Ancient placozoan cells tell the origin story of neurons

A tiny blob-like marine animal has cells that share striking similarities with neurons. It is possible these cells gave rise to neurons in humans and other animals.
Rupendra Brahambhatt
Placozoan (Trichoplax) nuclei under microscope.
Placozoan (Trichoplax) nuclei under microscope.

Sebastian R. Najle/Centro de Regulación Genómica

The human brain is made up of 100 billion neurons and each neuron could be linked to 10,000 other neurons. Sounds crazy, right? But did you ever wonder where all these neurons come from? 

A new study from scientists at the Centre for Genomic Regulation (CRG) in Barcelona suggests that neurons may have originated from peptidergic cells, special secretory cells found in blob-like marine organisms called placozoans.

“Placozoans lack neurons, but we’ve now found striking molecular similarities with our neural cells,” said Xavier Grau-Bové, one of the study authors and a postdoctoral researcher at CRG

What are placozoans?

Placozoans are ancient animals that inhabit warm oceans. They’ve been around for 800 million years and are considered the “simplest animals of Earth.” Their millimeter-sized body (~ the size of a sand grain) lacks any organs and only comprises different types of primitive cells. 

“The quirkiness of placozoans reminds us that, when you look at the whole of the animal kingdom, we can find many organisms with radically different biologies from ours. And still, they can teach us many lessons about our own biology and evolutionary history,” Xavier Grau-Bové told Interesting Engineering.

Bové and his team mapped all placozoan cells and studied how DNA in these cells regulated different genetic characteristics. This information enabled them to understand the function and working mechanism of each cell.

“What we have now done is draw a first draft of the placozoan cell-cell communication network: we have a clearer understanding of which cells express which neuropeptides, and which cells express receptors in which each neuropeptide could fit,” Bové said.

“From a chemical point of view — it's sort of a system of keys and locks. This is an important first step towards understanding how these tiny, brainless organisms coordinate their complex behaviors,” he added.

Next, they compared the placozoan cells with different cells of complex organisms like humans to check for any similarities. 

Neuron-like features in placozoan cells

In addition to discovering neuropeptides and their receptors, the researchers found numerous similarities between the peptidergic cells in placozoans and human neurons.

For instance, peptidergic cells contain nearly all the genes necessary to construct the presynaptic scaffold, a protein complex that facilitates neurotransmitter release in signal-transmitting neurons found in complex animals. 

Basically, it is the scaffold that makes the synapse in our brains work. Surprisingly, peptidergic cells also have the recipe to develop their own synapse.

Another similarity is that, akin to neurons, placozoan secretory cells employ neuropeptide-like cell signaling molecules to detect external signals for communication and carrying out various physiological activities. 

Moreover, the differentiation of peptidergic cells into progenitor epithelial cells occurs via signals like those seen in neurogenesis, the process that gives rise to new neurons in complex animals ranging from jellyfish to humans.

“We have seen that the process by which these cells develop is strikingly reminiscent of the neurogenesis process in other species, with key cellular signaling mechanisms involved in both,” Bové told Interesting Engineering.

The researchers suggest that although peptidergic cells are far from being true neurons, these resemblances indicate that they may have possibly played a role in the evolution of neurons.