Organs in the human body have complex networks of fluid-filled tubes and loops. They come in different shapes and their three-dimensional structures are differently connected to each other, depending on the organ. During the development of an embryo, organs develop their shape and tissue architecture out of a simple group of cells. Due to a lack of concepts and tools, it is challenging to understand how shape and the complex tissue network arise during organ development. Metrics for organ development have now been defined.