Raymond E. Keller
Primary AppointmentProfessor, Biology
Cellular and molecular mechanisms of morphogenesis
Our analysis of the convergent extension movements in the embryo of the frog, Xenopus laevis, serves as an example of this approach. The dorsal tissues of the vertebrate embryo narrow (converge) and elongate (extend) greatly during gastrulation and neurulation in movements collectively called "convergent extension." We used videomicroscopy of fluorescently labeled cells to show that cells bias their protrusive activity in the mediolateral direction, exert traction on adjacent cells in this direction, and pull themselves between one another along the mediolateral axis, to form a longer, narrower array. Mechanical measurements showed that the tissue becomes stiffer as it extends, enabling it to push strongly enough to stretch the remaining passive tissues of the embryo without buckling. Our current work seeks to learn the molecular and mechanical basis of this directed protrusive activity and stiffening, and also how these properties are induced. We inject RNAs coding for proteins that act as dominant inhibitors of molecules thought to be important in organizing the directed protrusive activity, along with RNA coding for green fluorescent protein (GFP). This enables us to visualize the resulting changes in behavior of the affected cells, with low light fluorescence videomicroscopy. Correlated changes in the forces produced and in tissue mechanics are measured, and changes in the terminal cell phenotype are monitored with molecular marker expression. With this multilevel, integrated approach, we can perturb a molecular function and directly analyze the effect on cell motility, tissue mechanics, patterning, and cell differentiation, enabling us to learn what components function in a particular morphogenetic event, and the mechanism of that function.