Arsenic is found in drinking water and rice, resulting in exposure to millions of people throughout the world. Exposure to arsenic during embryogenesis is associated with low birth weight, altered locomotor activity, and reduced weight gain, likely because of reductions in the number of skeletal muscle progenitor cells. Indeed, our data indicate that arsenite exposure to stem cell-derived embryoid bodies reduces the expression of MyoD, Myf5, and myogenin, all transcription factors needed to differentiate stem cells into skeletal myocytes. We have also shown that arsenic impedes the ability of myocytes to differentiate into mature myotubes. Muscle progenitor cells, or satellite cells, arise during embryogenesis and are needed for new fiber formation later in life. We have been using the killifish as a model species to examine changes in muscle development following embryonic arsenic exposure. Our preliminary data indicates that arsenite exposure only during embryogenesis reduces weight gain in offspring even 4 months later. This reduced weight gain appears to be due to reductions in the total muscle fiber numbers, as embryonically-exposed fish have 15-20% less fibers than control fish at 4 months of age, even when accounting for difference in size. Thus, the goal of this application is to assess the mechanism by which arsenic reduces muscle fiber number by determining the number, location, and function of satellite or muscle stem cells. We will examine these processes from the juvenile period and into adulthood, and during both normal development and during recovery after an injury. In the first aim, we will determine whether exposure to low arsenite concentrations during embryogenesis reduces the number and/or localization of the satellite cells as the organism develops into an adult. These results will enable us to determine if the satellite cells are targeted, and also determine whether muscle mass is inhibited. The goal of the second aim is to determine whether organisms exposed embryonically to arsenic can recover from a muscle injury later in life, which will indicate whether arsenic exposure during embryogenesis has latent effects that only become apparent in adulthood. Our long-term objective is to understand why arsenic-exposed populations are at increased risk for defects in muscle development, and how this leads to functional changes such as reduced weight gain. It will also help to determine whether a standard for arsenic levels in food should be set - one that will be protective of embryonic health.