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| Heidelberg,
Wednesday 13 July 2005 |
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Actin moves chromosomes
Discovery changes fundamental thinking |
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| The
starfish is an excellent model for Dr. Péter
Lénárt and Dr. Jan Ellenberg's
studies on oocytes. |
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![A three dimensional reconstruction of a dividing oocyte showing that the microtubules [red] are too short to reach all the chromosomes [green].](../../../../../images/press/press05/press12jul05pic2s.jpg) |
| A three dimensional reconstruction of a dividing oocyte showing that the microtubules [red] are too short to reach all the chromosomes [green]. |
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Press
Release 13 July 2005 [PDF]
[Deutsch]
Scientists first to show that microtubules are not solely responsible for
chromosome movement in egg cells
Microtubules need a helping hand to find chromosomes
in dividing egg cells, scientists have discovered.
Although it was generally accepted that microtubules
act alone as the cellular ropes to pull chromosomes
into place, a new study by researchers at the European
Molecular Biology Laboratory [EMBL] shows that this
is not the case. They found that in large cells
such as animal eggs, something else is needed to
move the chromosomes into the correct location –
fibres of the cytoskeletal molecule actin [Nature,
13 July 2005]
"No one has ever shown that actin moves chromosomes,"
says Dr. Jan Ellenberg, the EMBL researcher whose
group carried out the research. "We were able
to do so because our group is one of the few that
studies cell division in starfish – an ideal
model for observing division in living animal eggs."
The starfish is an excellent model for studying
oocytes, the cells that give rise to egg cells.
In this marine animal, these cells are transparent
and mature quickly outside the body, and can be
kept alive in a drop of seawater. That's why EMBL
scientists performed some of their experiments with
collaborators at the Marine Biological Laboratory
in Woods Hole, MA, USA – working with animals
fresh from the ocean.
Ellenberg and PhD student Péter Lénárt
studied the starfish oocytes as they underwent meiosis,
a special cell division that is needed to halve
the number of chromosomes in an egg before it unites
with a sperm. When the protective nuclear membrane
surrounding the chromosomes breaks down during meiosis,
it was thought that microtubules capture the chromosomes
and act as ropes to pull them to the surface and
expel half of them from the cell.
But when the EMBL researchers
measured the microtubules, they discovered that
they were, in fact, much too short to transport
the chromosomes over the long distance to the surface
of the large oocyte. By using a chemical to disable
the microtubules, they found that cells were still
able to pull chromosomes into the proper positions.
So what was moving the chromosomes?
When they repeated
the experiment with a chemical that breaks down
the other major type of cellular fibres, actin,
the cells lost track of their chromosomes and the
new cells had unequal amounts of genetic material.
This condition, called aneuploidy, is thought to
be a major cause of miscarriages and some types
of birth defects.
Lénárt spent 18 months optimizing
an imaging technology, with help from collaborators
at the German Cancer Research Center [DKFZ], to
visualize the delicate actin fibres before he could
confirm the group's fundamental breakthrough. He
observed a network of filamentous actin forming
in the region where the nuclear membrane breaks
down. This network acts as a fishnet to gather all
the chromosomes together and drag them close to
the short microtubules. Only then, when the chromosomes
are close enough, can the microtubules latch on
and pull half of them outside the cell.
The implications for this pioneering work are clear.
Starfish oocytes have many similarities to those
of other animals, including humans. Because this
mechanism is essential to prevent chromosome loss
before fertilization, advances in this field could
help to explain the causes of pregnancy loss and
birth defects in humans.
Source Article A contractile
nuclear actin network drives chromosome congression in starfish oocytes.
P. Lénárt, C. Bacher, N. Daigle, A.
Hand, R. Eils, M. Terasaki, J. Ellenberg.
Nature. Advanced on-line publication July
13 2005.
DOI: 10.1038/nature03810.
Press Contact
Trista Dawson
EMBL Press Officer, European Molecular Biology Laboratory,
Meyerhofstrasse 1, 69117 Heidelberg, Germany
Tel: +49 [0] 6221 3878452
E-mail: trista.dawson@embl.de |
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