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.
Beilby, Virginia Shepherd
and Chris Cherry-Gaedt