All animals move -- cheetahs faster, snails more slowly. Muscle contractions are the basis of movement in many, but not all, species. Some animal groups don't have any muscles at all, as they branched off from the evolutionary path before muscle cells evolved. Yet these animal groups -- for instance, the sea sponges -- are not incapable of movement. Sponges are able to contract without muscles. But which cells in sponges are actually contracting?
A group of scientists headed by associate professor Dr. Michael Nickel of Friedrich Schiller University Jena (Germany) is looking into movement without muscles. The scientists from the Institute of Systematic Zoology and Evolutionary Biology are especially interested in the question of which evolutionary forerunners did muscle cells derive from.
In a new study published in the Journal of Experimental Biology, the evolutionary biologists are offering new answers to this question. In their paper, the researchers described how they generated three-dimensional (3-D) images, with the help of synchrotron radiation-based X-ray microtomography. Using this technique, the Jena scientists, in co-operation with the Helmholtz-Zentrum Gesthacht at the Deutsches Elektronen Synchrotron Hamburg, were able to compare and visualize the 3-D structure of contracted and expanded sponges.
"A key feature of our approach is the use of 3-D data for measuring the volume and surface of our sponges," says Nickel. "Although the 3-D volumetric analysis is widely known and used in the technical sciences, it has rarely been used in zoology -- in spite of its enormous information potential."
Nickel's team was able to show that the inner and outer surfaces -- and therefore the epithelial cells, so-called pinacozytes -- cause the strong body contractions of the sponges. Ultimately, the Jena scientists believe they have also settled a hundred-year-old debate about the cause of cellular contractions. Until recently, spindle-shaped cells in the tissue of sponges as well as epithelial cells were thought to be possible candidates. But now, the Jena scientists have been able to identify the true initiator of the contractions.
These findings offer new approaches to understanding the evolutionary development of musculature. "The early evolution of muscles has not been fully understood so far. According to current scientific knowledge, muscle cells seem to have surfaced from nowhere," Nickel says. "But surely there must have been evolutionary predecessor systems, that have been unknown until now." The sponge epithelial cells are now moving to the forefront in the evolutionary biologists' continuing research in this field. "There is a lot of evidence that the sponge epithelial cells and the muscle cells of all the other animals are going back to a common contractile cellular predecessor." In future, scientists hope to test this hypothesis using genome and gene expression-related data.
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