Turritopsis dohrnii is a species of jellyfish with significant ability and is part of the class of Hydrozoa. Large bodies of research on the unique survival strategy and cellular mechanisms of this jellyfish concluded in the 1990s that it is able to revert to its immature form when needed, ultimately hitting a 'reset button'. Turritopsis is the only known genus that has this ability to perform 'reverse metamorphosis'. Therefore, it is hypothesized for the Turritopsis jellyfish can't die of old age, hence its reputation for being 'immortal'. This has led to significant research in the areas of biomedical science, specifically about the possible implementation of human stem cell research.
Turritopsis dohrnii is approximately only 3 mm in height and diameter in full adult form (medusa). Adult medusae have approximately 80-90 tentacles. The basic anatomy of T. dohrnii is the same as any other Cnidarian body.
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Turritopsis dohrnii was first discovered in the 1880s in Mediterranean waters, off the coast of Italy. It was then found to be in temperate warm waters near Panama and Japan, but it is thought that the populations found in these waters were invasive and non-native to the area. It is since been discovered that T. dohrnii attaches themselves to large vessels such as cargo ships when undergoing regeneration, thus distributing them outside of their natural habitat
Most organisms, both plants and animals, have the same three stages in their life cycles. This process begins as a fertilized egg or seed, then develops into an immature juvenile, which finally matures into a fully developed adult. Once the organism is at full development, reproduction will then occur. This is a 'one-way' cycle. However, the life cycle of Turritopsis dohrnii has a two-way cycle.
The life cycle of T. dohrnii is perhaps the most complex part of its existence, as it involves both asexual and sexual reproduction. A T. dohrnii egg is fertilized by the mass amounts of sperm and eggs released into the water by adult medusa. Once an egg is fertilized, it will develop into a planula (a free-swimming larva), which then resides on the sea floor. From there, they mature into a polyp, with hollow tube structures. These polyps then asexually reproduce more polyps, creating a colony, and young medusae begin to form. Once developed enough, the medusa detaches from the polyps and sexually matures into adult medusae which can reproduce. Most organisms will die after reproduction, however, T. dohrnii has an additional cycle that allows for regeneration. The regeneration process is triggered when a Turritopsis is stressed, injured, or in imminent death. Research has shown that this process can occur in both adult and newly released medusae. This switch means they can cycle back and forth, therefore allowing them to avoid unfavorable conditions, and thus avoid death.
The apoptotic process involves the degeneration of existing, dominant cells due to programmed cell death. Apoptosis occurs in T. dohnrii at the start of its reverse ontogeny, ridding the stressed/injured medusa of its tentacles to converting the medusa to a cyst-like structure that is ready to attach to a surface and grow into a polyp again. Apoptosis is the main cause of the organisms’ dramatic reorganization, both cellular and physical.
Transdifferentiation is not unique to Turritopsis dohnrii, however, it is the only known animal that transdifferentiates all its cells. Cellular transdifferentiation occurs when a mature pluripotent cell changes into another mature cell. For cellular transdifferentiation to occur, a previously differentiated cell is required to be de-differentiated (returned to a simpler form) before being re-differentiated to another structure and cell fate. The new cell has a different function and gene expression. For example, the conversion of a muscle cell into a nerve cell. The cell change process in Turritopsis dohrnii does not involve stem cells, which is unlike most research that occurs in tissue production research. By 'skipping' the use of stem cells, allows the process to be completed much faster. Therefore, transdifferentiation in T. dohrnii only takes approximately 1-2 weeks. This was observed in a controlled, experimental setting in laboratories. One study found that the intermittent period between regeneration was less than one month. This demonstrates the significant ability of adaption T. dohrnii have to the environment around them when considering their survival strategy.
Although Turritopsis dohrnii can perform reverse metamorphosis to avoid unfavorable environments, this does not eliminate all threats to the possibility of death caused by predators. The most common predators of T. dohrnii are other species of jellyfish. Other common predators of T. dohrnii include sea turtles, swordfish, and tuna. T. dohnrii can also be affected by diseases caused by bacteria and pathogens that inhabit seawater. One study found jellyfish are often affected by bacteria such as Alphaproteobacteria, Flavobacteriia, Gammaproteobacteria, and Synechococcophycidae. Further information proves that the jellyfish may also die when they are a polyp if they are starved or sick in this form, and therefore will not regenerate.
There have been numerous studies focusing on the sequencing of the T. dohrnii genome to gain further understanding and identify species that may have similar survival mechanisms. Research indicates that as a medusa reverts into the polyp, the resulting medusa produced are exact replicas of the original, containing the same DNA and genetic makeup. This finding has led scientists to question whether this is considered immortality, regarding that all of the cells in the organism are replaced. However, they are replaced with the same copies. Although it has been hypothesized that T. dohrnii can regenerate limitless amounts of time, this has led researchers to question if the same risks of constant cell regeneration that apply to humans also apply to these jellyfish. When cells are consistently regenerated, there is an increased risk of an underlying gene mutation occurring, which may lead to serious conditions such as cancer. For example, the constant regeneration of ovarian cells may increase the possibility of a mutated gene (BRCA1 or BRCA2) sequencing the new cells, thus increasing one's risk of developing ovarian cancer.
The research question investigated concludes that Turritopsis dohnrii are only biologically immortal, and some circumstances have not been considered when labeling this creature as 'immortal'. These factors include natural predation from other species that rely on jellyfish for food sources, such as sea turtles, swordfish, and other jellyfish. Another factor includes diseases caused by bacteria. As previously stated, there is the question of whether the new medusa produced after a polyp, although genetically identical, does not represent the same medusa as before regeneration. This theory is however largely proven wrong due to the transdifferentiation of the cells and replica DNA. Researchers have also hypothesized that there may be an eventual limit to the number of times that regeneration can occur, due to constant cellular regeneration and how this increases the risk of mutation, which may eventually kill T. dohnrii.
The case of Turritopsis dohrnii is remarkable to modern research because scientists are constantly studying ways to generate new tissue. Discovering how these cells can transform from one type of cell to another may lead to massive insight into how this phenomenon could be applied to human biology. This is specifically relevant to cancer, aging, and biomedical research. Thus, the cellular mechanisms and survival strategies of Turritopsis dohnrii are still a paramount focus for these specific scientific research fields.
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