Earthworms, Salamanders and Humans: A Pathway to New Discoveries

Abstract

Our world is one of much diversity, with over thirty million species of insects alone. What makes this diversity more incredible is the amount of similarities that many organisms share. These parallels can be harnessed in ways that allow us as humans to take advantage of our similarities with other organisms and use them in ways to better our lives. An important example of the applications of similar genes is that of the human spinal cord relating to the ventral nerve cord of an earthworm. If applied to the human problem of lack of such extensive regenerative abilities, these similarities can potentially allow humans to heal injuries that were previously untreatable using the regenerative powers of earthworms. Before doing so, we must gather as much information as possible about the regenerative abilities in earthworms.
 

Introduction

Many organisms try to escape dangerous situations as fast as possible to avoid damage to themselves. This response is known as a rapid escape response, and can be triggered when the organism is placed in an inhospitable environment. This experiment utilizes the rapid escape response mechanism in earthworms by comparing their reactions to being placed in distilled water versus the reactions when they are placed in a sodium chloride solution. Earthworms were used in this experiment because of their incredible regeneration abilities and because of the parallels one can draw between their genomes and those of humans. Some fellow researchers and I performed this experiment at the Homi Bhabha Center for Science Education in Mumbai, India.
 

Background

The human nervous system consists of two parts: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord; the brain interprets the signal received from the nerves and sends out more signals to the spinal cord, which then conveys the brain’s instruction to the specified body part. It is able to do so because of the gray matter contained within the bony vertebrae that make up the spinal column. The gray matter contains interneurons, which have both short and long axons connected to them, thus allowing the impulses to travel to various cell groups ⁽¹⁾. These essential connections provided by the matter make it so that the spinal cord becomes invaluable in protecting those connections. If it is damaged in any way, the stress on the spinal column could break the surrounding bones, thus affecting the spinal cord and nerves ⁽²⁾. Harm done to the nerves in the spinal column has a radiating effect on the rest of the body; it may seem small in the spinal column, but because of the network of connections it forms with the rest of the body, it can lead to the inhibition of regular functioning in several body parts. The nerves in the spinal column are both sensory and motor, so damage to them can decrease one’s sensory abilities and one’s motility. The biggest problem surrounding these injuries is that there is no way for the body to heal itself once such damage has occurred. As said on brainandspinalcord.org, “Injuries to the spinal cord are extremely complicated, and affect highly individual cells that are so specialized they are unable to repair or regenerate” ⁽³⁾.
Earthworms share many similarities in their nervous systems with humans. Wigglyworld.org states that “The earthworm’s nervous system is controlled by its cerebral ganglion, which functions as a simple brain. The ventral nerve cord is attached to the ganglion in the anterior segments, and it runs through the length of the body. In each segment of the body, segmental ganglion attach to the ventral nerve, forming a complete nervous system” ⁽⁴⁾. Human spinal cords can be compared to the ventral nerve cords in earthworms. Like the earthworm’s ventral nerve cord, the human spinal cord is the “highway” for signal exchange between the brain and body. If this highway is affected, information flow is disrupted throughout the body ⁽⁵⁾. The disruption caused by the damage of the ventral nerve cord was shown in the experiment when the lesions hindered the earthworms’ abilities to escape the sodium chloride solution. However, the key difference between the two is that while the earthworms’ ventral nerve cord can regenerate itself, the human spinal cord cannot.
 

Method

Before carrying out the experiment itself, a baseline experiment was performed to observe how earthworms would react when placed in distilled water and when placed in a sodium chloride solution. A minimum of ten earthworms must be used for the experiment. Each earthworm was tested in three milliliters of solution, which was just enough for it to be able to escape but also enough that it was fully immersed when dropped in. A sodium chloride solution was used as an inhospitable environment because of its effect on earthworms; earthworms have moist skin that helps them maintain homeostasis, and contact with salt draws the water out of their bodies by osmosis, thus decreasing their abilities to carry out regular functions and eliciting the desire to escape ⁽⁶⁾. After preparing the solution, ten test tubes were filled with three milliliters of sodium chloride solution and another ten filled with three milliliters of distilled water. The first trial was started by quickly dropping an earthworm into a test tube with three milliliters of distilled water, making sure it was fully immersed in the liquid and did not touch the sides of the test tube while going down. The earthworm was timed for two minutes to see if it came out of the liquid within that time or not. After the two minutes were over, the earthworm was removed. The same procedure was repeated except the earthworm was now placed into the sodium chloride solution. This entire process was done with each of the earthworms. The data gathered from the responses showed that the earthworms preferred to remain in the distilled water but that they came out of the sodium chloride solution, most escaping in under a minute. We inferred that this desire on the earthworms’ part to get out of the sodium chloride solution was a rapid escape response elicited by the salt in the solution. This experiment served to confirm that the earthworms tended to escape from an inhospitable environment but they remained in the more suitable environment.
The purpose of the actual experiment was to see how long it took for an earthworm’s ventral nerve cord to regenerate. Out of the ten earthworms used, five were given a lesion on their ventral side and five on the dorsal side. The five earthworms cut on their ventral sides therefore had a severed ventral nerve cord, which rendered many of their regular functions useless. The earthworms were given both dorsal and ventral lesions so that the differences in their responses could be attributed to the part that was cut; the dorsal cut did not affect the earthworm while the ventral cut did, an observation that was verified once the data showed the ventral cut as inhibitive to the earthworms’ escape abilities. Once all the earthworms were given lesions, they went through the same trial process as in the baseline experiment. The ventrally cut earthworms were not able to escape the sodium chloride solution, a result of their ventral nerve cords being severed. The earthworms were placed in this trial daily until those with the ventral lesions were able to escape. The ten days between when they were cut and when they regained the ability to escape represented the amount of time their nerve cords took to regenerate.
 

Results

Amount of time spent by earthworms in distilled water vs. in 0.2 M NaCl solution

Earthworm number	Time spent in distilled water (sec)	Time spent in 0.2 M NaCl solution (sec)
1	120	47
2	120	13
3	120	26
4	117	52
5	120	73
6	120	49
7	120	6
8	120	33
9	118	65
10	120	29
Standard deviation of time (sec)	1.08	21.69

Time spent by lesioned earthworms in distilled water vs. in 0.2M NaCl solution

Earthworm number	Lesion type	Time spent in distilled water (sec)	Time spent in 0.2 M NaCl solution (sec)
1	Dorsal	118	44
2	Dorsal	120	17
3	Dorsal	120	30
4	Dorsal	120	59
5	Dorsal	115	62
6	Ventral	120	120
7	Ventral	120	120
8	Ventral	120	120
9	Ventral	120	120
10	Ventral	120	120

Lesion of earthworm no. 9

Discussion

Earthworms entered the human health realm when researchers found that earthworms, as annelids, shared more protein sequences with humans than they did with other types of insects and worms ⁽⁷⁾. The fact that annelids and humans share genes, however, is not grounds enough for one to harness the abilities displayed in another. All creatures have a common ancestor, and there have been many diversifying factors since that have changed the genomes of the separate species. What allows us to explore the connection between annelids and humans is the discovery that “more than 60% of annelid introns divide protein-coding sequences at exactly the same positions as human introns” ⁽⁷⁾. This plethora of similarities between the two genomes creates a tangible possibility that there are genes in the annelids’ genome that could be harnessed to recreate their regenerative abilities. When researchers cut off parts of the worms and “watched as the creatures regrew the missing body part…they learned that interfering with the expression of one gene kept the tubules and pores from branching off a precursor structure and from re-forming. This suggests the gene plays a critical role in regeneration” ⁽⁸⁾. Using this information, humans can search for the microscopic molecules in the earthworms’ genomes that could allow us to harness shared similarities for the benefit of humans.
These regenerative properties seen in earthworms are also present in salamanders, which, as chordates, share more of their genomes with humans than annelids do. Salamanders are similar to annelids in that they can regenerate large portions of their bodies, but the difference lies in that the regeneration displayed in salamanders is that of more complex structures, such as their limbs. Unlike in human healing, “Salamander regeneration involves dedifferentiation of mature tissue into a mass of stem-cell like cells, which then redifferentiate into appropriate mature tissues to perfectly repair the damage” ⁽⁹⁾. The use of stem cells is something humans do when developing. However, as we develop, those stem cells are replaced by specialized adult cells that only carry out their specific functions. The lack of remaining stem cells is also attributed to the fact that since stem cells are so flexible with their functions, they have an increased chance of forming tumors, making them difficult to control ⁽¹⁰⁾. Because of this, the alternate way to induce regeneration would be for scientists to try and activate the gene and pathways needed only once regeneration is needed. Humans lack the information cells need to follow in order to make a whole new structure, information contained in the fibroblasts that salamanders have ⁽⁹⁾. The presence of stem cells in the regeneration of both earthworms and salamanders suggests that humans could utilize stem cells as a way to induce regeneration of our own. The fibroblasts in salamanders also present an opportunity to use information for regeneration and apply it to humans.

Conclusion

The results obtained from the experiment are the beginning to the journey towards human regeneration. The experiment supplies some of the puzzle pieces in determining how long it takes for earthworms to regenerate their body parts, information that gives us insight into the earthworms’ regenerative abilities and opens up future possibilities for discovering other factors involved in regeneration. Now, the rest of the puzzle lies in using that information to draw parallels between humans and earthworms to find the necessary molecules for regeneration. The same process can be applied to salamanders in trying to break down their regenerative processes part by part and trying to find areas of similarity where a similar process can be induced in humans. Spinal cord injuries have been extremely damaging to humans for some time, as the spinal cord is literally the backbone of our nervous system. Because of the severity of these injuries, any possibilities of curing them are quite welcoming. These parallels between earthworms, salamanders and humans show how the natural world is full of relations between microscopic and macroscopic features. Something as small as the genes in an earthworm shares similarities with us humans, similarities that could be very significant in terms of taking the abilities of one creature and applying them to the other.
 
 

References

1. Ratcliff, Graham, and Duane E. Haines. \”The Spinal Cord.\” Encyclopædia Britannica. May 03, 2017. https://www.britannica.com/science/human-nervous-system/The-spinal-cord.
2. \”Spinal Cord Injury: What Is It and What Does It Affect?\” AbilityLab. https://www.sralab.org/lifecenter/resources/spinal-cord-injury-what-it-and-what-does-it-affect.
3. \”Is There a Cure for Paralysis, and Will My Loved One Ever Walk Again?\” Brain and Spinal Cord. http://www.brainandspinalcord.org/cure-for-paralysis/.
4. \”Earthworm Anatomy.\” Wiggly World. http://www.wigglyworld.org/earthworm-anatomy/.
5. \”How the Spinal Cord Works.\” Reeve Foundation. https://www.christopherreeve.org/living-with-paralysis/health/how-the-spinal-cord-works.
6. Jettycox. \”Posts about Why Do Earthworms Die When Salt Is Sprinkled? on Sciencejetty.\” Sciencejetty. https://sciencejetty.wordpress.com/tag/why-do-earthworms-die-when-salt-is-sprinkled/.
7. Fedorov, Alexei, and Larisa Fedorova. \”Where Is the Difference between the Genomes of Humans and Annelids?\” Genome Biology. 2006. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1431714/.
8. Toledo, Chelsea. \”Worm Regeneration May Lend A Hand in Human Healing.\” LiveScience. November 14, 2012. https://www.livescience.com/24791-worm-regeneration-research-nsf-ria.html.
9. Fior, Jonathan. \”Salamander Regeneration as a Model for Developing Novel Regenerative and Anticancer Therapies.\” Journal of Cancer. 2014. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4174516/.
10. Kiger, Patrick J. \”Why Can\’t Humans Regenerate Body Parts?\” Seeker. August 26, 2013. https://www.seeker.com/why-cant-humans-regenerate-body-parts-1767727453.html.
 

About the Author

Anindita Lal, 16, USA

Anindita Lal is currently a junior at the Acton-Boxborough Regional High School. She loves biology and is considering medicine as a future career path. Over the summer, she spent two weeks doing lab work in Mumbai with the Homi Bhabha Center for Science Education. The experiments she did there inspired her to apply the implications of the data to future innovations in science.