Differentiation inside a developing embryo can be observed by a variety of optical methods but hardly so in opaque organisms. Embryos of the frog Xenopus laevis - a popular model system - belong to the latter category and, for this reason, are predominantly being investigated by means of physical sectioning. Magnetic resonance imaging (MRI) is a noninvasive method independent of the optical opaqueness of the object. Starting out from clinical diagnostics, the technique has now developed into a branch of microscopy - MR microscopy - that provides spatial resolutions of tens of microns for small biological objects. Nondestructive three-dimensional images of various embryos have been obtained using this technique. They were, however, usually acquired by long scans of fixed embryos. Previously reported in vivo studies did not cover the very early embryonic stages, mainly for sensitivity reasons. Here, by applying high field MR microscopy to the X. laevis system, we achieved the temporal and spatial resolution required for observing subcellular dynamics during early cell divisions in vivo. We present image series of dividing cells and nuclei and of the whole embryonic development from the zygote onto the hatching of the tadpole. Additionally, biomechanical analyses from successive MR images are introduced. These results demonstrate that MR microscopy can provide unique contributions to investigations of differentiating cells and tissues in vivo.
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Acknowledgments This work was supported by the Basic Science Research Program and the National Research Laboratory Program (Grants to C. C.), by the Center for Biological Modulators of the 21st Century Frontier R&D Program (Grant to K. S. H.) and by NRDP (Grant to W. L.) from the Ministry of Science and Technology, Republic of Korea. K. S. H. and C. L. were supported partially by Korea Basic Science Institute projects. We thank P. Callaghan (New Zealand), P. Glover (U.K.), D. Weisblat (U.S.A.) and F. Volke (Germany) for critically reading earlier drafts of this manuscript.
- MR microscopy
- Subcellular resolution
- Xenopus laevis