A combination of genetic and embryological amenability has placed zebrafish at the forefront of attempts to understand how genes function to control vertebrate development. The optical transparency of the zebrafish embryo provides the ability to visualise every cell in the forming embryo by simple optical inspection as well as enabling the use of a host of cell labeling and transgenic approaches to dissect embryonic development. Furthermore the large scale mutagenesis of the zebrafish genome has also produced many different classes of mutations which disrupt gene function. We use the many advantages of zebrafish embryology to dissect molecular mechanisms which act to pattern the vertebrate embryo. In particular we are interested in how specific muscle cell types are determined within the developing embryo.

Skeletal muscle of the vertebrate body derives from segmented arrays of mesodermal structures termed somites which form from the paraxial mesoderm in a stereotypic rostral to caudal progression. Each somite is then partitioned into dorsal and ventral compartments that contain progenitors for individual structures of the developing embryo. The ventral portion of the somite undergoes a mesenchymal transition and gives rise to the cells of the sclerotome which will migrate and differentiate into components of the axial skeleton. The dorsal segment of the somite remains epithelial and forms the dermomyotome. The dorsal portion of the dermomyotome gives rise to the dermis. The remainder forms the myotome which will give rise to skeletal muscle of the trunk and tail.

Research in my laboratory focuses on attempts to understand how the cells of the vertebrate myotome are specified to become individual muscle cells later in development. To do this we utilise the small, genetically tractable and embryologically amenable teleost, zebrafish. Three basic cell types differentiate in the mature zebrafish myotome. Slow muscle cells, fast muscle cells and muscle pioneer cells. These different cell populations have different embryonic origins. Slow muscle cells arise and differentiate next to the notochord, and following this differentiation the majority of slow muscle undergoes a remarkable migration, traversing the entire extent of the forming myotome to produce a superficial subcutaneous layer of differentiated slow twitch muscle cells. Fast muscle cells differentiate behind this wave of migrating slow twitch muscle cells, arising from the remainder of the cells of the myotome. Muscle pioneer cells are the first differentiating cells of the zebrafish myotome and form as slow twitch muscle cells that express the Engrailed homeodomain proteins and that remain axial up to 48 hrs of development.

We are using a number of genetic and molecular approaches to dissect the events underlying the specification of these cells. Firstly we have identified families of secreted peptides that are expressed within tissues that are known to control specification of cells of the myotome. Many studies have implicated signals from the notochord in the induction of cells of the vertebrate somite. Specifically, we have implicated members of the Hedgehog family of secreted glycoproteins secreted from the notochord, in the specification of slow muscle cells. Specific members of this protein family are responsible for the specification of the most medial of myoblasts, the "adaxial" cells to produce either slow twitch muscle cells or muscle pioneer cells. This is an instructive cell fate choice as slow twitch muscle cells are expanded at the expense of fast twitch muscle cells when one member of the HH family, Sonic hedgehog (SHH), is over expressed in the developing zebrafish embryo. We continue to analyse the mechanisms by which HH proteins determine cell fate within the myotome and have instigated studies in vitro within primary myoblast cell cultures to dissect HH signal transduction.

The large scale mutagenesis of the zebrafish genome has produced a vast array of different mutations affecting all aspects of its development. In particular one class of mutations, termed the you-type mutants, disrupts the formation of cell types induced by HH proteins. We are currently investigating how these mutations disrupt cell type specification within the myotome, how this relates to the generation and transduction of the HH signal, and whether or not novel genes are mutated within this class of mutations.