All animals have the ability to regenerate tissue and repair their body after it has been damaged or a part of the body has been lost—to varying degrees. Higher vertebrates, including mammals, are able to regenerate only very small parts of their bodies, such as portions of skin or muscle, while lower animals can regenerate entire body parts. Dugesia tigrina, or brown planaria, can regenerate a whole body from a piece one-eighth the size of its original body.
Regeneration consists of three basic processes, that sometimes occur simultaneously and at other times sequentially at the site of the injury:
- Loss of differentiation in tissues near the injury and migration of new cells to the injury, resulting in a mass of undifferentiated tissue at the site.
- Redifferentiation of cells into various tissues.
- Reorganization of tissue into the original form by differential growth (the growth of newly generated tissues at different rates than the rest of the body’s tissues.)
The formation of pigments is one of the last steps in a cell’s specialization process. Because of this, newly regenerated tissue can be recognized easily by its lack of pigmentation.
The decreased regenerative ability in higher animals is due primarily to the increased specialization of their cells; as animals evolved, their tissues became highly specialized to meet the demands of the environment. Although this increase in cell specialization was essential to the evolution of higher animals, it was in direct conflict with the first step in the regeneration process — loss of differentiation. As cells increased their degree of specialization, it became increasingly difficult to lose the specialized functions and become undifferentiated.
One of the most important concepts of animal regeneration is the “axis of polarity”, which is the line, and direction, along which newly regenerated tissue is organized. Using planaria as an example, a piece taken from the flatworm’s body will regenerate a head only at the front end, not at the side or back of the piece.
Another important control is the “field of dominance”. Tissue that is closest to the head, or anterior, controls the regeneration of all the posterior tissue. Anterior pieces tend to regenerate faster than those from posterior positions. The anterior field of dominance has been demonstrated with planaria by grafting a bit of the head of one flatworm to a posterior position on another. The grafted pieces regenerate a new head and also influence the surrounding tissue to form essential internal organs.
A great deal of time has been spent researching ways to increase the regenerative ability of higher animals. It has been found that a higher animal’s regenerative ability depends on the proportion of nerve tissue in the area of regeneration. In the case of the salamander, for example, one half of the total mass of nerve tissue is in the brain; the other half is spread throughout the body, enabling the salamander to regenerate tails, legs, and even heart tissue. Humans, however, have 70% of their total nerve tissue concentrated in the brain, greatly reducing human regenerative ability.
Scientific investigations with electrical activity have shown that a small current applied to an amputated area stimulates cell change, achieving a very small partial regeneration. At the present time, lower animals such as the planaria are still the masters of regeneration.
Did You Know?
- A great amount of research has been performed with Dugesia. Of particular interest is the regenerative ability of the flatworm. Scientists believe that insights into the mechanism of the regeneration and re-differentiation of flatworm cells may be useful in spinal cord injury and brain trauma treatments.
- Regeneration is a key factor in one of the reproductive mechanisms of planaria. Through the process of fragmentation, single individuals split into two parts, with one part regenerating a head and the other a tail. The end result of fragmentation is the formation of two genetically identical worms. This is one example of asexual reproduction.
Also see our article, Working with Planaria.
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