Regeneration

Regeneration is the ability to replace lost or damaged body parts.

This ability varies greatly among living things.

Plants

Plants can regenerate all body parts from precursor cells. Many trees, for example, can be cut off at the ground and, in due course, sprouts appear at the margins of the stump. These go on to develop new stems, leaves, and flowers.

In the laboratory, entire plants can develop from a mass of undifferentiated cells growing in culture.

For example, a fully-differentiated carrot root cell when grown in a suitable culture medium, begins to divide repeatedly, losing its differentiated structure as it does so. Then its descendants begin to differentiate, and they finally form all the organs of a mature carrot plant.

The photo (courtesy of Roy De Carava and Scientific American) shows a carrot plant that grew in a flask from fully-differentiated root cells that had been isolated and induced to undergo mitosis.

Invertebrate Animals

Sponges

Hydra

This cnidarian can also regenerate its entire body from cells. The cells that do the job are totipotent stem cells residing in the animal's body.

Planarians

These flatworms can regenerate the entire body from just a small piece. They do this by the proliferation and differentiation of the totipotent stem cells that it retains in its body throughout its life.

Planarians are valued for research on regeneration as individual genes that might participate in the process can be knocked out by RNA interference (RNAi).

Starfish

These echinoderms can regenerate the entire organism from just one arm and the central disk.

I have read that at one time oyster fishermen used to dredge up starfish from their oyster beds, chop them up in the hope of killing them, and then dump the parts back overboard. They soon discovered to their sorrow the remarkable powers of regeneration of these animals.

The photo (courtesy of Dr. Charles Walcott) shows a starfish regenerating an arm.

Vertebrates

Newts and Salamanders

These amphibians can regenerate a missing tail, legs, even eyes. This remarkable ability is particularly pronounced in the larval stage. For this reason, larval salamanders are favorites for doing research on regeneration.

• A layer of epidermal cells grows over and covers the stump.
• A mass of undifferentiated cells - called the blastema - develops just beneath.
• Muscle and cartilage form in the regrowing tail.
• The notochord and spinal cord grow out into the regrowing tail.
• After a few weeks, a new, fully-functional and anatomically-correct tail is complete.

The Mechanism

• a population of pluripotent stem cells that have resided in the animal body prepared for such an event (as occurs in the hydra) or
• the dedifferentiation of specialized cells, e.g. muscle and cartilage cells, in the stump.

The answer appears to be both.

• Stem cells in the spinal cord migrate into the regrowing tail and differentiate into several cell types, including muscle and cartilage. Although the stem cells are ectoderm, they are able to develop into mesoderm.
• Muscle cells in the stump migrate into the blastema while
  • reentering the cell cycle to produce thousands of descendants;
  • dedifferentiate as they do so; that is, they lose the characteristic proteins, etc. of muscle cells.
• Even though there is as yet no sign of a tail, its final pattern is established during this process for if the blastema is removed and transplanted elsewhere, it will continue the process of regenerating a tail.
• Finally the cells of the blastema differentiate into all the cell types - nerve, muscle, cartilage, skin - used to build the regenerated tail.

Mammals

Don't we wish that we had the same powers of regeneration that salamanders do: able to regenerate a severed spinal cord or grow a new heart!

But unfortunately, we cannot. We can regenerate some skin and a large amount of liver. But that's about it.

Just why we are so limited is not known (but is the subject of intense research). Much of the excitement of research on stem cells is because of the hope that they may provide a means of regrowing damaged or lost tissues or even organs.