The genetic blueprint for the three-dimensional body

In order for vertebrate embryos to develop their body axes, they require what is known as an embryonic signalling centre. This group of cells provides the instructions that determine where »up« and »down«, »left« and »right«, and »front« and »back« are. Biologists at Friedrich Schiller University Jena have now discovered that even comb jellies—which , according to current understanding, form the sister group to all other multicellular animals—possess this fundamental coordinate system. The Jena-based scientists report on their research in the latest issue of the journal »Nature«.

Together with his student Hilde Mangold, the biologist Hans Spemann discovered the so-called »organiser« during embryonic development in 1924. In doing so, they transplanted tissue from a specific subject area—the blastopore—of an amphibian embryo into another and observed that the resulting animal developed a second axis. The two researchers concluded from this that the transplanted group of cells organizes the three-dimensional structure of a multicellular animal during an early phase of embryogenesis. Hans Spemann was awarded the Nobel Prize in 1935 for the discovery of this elementary coordinate system—Hilde Mangold, a former student at the University of Jena, had died in a fire in 1924 at the age of 25.

A team from the University of Jena has now repeated this experiment on comb jellies and discovered that one of the earliest diverging lineages of animals living today develops according to this blueprint. This is because the Jena scientists transplanted parts of the blastopore from the embryo of one comb jelly into another, thereby also triggering the formation of a second body axis. As they had stained the transplanted cells, they were also able to show that these induced other cells in the new embryo—in other words, they influenced them.

»It’s like dissecting clouds«

»Through our experiments, we were able to show that this key mechanism, which coordinates the axes of the entire body, dates back in evolutionary terms to the dawn of animal multicellularity,« explains Prof. Dr Andreas Hejnol, an evolutionary biologist and head of the Jena research team. »This is because, according to current understanding, the lineage of the Ctenophora—the scientific name for comb jellies—diverged from ours around 700 million years ago.«

The Jena researchers needed the utmost delicacy for their experiments. The species of comb jelly they used grows to a size of up to twelve centimetres. Their embryos, however, measure only slightly more than the diameter of a human hair—namely 120 micrometres. It was only with great manual dexterity that the Jena-based biologist Dr Stanislav Kremnyov succeeded in transplanting the tissue sample, which measured around 20 micrometres, particularly as he had to integrate the cells directly into the recipient tissue to prevent the target embryo from rejecting them. »The editor of ›Nature‹ suspected that these experiments must have felt like dissecting clouds«, says Andreas Hejnol.

Transplantation between animal lineages separated for 60 million years

Another experiment by the Jena team demonstrates just how fundamental the organizer’s key mechanism is to the evolution of multicellular organisms: The experts succeeded not only in transplanting the organizer from the comb jelly into another comb jelly embryo, but also into the embryo of a sea anemone. Here, too, it triggered the formation of an additional body axis—an astonishing success, given that the lineage of these cnidarians only diverged from that of the ctenophores in the animal phylogeny some 60 million years later.

»Such a xenotransplantation—that is, the transfer of tissue from one animal group to another—across so many millions of years has never been demonstrated before«, says Andreas Hejnol. In this way, the team at the University of Jena also identified, for the first time, the gene responsible for the formation of the organizer in the sea anemone.

The results show that a central mechanism of embryonic development—the so-called organizer—already exists in animal groups such as the comb jelly and even functions across species, for example in the sea anemone—an indication that the fundamental building principles of complex bodies arose as early as the origin of multicellular animals.