding the cellular and molecular mechanisms underlying this crucial stage of vertebrate and human development. In a larger perspective, analysis of these mutants will help to explain the genetic basis for disorders in very early stages of human development, miscarriages and birth defects.
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