Experiments with Planaria

Instructor: Brenda Steadham

Brenda has worked with K-12 students in life science, chemistry, and language arts. She holds a master's degree in Biological Sciences.

They may be tiny, but planarians are a pretty big deal in the scientific community. Read on to learn about several planarian research experiments and their outcomes.

What Are Planarians?

Planarians, commonly called flatworms, are frequently used in all kinds of laboratory research. They are popular research subjects for several reasons, including reproductive capabilities, tissue regeneration, and conditioned cell memory.

Platyhelminthes flatworm
Platyhelminthes flatworm

Sexual and Not-So-Sexual Reproduction

One big reason researchers use planarians in experiments is for their varied reproductive capabilities. Most all planarians are capable of both sexual reproduction, which involves another organism, and asexual reproduction, which happens within the single organism. Let's take a quick look at both options.

They Have It All

Planarians are hermaphroditic, which means that they have both sex organs (one ovary and one testis). This means that it is possible for a planarian to simultaneously become pregnant and impregnate another planarian. It can all happen at once!

Internal sexual anatomy of a planarian.
Duo Sex

Extreme Makeover

If you thought that option was strange, listen to this: During asexual reproduction, the flatworm literally pulls itself apart in a process called transverse fission. During this process, a constriction point forms roughly halfway between the ends of the planarian. Little by little, the constriction site pinches together and the two halves pull away from each other. But you don't have to feel bad for the halves--they will be as good as new after they regrow, or regenerate, lost tissues.

A constriction site (represented by dashed line) pinches the flatworm into two unattached segments.
Transverse fission

Reproductive Experiments

Frequently, planarian reproductive studies investigate how outside forces, or external stimuli, change or affect the rate of reproduction as well as type (asexual/sexual). We don't have room to cover everything here, but here are a few common experimental external stimuli and their effects:

External Stimuli Sexual Asexual
pH No effect (lethal in extremes) Depressed in low pH (lethal in extremes)
Temperature Increased as temperature increased Depressed in low temperatures/high temperatures had no effect
Light/Dark Increased in absence of light Increased in absence of light

Regeneration Renegades

Almost all species of plants and animals experience cell turnover, which happens when dying and dead cells are replaced with new healthy cells. This is also known as physiological turnover, and even though it sounds bad, it mostly involves cell replacement and wound healing. However, planarians take this to a whole new level: not only are they capable of replacing dead and dying cells, they are able to fully regenerate lost or damaged tissues. This would be like a human regrowing an amputated leg or fingers. Amazing, right?

Planarians are capable of near full body regeneration or regrowth--even if only 1/200th of the original specimen is left, a planarian can completely regrow all missing sections. This magic happens through pluripotent cells, also known as stem cells. Pluripotent cells have the ability to become any type of cell or tissue, which means that they can regenerate whatever is needed. In most species, pluripotent cells are only present during the embryonic phase of life, and cells eventually become limited to routine physiological turnover, so they still repair damage and loss but they don't replace large amounts of lost tissues.

Regeneration Experiments

One reason this regeneration is so interesting to researchers is because planarians and humans share several types of tissues and cells. So if they can do it, theoretically, we can, too! Harnessing the regenerative abilities of planarians has great implications in human health. Here are a few noteworthy experiments:

  • Gene Mapping: Planarian gene mapping has shown that a little more than 200 hundred genes are involved in tissue regeneration. Many of the 200 genes are orthologs to human genes, which means that both are derived from a common ancestral gene.
  • Identification of DNA Transcription Sites: Once researchers identified and mapped the genes involved in tissue regeneration, the next step was to identify, isolate and manipulate the mechanisms controlling DNA transcription. Scientists figured out how to stop and start the DNA transcription, which stopped or started tissue regeneration.

Conditioned Cell Memory

As they worked with planarians, researchers found that certain species learned more readily than others and could be trained. Most studies focused on the use of positive and negative reinforcement to train planarians to maneuver through simple Y-shaped course. The positive reinforcement, like a reward, encouraged planarians to move toward an area and negative reinforcement, or punishment, deterred movement toward a maze section.

Planarians respond well to food as positive reinforcement
Y-maze Planarian

Of the trainable of species of planarians, some of the successful reinforcements include:

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