Surprising marine animals reveal clues about the evolution of the nervous system

2023-05-17 18:46:13

The origin of the nervous system, that set of cells that allow us to relate to the environment, remains a mystery. So clear.

Sponges, the simplest animals, have nothing like it. However, very primitive animals such as jellyfish and polyps already have a complete nervous system, with sensory organs, neurons, synapses, and neurotransmitters.

To this day we still do not know what made the development of our nervous system possible, something that marks an essential difference between animals and other complex organisms, such as plants and fungi. The nervous system is what allows us to be active in the tasks of detecting food, capturing it and avoiding becoming food for other animals.

What did seem to be clear is that it arose in the ancestral of all animals. Eumetazoos, characterized precisely by having a nervous system as well as muscles and a digestive cavity. It was taken for granted that it had appeared only once in evolution.

But this idea has recently been called into question. A group of animals, the ctenophores, have such a peculiar nervous system that they could even, at least in part, not conform to the classical model established by Santiago Ramón y Cajal and universally accepted. It is an unusual find.

The amazing ctenophores and their strange nervous system

In several recent scientific papers, the last one published a few days ago, it is shown that the ctenophores, very abundant animals in the seas around the world, have a very different nervous system from the rest of the animals. And this has led to the proposal that its origin occurred independently and parallel to that of the rest of the Eumetazoos.

Ctenophores are fascinating animals. Your name (cteno= pain, phoros=carry) is due to the bands of hair cells that run through the body. These bands, due to a phenomenon of refraction, produce iridescences that run through the body. Ctenophores were considered evolutionarily close to cnidarians (jellyfish and polyps). However, when in 2014 the genome of a ctenophore was published for the first timethe surprises were sensational.

The ctenophore genome displayed many peculiarities. In particular, the genes related to the development and functioning of the nervous system did not appear or were not expressed in the neurons, nor were the usual neurotransmitters found. The authors of this article already advanced the idea that the nervous system of the ctenophores would have had an independent origin from that of other animals.

Mnemiopsis leidyi, the ctenophore whose nervous system has been described in detail.
Steven G. Johnson – CC BY-SA 3.0

That article generated a great deal of controversy, since it seemed difficult to understand that something as complex as the nervous system could have appeared in two independent animal lineages. Many rejected this possibility, but more papers have recently been published that seem to point in the same direction, or at least insist on the fundamental differences between the nervous systems of ctenophores and other animals.

A group of biologists from the universities of Oxford, Bergen and Jena]published in 2021 a more detailed study of the nervous system of a ctenophore, confirming that its neurons use peptides as neurotransmitters and showing that the neurites, neuronal projections, form anastomoses among themselves, constituting a complex network. What was not known was the way in which the neurons connected with each other, or with sensory cells.

Neurons that form a continuous network

This question has just been revealed by another investigation directed by Maike Kittelmann, from the University of Oxford.

The study of the ctenophore larva Mnemiopsis leidyiusing 3D reconstruction of a huge number of electron microscopy images, has shown that the neurons of the subepithelial nervous system (SNS) that travel the surface of the body form a syncytium, a continuous network, and lack synapses.

Remember that synapses are the fundamental structures used by neurons to transmit the nerve impulse. The team not only showed structural evidence of neural continuity. When they injected a fluorescent tracer into one of the first two cells of the embryo, only half of the body fluorescent, but the subepithelial nerve network on the other side of the body also fluoresced, proving that the fluorescent molecule had been transported across the cell. continuity between neurons.

In addition to the subepithelial nervous system, the ctenophores have other, deeper neurons, the processes of which closely contact the SNS, but again without synapses. Synapses were indeed observed in the sensory cell projections to the SNS, and in the SNS projections to the hair cells of the combs. Except for these cases, the nervous system of this ctenophore larva is characterized by functioning for the most part without synapses, something totally unthinkable until now.

Many questions remain to be answered. How do subepithelial neurons act? The possibility is pointed out that they function as a kind of neuroendocrine system, releasing peptides into the environment that act locally.

And how does the subepithelial nervous system develop? Are connections maintained as neurons divide, or are connections established between independent neurons? And above all, is this usual? So far only a group of neurons from a larva has been studied. Is the network maintained in the adult? And in other species of ctenophores?

The differences between the nervous systems of the ctenophores and the rest of the animals (Eumetazoans) suggest that their origin occurred twice in evolution.

Let us remember Camillo Golgi and Santiago Ramón y Cajal

There is a curious fact that has not gone unnoticed by the authors of the study. At the end of the 19th century, two visions of the structure of our nervous system competed. The reticular theory, defended among others by Camillo Golgi, claimed that neurons formed a continuous network. Instead, the jobs of Ramon y Cajal they showed that neurons were independent cells that established synaptic connections.

Golgi and Cajal shared the prize Nobel and 1906, although Cajal’s proposal was universally accepted. It is striking that the subepithelial nervous system of ctenophores provides us with an unexpected example of a reticular system like the one proposed by Golgi… More than a century later.