Imaging: X-ray Tomography on a Living Frog Embryo


X-rays can image soft tissues throughout early embryonic development of vertebrates. Related to this, a German-American-Russian research team led by Karlsruhe Institute of Technology (KIT) presented a new X-ray method. For periods of about two hours, time-lapse sequences of cellular resolution were obtained of three dimensional reconstructions showing developing embryos of the African clawed frog.

Instead of the absorption of X-rays, the method is based on their diffraction. “X-ray diffraction enables high-resolution imaging of soft tissues,” explains Ralf Hofmann, one author of the study. “In our work, we did not only manage to resolve individual cells and parts of their structure, but we could also analyze single cell migration as well as the movement of cellular networks.”

Using X-ray diffraction, similar tissues can be distinguished by minute variations of their refractive index. However, in contrast to classical absorption imaging, this does not require any contrast agent, and X-ray dose is profoundly reduced. The method is of particular advantage when probing sensitive tissues in living organisms, such as frog embryos. In their study, the researchers concentrated on the motion and shape changes of tissues, cavities, and single cells during the developmental milestone of gastrulation.

During gastrulation, germ layers are formed and organized in their proper locations. Thereby, an initially simple spherical ball of a few hundred cells turns into a complex, multilayered organism with differentiated tissues eventually turning into the nervous system, muscles, and internal organs. Quoting the renowned developmental biologist Lewis Wolpert: “it is not birth, marriage, or death, but gastrulation that is the most important event in your life.”

“Employing X-rays, we were able to watch joint and individual cell movements during gastrulation,” zoologist Jubin Kashef points out. For the first time, it was appreciated how cells interact with each other in a living embryo and how regions void of cells form and disappear. “It is like the migration of peoples. Stimulated by the migration of individual cell groups, other cells join in. They form functional cellular networks, which adjust to their changing environment. During migration, cells specialize to form progenitor tissues of future organs, e.g. the brain or skin.”

“It is fascinating to have digital capabilities to observe and analyze these processes in an individual living frog embryo,” Hofmann and Kashef emphasize. “In this way, fundamental results are obtained.” The new method not only reveals morphological and dynamic aspects of embryonic development but also provides insights into their underlying molecular biology obtained by comparing the development of wildtype embryos and morphant phenotypes.; Source: Karlsruhe Institute of Technology (KIT)