We offer you one "origin of innovation" a day in a compact Telegram message. Seven days a week, delivered around 8 p. Straight from our newsroom. Subscribe here, it's free! To study the role of these synaptic genes in sponges, the Arendt lab established microfluidic and genomic technologies in the freshwater sponge Spongilla lacustris.
Sponges use their digestive chambers to filter out food from the water and interact with environmental microbes. To understand what the cells expressing synaptic genes do, the Arendt group joined forces with six EMBL teams as well as collaborators in Europe and worldwide. The scientists captured three-dimensional snapshots of cells crawling throughout the digestive chamber to clear out bacterial invaders and sending out long arms that enwrap the feeding apparatus of specific digestive cells.
This behaviour creates an interface for targeted cell—cell communication, as it also happens across synapses between neuronal cells in our brains. Cells frequently communicate with one another, and neurons do so by passing electrical or chemical signals through tiny, targeted connections called synapses.
They found that the sponge has 18 distinct cell types. The researchers then used X-ray imaging and electron microscopy to study one of these cell types, which they called secretory neuroid cells. Electron microscopy reveals that neuroid cells purple and red can stretch out their arms to communicate with digestive cells blue, green and yellow.
On the basis of the proximity of the two cell types and the expression of genes that might allow for the secretion of chemicals, the researchers think that these arms enable neuroids to communicate with choanocytes, so that they can pause the water-flow system and clear out any debris or foreign microbes.
However, these neuroid cells are not nerves, and there is no sign of the synapses that enable neurons to communicate so quickly.
Instead, this cell type might represent an evolutionary precursor to a true nervous system, says Jacob Musser, an evolutionary biologist at EMBL, who co-authored the study. Some scientists say that calling these cells a precursor to a nervous system is a stretch. She says it will be difficult to prove whether nervous systems evolved from this cellular communication system or arose earlier or even multiple times, as some groups have proposed.
Indeed, many other organisms, including unicellular eukaryotes 3 , contain the same synaptic genes, says Sally Leys, a marine biologist at the University of Alberta in Edmonton, Canada. Whether other sponges use a similar cellular communication system, she adds, remains a key unanswered question.
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