Growing It Back

Healing is not the same as regrowing

Cut your hand and it heals. Cells at the edges of the cut divide, each one splitting in two, and fill the gap. But what fills it is usually not the skin that was there before. It is scar tissue: a tough patch, made mostly of a protein called collagen, that closes the wound quickly and never quite works like the original. No hair grows from a scar, and it does not sweat.

Regeneration is something more: rebuilding a lost part as it was, with the right kinds of cells in the right places. An axolotl, a salamander from the lakes of Mexico City, can lose a whole leg and grow a new one, bone, muscle, nerves and all. People cannot. But scientists are learning a great deal about it: the ability is there in pieces, and scientists are finding out what switches it on and off.

Who can regrow what

Tap an animal to see how much it can rebuild.

How a leg grows back

When an axolotl loses a leg, the regrowing happens in four stages. Tap each one in order.

What tells the cells what to build

A blastema is a crowd of cells that could become many things. Something has to tell each cell what to become, and where. Scientists are finding three kinds of signals, and each has been the key to a recent experiment.

• Chemical signals. Proteins called growth factors pass from cell to cell and tell them to divide, to gather, or to become bone.
• Genetic switches. Some genes act like switches: when they are on, regrowing can happen; when they are off, it cannot.
• Electrical signals. Every cell has a small voltage across its membrane, and the pattern of those voltages across a tissue helps tell it what shape to build.

Tap an experiment to read what it found.

Building a part instead of regrowing it

There is a second road, called tissue engineering: grow the part outside the body, from the patient’s own cells, and then put it in. In 2006 a team led by Dr. Anthony Atala reported that seven young patients, aged 4 to 19, had their own bladders enlarged with bladder tissue grown this way. The doctors took a small sample of each patient’s own bladder cells, grew more of them in the laboratory, and laid them on a frame shaped like a bladder, which the body slowly broke down as the new tissue took its place. The patients were followed for up to five years, and the new tissue was working.

Because the cells are the patient’s own, the body does not attack them as foreign. Today, 3D bioprinting takes the same idea a step further: a machine lays down living cells and the material that holds them, layer by layer, in the shape of a tissue. Simple, thin, hollow structures are the easiest to make this way. Solid organs like a kidney, with many kinds of cells and a web of blood vessels, are still far off.

Reading a regeneration experiment

The GED science test often gives you an experiment and asks what it shows. Here is the frog experiment from Tufts University, reported in 2022, laid out the way a test question would give it.