ABSTRACT
Drugs typically replace or correct a missing function at the molecular scale; by contrast, regeneration templates replace the missing function at the scale of tissue or organ. An organ may be lost to injury or may fail in disease: The usual response of the organism is repair, which amounts to contraction and synthesis of scar tissue. Tissues and organs in the adult mammal typically do not regenerate. There are exceptions, such as epithelial tissues of the skin, gastrointestinal tract, genitals, and the cornea, all of which regenerate spontaneously; the liver also shows ability to synthesize substantial organ mass, though without recovery of the original organ shape. There are reports that bone and the elastic ligaments regenerate. These exceptions underscore the fact that the loss of an organ by the adult mammal almost invariably is an irreversible process, since the resulting scar tissue largely or totally lacks the structure and function of the missing organ. The most obvious examples involve losses due to injury such as the loss of a large area of skin following a burn accident or the loss of substantial nerve mass following an automobile accident. However, irreversible loss of function can also occur following disease, although over a lengthy time: Examples are the inability of a heart valve to prevent leakage during diastole as a result of valve tissue response to an inflammatory process (rheumatic fever), and the inability of liver tissue to synthesize enzymes due to its progressive replacement by fibrotic tissue (cirrhosis).
