Distinct secretory cells have been found in ancient placozoans. These cells could have been the prelude building blocks of neurons in other creatures.
Placozoans: Tiny Grain-Sized Creatures
These tiny creatures roughly have the size of a sand grain. They are extremely simple creatures who do not have any organs as they mainly consist of slightly more than a colony of various cells.
Placozoans were thought to first appear on Earth roughly 800 million years ago. Similar to how they maneuver today, placozoans from back then used to browse algae and hunt microbes in warm and shallow sea regions. In order to achieve such a feat, they depend on their cells for bodily coordination.
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Uncanny Similarities Between Neurons and Placozoan Peptidergic Cells
Cell biologist Sebastián Najle from the Centre of Genomic Regulation and colleagues found that these peptidergic cells, as they are called, share similar characteristics with neurons.
Naike expresses how astounding they found the uncanny similarities, adding that they resemble primitive neuronal cells even when they were not exactly there. This could be an "evolutionary stepping stone" of some sort.
Microscoping screenings and genetic analysis revealed that the tiny creature's behavior was coordinated by the cells. These cells did so by expelling peptide signals, which is similar to how neurons function.
The study identified 14 various peptide cell types in these tiny aquatic creatures. The researchers then compared the tissues with four known species of placozoa and other early animals.
Doing so enabled the team to discover that such cells carry some of the same genes as human neurons. However, they did not have the specialized components present in human neurons that are responsible for generating electric signals or receiving peptide ones. Rather than this, the peptidergic cells utilized GPCR, or receptor proteins, for accepting the chemical messages. This mechanism is common to various cell types across animals; it is not unique to neurons.
The peptide-releasing cell genes were conserved significantly among placozoans. However, for other early animals, including comb jellies and sponges, this was absent.
Hence, this suggests that the evolution of the peptide-secreting cells could have come before neuron-like ones.
However, there are still questions that are yet to be addressed. This includes questions pertaining to neuron divergence and evolution and whether each piece could have had a unique origin. Nevertheless, Placozoans, among other creatures, hold secrets that the scientific community is just starting to unravel.
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