RIGENERAZIONE (SCIENZA). — One of the general properties of living organisms is regulation (v.), a property that permits the preservation of the unity of the organism, that is, of the individual. The phenomena of r. are one aspect of this property.
During the stages of an organism’s development and throughout the subsequent periods of adulthood and senescence, until death, cells and parts are continually destroyed physiologically, and the organism continuously reconstructs them through a physiological r. always suited 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 it to remain alive, regenerates the part to a greater or lesser degree according to its position in the zoological scale in the case of animals, or, through a more limited regenerative process, repairs it by cicatrization.
A distinction must therefore be made between physiological and traumatic r. R. and cicatrization are two aspects of the same repair process, and cicatrization, like r., also involves the r. of connective tissue and epithelium; nevertheless, in organisms endowed with a great regenerative capacity, cicatrization represents a process antagonistic to the r. of the organ: if the tail of an aquatic salamander is cut off, it is regenerated, but if, after amputation, the skin edges are sutured so as to accelerate cicatrization, no r. of a new tail takes place.
Although, in general terms, the regenerative processes of animals are greater the lower they are situated in the zoological scale, there are nevertheless notable exceptions, in that animals belonging to lower groups, both among invertebrates and vertebrates, sometimes display very little regenerative capacity. Thus it is observed that forms endowed with extensive regenerative capacities, for example of the tail and limbs, often also display a great facility for autotomizing these organs, as in the limbs of decapods or the tail of lizards.
Regenerative phenomena, which lead to the reintegration of the diminished individual, appear to be typically teleological. Nevertheless, certain conditions may apparently cast doubt on this concept. The multiple r. are well known in this regard, whereby several organs regenerate in place of one that has been removed, so that functionality remains severely impaired. Cases of multiple r. of the limbs of amphibians, the tail of lizards, the crystalline lens of the eye of newts, etc., are well known.
In physiological r., of which the continuous formation of the epidermis (also in humans) may be considered an example, as the outermost layers become keratinized and are shed, there is in the tissue a layer or zone with embryonic characteristics that continuously reforms the new tissue through cellular reproduction. These are called germinal or cambial zones.
In traumatic r., according to Morgan’s now classical concepts, two conditions are considered: epimorphosis and morphallaxis. In the first case, tissue with embryonic characteristics becomes active in the wounded region of the stump and, through cellular multiplication, reproduces the organ that has been removed; in the second case, by contrast, it is the old cells that reorganize themselves, differentiate, and regain reproductive properties. In either case, the organ reproduced through these processes does not retrace the embryonic mechanism by which it was originally formed. A particular case of r. is heteromorphosis, whereby one organ regenerates in place of another; a typical example is that of prawns (Palemon), which regenerate an antenna in place of an eye.
As has been said, the higher one rises in the zoological scale, the more limited the regenerative processes become; this must be related to the antagonistic relationship between differentiation and r., differentiation always increasing as one ascends the zoological scale.
In mammals and humans, these processes are reduced to physiological r. affecting labile tissues, such as epithelia, certain glands, and the blood, whereas traumatic r. is found only in bone tissue, through the periosteum, in connective tissues generally, and in epithelia through processes of cicatrization. In nervous tissue, there is no r. of cells, whereas the fibers may regenerate, especially those following a peripheral course in the nerves. In the centers, there is r. of neuroglia.
The regenerative processes of certain animals and many plants link this process with reproduction, specifically with vegetative reproduction. Such are the cases of division or fission in protozoa, annelids, many coelenterates, etc. This may be a natural process by which certain forms reproduce, in addition to reproducing through the sexual mechanism (v. RIPRODUZIONE), and it may also occur traumatically and accidentally. Thus, if an earthworm is cut in two, one part regenerates the head and the other the caudal portion, and two complete individuals are formed. Another case of r. is reconstitution. This was described by Wilson in sponges: if a sponge is passed through a sieve, thereby isolating the various cells, and they are collected on a support, they reaggregate; most of them die, but those that survive, endowed with multiple potentialities, reconstitute the new animal body.