When is a Cell Not a Cell ? When it’s a Ventricaria !
Optical sections of a Ventricaria cell made using a confocal microscope. The cell structure shows a highly unusual and complex topology. Cells are spherical and contain an inner layer of cytoplasm, structured like a sponge, surrounding a central space called the vacuole. This contains a mucilage made up of sulphated polysaccarides. In the photo the light areas are the multiple nuclei (an) in the cytoplasm (c) with the vacuole (v) interpenetrating like the holes in a sponge. The optical sections are only 1.8 microns apart in each row. The optical slicing reveals the complex and irregular interface between cytoplasm and vacuole. This explains the unusual measurements of electrophysiology, that are unlike any other cell.

The giant spherical cells (up to 3 cm diameter) of siphonous algae like Ventricaria grow in tropical reef environments. They have been popular with researchers since the turn of the century, because it is relatively easy to impale them with microelectrodes. However they are unlike any other plant cells, because they appear to have a positive potential difference across the main cell membrane. This presents a puzzle to researchers today, because the “normal” plant cell has a highly negative potential difference across the membrane (eg. –300 mV). Mary Beilby, Virginia Shepherd, Chris Cherry-Gaedt and collaborator Professor Mary Bisson from New York State University at Buffalo, have been pursuing the answers to this puzzle for some years now. The answer appears to lie in a truly weird cell structure. In fact, the new interpretation reached by these researchers is that Ventricaria is not actually a cell, but a spherical colony of interconnected individuals, each with a nucleus, that are bound together like a sponge in the mature cell. When wounded or stressed, the cytoplasm of a Ventricaria sphere contracts and breaks up into individual units, that slide along protein strings, clustering together to eventually form hundreds of tiny replicas of the original. At first these clusters have no cell walls but within a few hours a new cell wall is formed. The clusters then pump a polysaccharide mucilage both into their internal space (the vacuole) and to the outside. This mucilage draws them together, as clusters of clusters, or superclusters. The strange electrical properties are a consequence of the strange spongy construction, where the inner space (vacuole) interpenetrates the cytoplasm like holes in a sponge.

Mary Beilby, Virginia Shepherd
and Chris Cherry-Gaedt

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