Discovery challenges our understanding of nervous systems and their evolution

Using advanced technologies, a team of scientists led by Pawel Burkhardt from the Michael Sars Centre, University of Bergen and Maike Kittelmann from Oxford Brookes University, have revealed the nervous system connectivity of ctenophores, one of the most ancient animal lineages. Reconstructing neurons from the nerve net through 3D electron microscopy, they discovered an extraordinary architecture: a continuous neural network. These findings challenge our understanding of nervous systems and their evolution.

Changing neurobiology theory

Ever since the work of scientists Santiago Ramón y Cajal and Fridtjof Nansen in the 19^th century, neurobiology research is interpreted through the lens of the neuron doctrine. This theory states that nervous systems are composed of discrete individual cells.  
Camillo Golgi challenged this theory by putting forward the idea that neurons within a nervous system are connected as a continuous network. Cajal and Golgi shared the Nobel Prize in 1906 for their extraordinary findings, though they were fierce competitors throughout their careers.

Cajal’s theory was finally proven correct by identifying neuronal junctions, so called synapses, through the invention of electron microscope in the 1950s, thereby disproving Golgi’s theory. These new findings now prove that Golgi was also right.

Why ctenophores?

The ctenophore Mnemiopsis leidyi, visualized using illumination from above, which causes iridescence of the comb plates.

Photo:

Alexandre Jan

Ctenophores, also known as comb jellies, are fascinating organisms that have been living in the world’s oceans for approximately 600 million years. When the first animals evolved, ctenophores were one of the first animal l­ineages on the planet. Within early evolution of neurons and nervous systems, multiple ways to make a nervous system were possibly established.

Previous attempts to describe ctenophore nervous systems connectivity had proven difficult because the organisms are delicate and very fragile, and investigating their anatomy was very challenging.

Applying new technology sparked curiosity.

Pawel Burkhardt’s collaboration with Maike Kittelmann, a 3D electron microscopy expert, led to the important observation that one single neuron in the ctenophore nerve net had made a small network by fusing its neuronal processes, also known as neurites, to each other.

Curious to explore this irregularity, Pawel and Maike collected a much larger 3D dataset.

“At the Centre for Bioimaging at Oxford Brookes University we have a Serial Block Face SEM (Scanning Electron Microscope) that allows automated collection of hundreds of images of one animal. One of the datasets that we now have available includes five nerve net neurons and their widely branching neurites”, says Maike. Reconstructing these cells revealed an extraordinary architecture: they form a continuous neural network.

“We found fundamental differences between the nerve net of ctenophores and that of cnidarians and other animals. This is extremely exciting. One could argue: is it even a nervous system?” Pawel Burkhardt

Despite its unique architecture, the ctenophore nerve net displays key characteristics found in nervous systems such as neuropeptides and ion channels generating membrane potentials.

What do the findings mean?

The characterization of the ctenophore nerve net has the potential to provide key information on the evolutionary origin of nervous systems. By revealing the unique anatomy of the ctenophore nerve net, the teams offer a novel way of thinking about nervous system architectures, thereby paving the way for a new period of comparative neuroscience research.