This three-day-old zebrafish embryo is only 3.5 mm long. If you colour its cell nuclei and its cell membranes, it turns into a mini work of art. | Image: Robert Bill

“I like this image because it is both informative for my research and fun to look at”. Robert Bill is a doctoral student at the University of Zurich and says this with a touch of guilt in his voice. He sometimes takes a break from work to contemplate his photos. And he has many more like it. Because they help Bill to understand better the mechanisms by which zebrafish skin cells are organised.

Here we see a three-day-old, 3.5-millimetre-long zebrafish embryo observed under a confocal laser scanning microscope. Or rather its 2D reconstruction, using the maximum intensity projection method, after the microscope has assembled images of several horizontal sections of the embryo. The raw image was not in colour, as the microscope only detects a signal or the absence thereof.

It’s Bill who chose to add the colour to see more clearly what he was interested in, i.e., the nuclei of the skin cells in red (top image) and their membranes in blue (middle image). Bill used fluorescent antibody staining to highlight the nuclei of the cells and genetic engineering to make the embryo express fluorescently labelled proteins, which allows the membranes to be visualised.

By merging the two embryo images obtained, Bill has created the lower one. He is pleased with this image, and not just for its aesthetic appeal: “This image demonstrates that our technique is capable of visualising the architecture of the entire skin of the zebrafish at the nuclear and cellular level. It also shows how the nuclei and cell membranes of the skin have already become organised with great precision after only three days of development, which is not the case for all the cells and tissues of this organism. The next step is to uncover the molecular mechanisms of this organisation.