Creating 3D digital replicas of ascidian embryos from stacks of confocal images

During embryonic development, cell behaviors that are tightly coordinated both spatially and temporally integrate at the tissue level and drive embryonic morphogenesis. Over the past 20 years, advances in imaging techniques, in particular, the development of confocal imaging, have opened a new world in biology, not only giving us access to a wealth of information, but also creating new challenges. It is sometimes difficult to make the best use of the recordings of the complex, inherently three-dimensional (3D) processes we now can observe. In particular, these data are often not directly suitable for even simple but conceptually fundamental quantifications. This article describes a process whereby image stacks gathered from live or fixed ascidian embryos are digitalized and segmented to produce 3D embryo replicas. These replicas can then be interfaced via a 3D Virtual Embryo module to a model organism database (Aniseed) that allows one to relate the geometrical properties of cells and cell contacts to additional parameters such as cell lineage, cell fates, or the underlying genetic program. Such an integrated system can serve several general purposes. First, it makes it possible to quantify and better understand the dynamics of cell behaviors during embryonic development, including, for instance, the automatic detection of asymmetric cell divisions or the evolution of cell contacts. Second, the 3D Virtual Embryo software proposes a panel of mathematical shape descriptors to precisely quantify cellular geometries and generate a 3D identity card for each embryonic cell. Such reconstructions open the door to a detailed 3D simulation of morphogenesis.