What Scientists Know About How Regeneration Occurs
Some organisms regenerate by first forming a mass of many types of dividing cells, including adult stem cells, at an injury. In plants, this mass is a structure called a callus, which grows into new leaves, flowers, stems, or roots. In some animals, such as salamanders, the structure is called a blastema. After an amputation, blastema cells rapidly divide to form the skin, scales, muscle, bone, or cartilage needed for growing a new limb, fin, or tail.
Most mammals, including humans, don’t form blastemas. Their limited regenerative abilities are the result of adult stem cells that differentiate into the cell types of the tissue or organ in which they are found, which enables the formation of the parts needing replacement due to injury or aging.
While humans can’t regenerate limbs, they can undergo compensatory hypertrophy, which is an increase in an organ’s size in response to damaged or removed tissue. When part of the liver is damaged or removed, for example, the remaining portion grows to the original size and allows the liver to function as it did before. Other organs, including the pancreas, thyroid gland, kidneys, adrenal glands, and lungs, compensate for damage in a similar but more limited way.
Studying Regeneration for Human Health
Human organs and tissues have varied capacities for tissue repair that gradually deteriorate with age. The brain, spinal cord, heart, and joints are among those with the least regenerative capacity. These limitations are partly the cause of conditions such as heart failure and degenerative nerve diseases. A better understanding of the regeneration process could help with treating age-related conditions as well as giving clues about repairing damaged organs or even, someday, replacing lost limbs.
What Are Stem Cells?
Stem cells are cells that can develop into the different, specialized cell types found in the body. During embryonic development, each of the stem cells in the embryo can differentiate into any of the cell types in the adult body, a characteristic known as pluripotency. Adult stem cells, like those involved in regeneration, can differentiate into the cell types of the tissue or organ in which they are found. This type of stem cell is important for the normal repair and maintenance of tissues.
Researchers have developed methods to reprogram mature adult human cells, like skin cells, to act more like embryonic stem cells. Scientists use these cells, called induced pluripotent stem cells, to study the basics of human aging, cell division, and growth, and to develop therapeutics.
