Reproduction of Planaria Worms

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  • 0:00 Introduction To Planaria Worms
  • 1:13 Planarian Reproductive Anatomy
  • 1:57 Planarian Sexual Reproduction
  • 2:44 Planarian Asexual Reproduction
  • 3:35 Biomedical Benefits
  • 5:14 Lesson Summary
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Lesson Transcript
Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

In this lesson, we'll discuss the reproductive cycle and regeneration ability of planarian worms. We'll also discuss medical and scientific implications of planarian regrowth.

Introduction to Planarian Worms

A drop of pond water might look clear and lifeless. But look again. There's something visible to the naked eye, like small black worms, writhing and slithering. Zoom in closer, and two beady -- but almost cute -- eye spots peer up at you.

The eye spots detect light.
Planarian worm overview

These are planarian worms, a type of flatworm in the Phylum Platyhelminthes and the Class Turbellaria. They are very common classroom organisms, with a simple body plan. The species in the photo here is Dugesia subtentaculata.

These worms are, famously, immortal when cut lengthwise or in half -- something that seems straight out of a science fiction movie. If you cut one of these little flatworms, the two halves will grow into two new, whole organisms with the same genetic pattern. Pluripotent stem cells allow the planarians to regrow and replicate. These stem cells have the ability to generate any planarian tissue, a process, known as regeneration. Regeneration has far-reaching effects on aging and medical research. In this lesson, we'll discuss planarian reproduction, which includes both sexual reproduction and the fascinating asexual process of regeneration.

Planarian Reproductive Anatomy

The image here highlights the simple reproductive anatomy of planarian worms. Planaria are hermaphroditic, meaning they have both male and female sex organs.

An overview of the planarian reproductive system anatomy.
Planarian reproductive system

The ovaries are located in a rostral direction, near the eyespots. This is, of course, very different from the location of the ovaries in the lower abdominal body cavity of humans! Planarian ovaries create a single egg cell.

Testes are located laterally, along the sides of the body, along with seminal ducts, which together create and transport sperm. Planarian worms also have a penis, which is housed in a genital chamber. Eggs are supplied with nutrients via specialized yolk glands.

Planarian Sexual Reproduction

Planarian worms possess both testes and ovaries, and can produce both sperm and eggs. However, sexual reproduction occurs when one flatworm transfers sperm via the penis duct to the seminal receptacle in the recipient flatworm.

The recipient's egg is fertilized in the seminal receptacle, and transferred outside the body into the planarian worm's liquid environment. Typically the egg is attached to a surface in the liquid by way of a thin mucous filament.

Not all planarian worm species can undergo sexual reproduction; some species are exclusively asexual. A benefit of planarian sexual reproduction is the proliferation of genetic diversity within the local population.

Planarian Asexual Reproduction

Planarian asexual reproduction, or regeneration, occurs when the flatworm experiences an injury that splits the worm. Planarian worms can be cut into as many as 1/279th of the original body plan and regenerate into fully formed genetic copies.

The remarkable ability of the planarian worm to regenerate is owed to about 20% of the adult planarian body being made up of pluripotent stem cells, which can become any type of adult tissue. Adult stem cells in planarians have far-reaching possibilities for biomedical research.

Planarian worms do not need to be fully cut in half in order to regenerate. In fact, a split head or tail can result in a double-headed or double-tailed flatworm, and the organism will often survive.

Biomedical Benefits

Much like human embryonic stem cells, the pluripotent stem cells in planarian worms can become any type of planarian tissue. This remarkable ability has placed planarians in the biomedical spotlight. How can the regenerative ability of planarian worms help biologists and medical experts treat disease? Or help us better understand aging?

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