Regeneration and Stem Cells

Identical twins

Identical Twins

Are you an identical twin?  Do you know any identical twins? The way that identical twins form is an amazing process that can teach us something about regeneration. Early in development, the embryo splits in half, and then each part grows into a whole embryo and each embryo grows into a whole baby. These babies are always the same sex because they are identical—they came from one fertilized egg and share all the same genetics.  In humans this process is only possible in the first three days after conception.  Human embryos are only flexible enough to divide in half and become twins in the first three days after conception. Older embryos, new born babies, and adults cannot do this—because our cells are no longer so flexible!  Once our cells start to differentiate into different cell types, they have “committed to their fate” and can’t become a different type of cell. So later in development, if you divide the embryo in two, each half cannot regenerate the other half. And certainly, in adults and kids, we can’t just cut ourselves in half and watch each half grow into a twin of the other half. But this is what planaria do!


Planaria are amazing regenerators!  You can cut an adult planaria in half and it will grow into two whole identical planaria. Even more amazing, you can cut a tiny fragment off of a planarian and that tiny piece will regenerate a whole new planaria—a fragment of planarian only 1/279th the size of a fully grown planarian can regenerate into a new planaria. Because planaria are easy to keep in the laboratory and have this amazing regenerating ability, they are used as model organisms to learn about regeneration. Model organisms are non-human organisms that scientists study to better understand something. The hope is that discoveries made on this organism can be used to help understand other organisms, including humans. One thing that scientists have learned by studying planaria is that they are such good regenerators because they have a lot of special stem cells.

Embryonic stem cells with fluorescent dye

Research with Stem Cells

Stem cells are undifferentiated cells that can make more of themselves and develop into different cell types (differentiate)—but may not be able to differentiate into all cell types. The stem cells in planaria are called omnipotent stem cells because they can become ANY other type of cell and thus can grow into an entire organism! Humans have omnipotent stem cells only in the first three days after conception—and this is the only time that you can divide a human in half and it will regenerate into two humans—become identical twins. Planaria are such good regenerators because they have omnipotent stem cells throughout their entire lives and they have a lot of them—20-30% of all planaria cells are these omnipotent stem cells. Scientists are currently studying planaria and other model organisms to learn how stem cells can promote health and lead to therapies for disease. Scientists are exploring how to use stem cells to grow organs for transplantation  and to treat neurological disease. For example, stem cells can be made to differentiate into neurons, including neurons which make dopamine . These neurons have been transplanted into the brain of model animals with a Parkinson’s-like disease; they survive, continue to make dopamine, and reduce symptoms of Parkinson’s disease (Barker et al. 2013). Stem cells and model organisms are currently being used to understand several other neurological diseases such as Huntington’s disease, Cerebral ischaemia, and Demyelinating disease. Neuroscience research has formed the basis for significant progress in treating a large number of disorders. Stem cells and model organisms may help us find cures for additional nervous system disorders.

Research with Animals

You may be wondering why scientists would do this research on animals rather than directly on people. Neuroscience research must be done in an ethical manner. Experiments on model animals can help us learn about the human brain and cure diseases without putting humans at risk. Testing chemicals, plant extracts, or new procedures on animals is an ethical way to determine their safety and likely effects on people. However, research on humans is an essential final step before new treatments are introduced to prevent or cure disorders.

Plants that Heal

Ginkgo biloba
Traditional healers around the world use plants to speed healing of cuts, scrapes, and broken bones. Some well-known plants used by different cultures to help the body heal include Astragalus (Huan Qi), Centella (Gotu kola or ji xue cao), Comfrey (called “boneknit” or “healing blade”) , Ginkgo biloba (bai guo), Hawthorne (Shan zha), St. John’s Wort, and Aloe Vera. You may have used one of these plants yourself—perhaps your family has an aloe vera plant in the kitchen and you quickly tear off a leaf and use the liquid inside to help heal burns. Or perhaps an acupuncturist has prescribed some of these herbs to help you heal from an accident. You might also think about how stimulants and depressants might affect regeneration. Stimulants (such as coffee, tea, ginger, or ginseng) speed the metabolism and thus may increase the speed of regeneration. However, these may also slow down regeneration by stimulating other body functions and thus taking nutrients and energy away from regeneration.

Scientists are only now studying the effects of some of these plants on healing. In the planaria regeneration lesson, you will use one of the decoctions or infusions that you made in the plant-extract lab to example speed of regeneration in planaria. Perhaps one of the plants you extracted will speed up or slow down the regeneration rate in planaria. Either of these effects could be helpful to scientists as they research ways to use stem cells to cure disease. Your experiment might lead to new information that could be used in treating neurological disease or helping people recover from accidents.

Ted Talk- Alan Russell and the promise of regenerative medicine

Stem Cells: Cells with Potential   Exploratorium

Newt Limb Regeneration Animation  HHMI Biointeractive, 1:21 minutes  

Zebrafish Heart Regeneration Animation  HHMI Biointeractive, 2:30 minutes

Tissue Regeneration in Animals—
HHMI Biointeractive 

Stem Cell-Based Therapies—HHMI Biointeractive
Planaria: A Window on Regeneration