What’s in a worm? Pluripotency and the planarian model organism

An exploration into the regenerative capacities of the Planarian flatworm and its applications in biomedical research.

Written by: Olivia Hil

Edited by: Elly Chaw

When was the last time you visited a freshwater lake? Did you by chance pick up a rock only to scream and throw it across the nearest plain? Was this because there, revealed to you on the surface, were some small, sticky, slug-looking creatures? If so, it is of great likelihood that the animal you were looking at was a planarian flatworm – an order of model organisms currently showing great prospects for research into both cancer and embryonic development.

These highly abundant creatures are one of several flatworms belonging to the class Turbellaria. Anatomically, they consist of a small brown body with a triangular head. A pair of eyes mark the anterior and a pharynx or mouth is found in the centre of their ventral side. These worms are of primary interest for their incredible regenerative capacity enabled by the presence of a permanent supply of pluripotent adult stem cells maintained within the creatures’ core. Individuals of this cellular population are termed as “neoblasts”.

Stem cells are found in multicellular organisms and can be described as self-renewing, highly proliferative cells that have the potential to generate various new cell types. During human embryonic development, the stem cell loses potency, the characteristic defining the limitations in the number and type of cellular fates it can eventually adopt. Most adult organisms possess only multipotent stem cells that differentiate into a small number of identities used to replenish specified tissues. Pluripotent cells are those which can differentiate to become all three germ layers comprised within the body of an embryo. In adult planaria, persistence of this stem cell population facilitates a number of unique anatomical feats.

One such phenoma is the ability of the worm to regenerate following bodily severance. In the event of amputation, a planarian is able to regrow an estranged body part within a timeframe of about one week. Many diversities of planarian flatworms exist, some of which are asexual, and others of which undergo sexual reproduction. The asexual species are able to harness this same mechanism in order to expand their population. By actually detaching the tail, half of the body from its anterior forms two semi-conserved, genetically identical worms. Those of a sexual variety, however, simply practice this aptitude as a healing response to wounding.

The relationship between the body tissue and stem cell reservoir is by no means static in the absence of physical distress. In fact, it has been shown that the organism’s body tissue is actually in a state of constant cellular turnover, permitted by the migration and replenishing of the neoblast population. This is exemplified in a unique response adopted by the organism during reduced nutrient availability. In times of nutritional deficiency, the worm is able to decrease in size, lowering its overall energy demand, thus getting smaller and smaller until it eventually dies of starvation. If the food supply increases, a larger overall size will be adopted once again, invoking the original higher energy requirement for general sustenance.

The implications for the use of this worm in biomedical research are vast with perhaps the most relevant being those with regard to cancer. There are more than 100 varieties of this deadly disease in humans, resulting in almost 7.98 million fatalities in 2010. Planaria have already been used to perform studies on stem cell migration within the organism, which could reveal much about the process of metastasis, and genetic markers for pluripotent stem cells have been determined. The identification of genes that induce mitotic division could be essential in the discovery of the molecular process underlying the establishment of tumorigenesis in previously differentiated adult cells.

Another area of interest is the molecular pathways underlying the development of the planarian organism, such as cellular differentiation. The asexual reproductive mechanism of some planarian subtypes proves useful in this research as it offers an alternative for the use of embryos, avoiding ethical issues regarding to whom the responsibility belongs to give consent for the use and manipulation of a potential being, and solving problems regarding short supply.

The planarian flatworm model organism presents numerous possibilities for the extension of our knowledge on stem cells. These are required for development and repair in all multicellular organisms. They can also be used to observe the mechanisms behind anomalous circumstances regarding cell growth and proliferation in various cancers. So perhaps the next time you visit that same lake, rather than flinging or even squashing these precious creatures, handle them with care. They just might hold the solution to one of the most problematic pathologies of our time.

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