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Nadine
Age: 42
Joined: 23 Jan 2006
Posts: 26
Location: Germany
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Planarians
Planaria sp., or flatworm, is a common member of the Platyhelminthes strain, class Turbellaria, order Tricladida.
Life is inseparably connected with a constant conversion of materials and energy. While this equally applies to protozoans and multicellular organisms, besides that, multicellular organisms have to be able to replace worn out or lost parts. Usually this is a comletely inconspicuous process like e.g. the renewal of the hair and skin cells. There are some examples of amazingly high regeneration abilites in the animal world.
Certain groups of the turbellaria family show maximum regeneration performance. This applies particularly to planarians.
Planarian species live in sea and fresh water, and also in water-filled soil gaps and damp land biotopes. Their body is flat and shaped like a band, whereby the ventral side forms a flat, even creeping sole, while the back side is somewhat curved. A head can be more set off or less clearly, there are often tentacles or auriculae at its sides. In the head there are no brain and sensory organs, and even the mouth opening is only rarely located there.
In 1774 the Prussian natural scientist P.S. Palls already described the ability of planarians to regenerate entirely from tiny body fragments. Around 1900 they became model organisms for modern regeneration research.
If a planarian is cut through with a razor-blade, both the parts are seemingly undisturbed. The anterior fragment forms a new tail, the posterior a new head. The European kind Dugesia gonocephala needs about two weeks at a water temperature of 9-10 °C to do so. The tropical kind D. tahitiensis needs only 59 hours if the conditions are optimal (water temperature 23°C). This kind also divides in two parts in nature, thus reproducing.
Special cells, called neoblasts, are responsible for this amazing regenerating ability. They are totipotent stem cells which are undifferentiated, divide easily and are capable to renew themselves in order to form new differentiated cells.
The neoblasts of D. tahitiensis are particularly numerous. Under all planarians this kind possesses the largest percentage of neoblasts, over 40% of the total cell number.
German version by Diana Wille |
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