REGENERATION (SCIENCE). - One of the general properties of living organisms is regulation (v.), a property that allows the preservation of the unity of the organism, that is, of the individual. The phenomena of r. are one of the aspects of this property.
During the developmental phases of an organism and in the subsequent period of adulthood and senescence, until death, cells and parts are continuously being physiologically destroyed, and the organism continuously rebuilds them by means of a physiological r. always adequate to the stage of the life cycle. Likewise, accidental or traumatic causes may injure an organism more or less deeply, and the organism, if the injury permits life, regenerates the part to a greater or lesser degree according to the position of the animals on the zoological scale, or, by a more limited regenerative process, repairs it by scarring.
A distinction must therefore be made between physiological and traumatic r. R. and scarring are two aspects of one and the same repair process, and scarring too, like r., involves the r. of connective tissue and epithelium; nevertheless, in organisms endowed with a great regenerative property, scarring represents a process antagonistic to the r. of the organ: if the tail of a water salamander is cut off, it is regenerated, but if, after amputation, the skin flaps are sutured so as to accelerate scarring, the r. of a new tail no longer occurs.
Although the regenerative processes of animals are, in general, the greater the lower they stand on the zoological scale, there are nevertheless notable exceptions, so that animals of lower groups, both among invertebrates and among vertebrates, display them very scantily. Thus it is observed that forms endowed with extensive regenerative properties, for example of the tail and limbs, often display a great facility for autotomy of these organs, such as the limbs of decapods or the tail of lizards.
The regenerative phenomena, which lead to the reintegration of the impaired individual, appear as typically finalistic. However, certain conditions may apparently cast doubt on this concept. Well known in this regard are multiple r., whereby in place of a single removed organ more than one is regenerated, so that functionality remains severely compromised. Well known are the cases of multiple r. of the limbs of amphibians, of the tail of lizards, of the crystalline lens of the eye of newts, etc.
In physiological r., of which the continuous formation of the epidermis (also in man) may be considered an example, as the outermost layers keratinize and desquamate, there is found in the tissue a layer or zone with an embryonic character that continuously, by means of cell reproduction, re-forms the new tissue. These are called germinal or cambial zones.
In traumatic r., according to the now classic concepts of Morgan, two conditions are considered: epimorphosis and morphallaxis. In the first case, a tissue of embryonic character is activated in the wounded region of the stump, which, by cell multiplication, reproduces the removed organ; in the second case, on the other hand, it is the old cells that rearrange themselves, differentiate and regain reproductive properties. In any case, the organ that reproduces by such processes does not follow the embryonic mechanism by which it was formed the first time. A particular case of r. is that of heteromorphosis, whereby in place of one organ another is regenerated; a typical example is that of prawns (Palemon), which regenerate an antenna in place of an eye.
As has been said, the higher one goes on the zoological scale, the more limited are the regenerative processes; this must be related to the antagonistic condition between differentiation and r., a differentiation that is always greater the higher one goes on the zoological scale.
In mammals and in man such processes are reduced to physiological r. involving labile tissues, such as epithelia, certain glands and the blood, while traumatic r. is found only for bone tissue, involving the periosteum, connective tissues in general and epithelia with scarring processes. In nervous tissue there is no r. of cells, whereas the fibers can regenerate, especially those with a peripheral course in the nerves. In the centers there is r. of neuroglia.
The regenerative processes of certain animals and of many plants link this process with reproduction and precisely with vegetative reproduction. Such are the cases of division or schizogony of protozoa, of annelids, of many coelenterates, etc. This may be a natural process by which certain forms reproduce, in addition to the sexual mechanism (v. RIPRODUZIONE), and it may also occur traumatically and accidentally. Thus if we cut an earthworm in two, one part regenerates the head and the other the caudal part, and two complete individuals are formed. Another case of r. is reconstitution. It has been described by Wilson in sponges: if a sponge is passed through a sieve, thus isolating the various cells, and they are collected on a support, they reaggregate; for the most part they die, but those that survive, endowed with multiple potencies, reconstitute the new animal body.