(I) The morphogenesis of the early mammalian embryo.

Eomesodermin BAC transgenic mouse embryos expressing GFP in the extraembryonic ectoderm and early primitive streak.

Early steps in primitive streak formation and function.
A primary objective is to provide a detailed and dynamic picture of events operating to, (i) establish and maintain the primitive streak in the mouse embryo, and (ii) generate and pattern cell types emanating from the primitive streak.

Tbx6 gene targeted mouse embryo expressing GFP in the primitive streak and paraxial mesoderm.

Morphogenesis of the paraxial mesoderm.
Mesoderm formation occurs in all vertebrates, taking place in a head-to-tail sequence. An epithelial to mesenchymal transition (EMT) occurs at the primitive streak with cells ingressing through the streak and emigrating as nascent mesoderm that can be subdivided into several distinct populations. The time and position at which mesoderm emerges from the primitive streak defines its fate, with the paraxial mesoderm believed to be the last population to exit the streak. The metameric organization of the vertebrate body plan is established by somitogenesis, a process by which the paraxial mesoderm becomes segmented into somites, which later will give rise to the vertebrae, skeletal muscles, and part of the dermis.

Wnt and FGF signaling are key elements in paraxial mesoderm morphogenesis. The Wnt signaling molecule Wnt3a, the T-box transcriptional regulator Tbx6 and the basic helix loop helix transcriptional regulator Mesogenin are the only molecules shown to be absolutely required for paraxial morphogenesis. These are the only three single gene mutants that exhibit a complete loss of paraxial mesoderm, thus they provide excellent molecular entry points into the study of the paraxial mesoderm and are the focus of our work.

Live imaging and gene expression within the visceral endoderm.

We have developed strains of mice that allow us to visualize the visceral endoderm in living embryos. We are exploiting these strains in a live imaging approach to follow the dynamics of visceral endoderm tissue movement and cell fate.

Future work will focus on clarifying the cell behaviors intrinsic to the visceral endoderm through (i) time-lapse imaging, (ii) marker expression, and (iii) fate mapping. These experiments will address both the precise cell dynamics within the visceral endoderm, and define the location of visceral endoderm-derived cells in the fetus, its extraembryonic membranes and placenta. To investigate the function of visceral endoderm-derived cells, we will identify mutants which perturb the cell behaviors we have uncovered.