What Is Planaria Regeneration In Ideal Temperatures

Planaria Regeneration in Ideal Temperatures vs. Cold Temperatures

Christina Malfas

Abstract Planaria are flatworms that live in quiet freshwater bodies. They live in a narrow range of temperatures, from 22-24°C. They regenerate asexually, specifically through a process called epimorphosis, in which they completely regenerate one half of their bodies after splitting themselves in two. The diffusion gradient model assumes that cells in an organism “know” their orientation on that organism due to concentration of morphogen surrounding them. The morphogen is most abundant at one end of the planaria, and diffuses down the length of it. In this lab, their regenerative capabilities will be experimented with. Since planaria live in such a narrow range of temperatures, their morphogen concentration gradients could be dependent on these specific conditions. If placed under colder temperature conditions, then the planaria will not regenerate correctly, because their morphogen concentration gradients will have been thrown off by the change in conditions. While at the end of the experiment, many of the planaria in the experimental did appear to be growing incorrectly, the results cannot be conclusive. The colder temperature also caused growth rate to be greatly inhibited, so the
planaria did not grow enough to be able to tell which end was growing back.

Introduction

Planaria are free-living flatworms. They live in still bodies of fresh water, like ponds or shallow streams, found under rocks and leaves. Their ideal temperature is around 22-24°C. Planaria prefer to live in shady, dark areas, as they are negatively phototactic, meaning they move away from light when they sense it. They move around by contracting and extending their bodies, first bunching themselves up by pulling their tail end to towards their head, and then moving forward by extending its head out (Kolbasova, 2013). The feature of planaria that will be observed and experimented with in this lab is their reproduction process. Planaria reproduce with a type of asexual reproduction, called epimorphosis. (Vervoort 2011). Epimorphosis is a process in which cells dedifferentiate to form a blastema, an undifferentiated group of cells. These cells then redifferentiate into the proper cells. First, to prepare for reproduction, a planarian will rip itself into two halves. It accomplishes this by latching onto something with its head and tail ends, and then pulling itself apart from each end. These two halves will become two identical, new and independent organisms. Before epimorphosis can occur, the planarian closes the wound from its severed half. Once the wound is closed, neoblasts gather at the site and a blastema forms. From this blastema, a new half will regenerate to form a complete planarian (Vervoort 2011). In this type of reproduction, the question of how cells know what half of the animal to grow into comes up.
The diffusion gradient model explains this phenomenon. It proposes that cells know their orientation on an animal by concentration of a chemical substance, called a morphogen in this case, surrounding them. The morphogen concentration forms a “gradient” down the organism, being most concentrated at one end and diffusing out down the length of the organism. When the blastema in epimorphosis forms, the cells that will redifferentiate into the new half of the organism sense the morphogen concentration in the area they are in, and thus know what half they are going to form (Vervoort
2011). In this lab, two samples of planaria will be experimented with. One will be kept under ideal conditions, while the other in much colder conditions. Their regenerative progress will be compared. Since planaria live in such a narrow range of temperatures, their morphogen concentration gradients could be dependent on these specific conditions. If placed under colder temperature conditions, then the planaria will not regenerate correctly, because their morphogen concentration gradients will have been thrown off by the change in conditions.

Materials and Methods

A jar containing many planaria, two petri dishes, and pond water were obtained. Pond water was put into the petri dishes. Four planaria from the jar were each put in the petri dishes. All of them in both dishes were cut evenly in half into a “head-end” and a “tail-end.” Both were kept in low light conditions. One dish, serving as the control, was put under room temperature conditions (about 24°C), while the other, the experiment, was placed into a refrigerator (about 2°C). The progress of the planaria’s regeneration was monitored over the course of a week. The water in the petri dishes was changed 24 hours after the initial set up, and from there after changed every 48 hours. At the end of the week, the overall regeneration progress was compared between the control and the experiment.

Results

The results of this experiment were qualitative, so no measurements were recorded. Instead, these results are based on observation of the differences between the experimental and control planaria’s regenerative progress. The control animals’ regenerative progress was overall as expected. From the severed tail-ends, heads could be clearly seen developing, as under a microscope, two sockets could be seen forming for their photoreceptors. Likewise, from the severed head-ends, tails could be seen forming, as there were no signs of photoreceptors developing, and the form was beginning to taper off to be narrower, as seen in a fully developed planaria. On the other hand, in the experimental group, the regenerative progress appeared to be greatly inhibited. Some of the cuts had no regeneration whatsoever. Many of the tail ends that did show growth were growing into another tail end. It is notable that there were no head-ends that were growing another head-end from their cut, even though there were many tails growing another tail-end. Very few of the specimens appeared to be regenerating correctly, but the amount regenerated was greatly reduced compared to the control group. There were also a few specimens that appeared to have died during the experiment, as when poked or brushed gently, they did not respond as they had before the experiment.

Discussion

Since the planaria kept under the cold conditions were greatly inhibited in their growth, and did not grow correctly, the hypothesis that colder conditions will affect the morphogen gradient in the planaria appears to be supported. However, this cannot be said with certainty due to other factors involved.
Looking at the results from the experimental group, there were several tail-ends that were growing into another tail-end, but there were no head-ends that were growing into another head-end. While this is certainly notable, it should be considered that there could have been head-ends that were actually growing another head-end, but because growth was so inhibited, it was not observable in the time frame that was allowed for growth. Also considering this, there could have very well been several cuts, regardless of which end they were, that were in fact growing correctly, but due to the fact that growth was so slowed, not enough had regenerated for this to be observable.
It is notable also that a few of the specimens kept under cold conditions died during the experiment. When comparing our results to those of other groups in the laboratory, no planaria in their experimental groups died during the week. The groups asked did not experiment with temperature, but with other variables such as light intensity and salt concentration. One explanation could be that the animals’ metabolism was slowed down to the point of death. The animals had already been starved for a week before the initial experiment, and also did not receive any food during. Their metabolism would have been slowed down greatly from that alone, but when the cold temperature was added, it slowed even more, to the point where the animal could not survive (Campbell 2013).
While the results of this experiment are inconclusive, if it was replicated but allowed for a longer time frame for regeneration, perhaps the planaria in the cold conditions would be able to grow enough to be certain whether or not they were regenerating correctly.

References

Kolbasova, Glafira M. "Biology of Pseudopotamilla Reniformis (Muller 1771) in the
White Sea, with Description of Asexual Reproduction." Invertebrate Reproduction and Development 57.4 (2013): 264-75. Biosis. Web. 10 Apr. 2015.
Reece, Jane B. "An Introduction to Invertebrates." Campbell Biology. 10th ed. Boston:
Pearson, 2013. 680-712. Print.
Vervoort, Michel. "Regeneration and Development in Animals." Biological Theory 6.1 (2011): 25-35. Biosis. Web. 10 Apr. 2015.