ABSTRACT
Magnetic resonance imaging (MRI) has two major strengths for in vivo imaging of developing embryos: the modality is noninvasive and nondestructive. MRI does use ionizing radiation, and the bioeffects are well understood and can be reduced to negligible levels for most MRI studies. MRI has been applied to developmental biology questions in several key model systems, including chicken, quail, rat, mouse, and frog embryos. Avian and amphibian embryos are popular subjects for MR microscopy studies, since they develop externally and can be incubated within the MR imager with careful preparation and experimental design. While three-dimensional imaging provides important information about the spatial arrangement of structures within the embryo, it often requires extensive data collection times. The cleft of Brachet is a well-defined landmark in the embryo, marking the boundary between the interior endo-mesodermal cells and exterior ectodermal cells and serving as a useful morphological landmark in defining the germ layer identity of the cells within labeled clones.
