REGENERATION (SCIENCE). - One of the general properties of living organisms is regulation (q.v.), a property that preserves the unity of the organism, that is, of the individual. Regenerative phenomena are one aspect of this property.
During the developmental phases of an organism and in the subsequent adult and senescent periods, up to death, cells and parts are continually physiologically destroyed, and the organism continuously reconstitutes them through physiological regeneration that is always adequate to the stage of the life cycle. Likewise, accidental or traumatic causes may more or less deeply injure an organism, and this, if the injury permits life, regenerates the part to a greater or lesser extent according to the organism’s position in the zoological scale, either by regenerating the part or, through a more limited regenerative process, by repairing it through scarring.
Therefore, physiological regeneration must be distinguished from traumatic regeneration. Regeneration and scarring are two aspects of the same reparative process, and scarring, like regeneration, also involves the regeneration of connective tissue and epithelium; however, in organisms endowed with great regenerative capacity, scarring represents a process antagonistic to organ regeneration: if the tail of a newt is cut off, it is regenerated, but if, after amputation, the skin flaps are sutured so as to accelerate scarring, the regeneration of a new tail no longer occurs.
Although, in general, regenerative processes in animals are greater the lower they are in the zoological scale, there are notable exceptions whereby animals of lower groups, both invertebrates and vertebrates, exhibit very limited regenerative capacities. Thus, it is observed that forms endowed with extensive regenerative properties, such as of the tail and limbs, often exhibit a great facility for autotomy of these organs, as in the limbs of decapods or the tail of lizards.
Regenerative phenomena, which lead to the reintegration of the impaired individual, appear typically teleological. However, certain conditions may cast doubt upon this concept. Notable in this regard are multiple regenerations, whereby more than one organ is regenerated in place of the one removed, so that functionality is severely compromised. Well-known cases include the multiple regeneration of limbs in amphibians, the tail in lizards, and the crystalline lens of the eye in newts, among others.
In physiological regeneration, which may be exemplified by the continuous formation of the epidermis (even in humans), as the outermost layers cornify and desquamate, a zone with embryonic character is found in the tissue, which continuously reforms the new tissue through cellular reproduction. These are called germinal or cambial zones.
In traumatic regeneration, according to the now classic concepts of Morgan, two conditions are considered: epimorphosis and morphallaxis. In the first case, embryonic tissue is activated in the wounded region of the stump, which, through cellular multiplication, reproduces the removed organ; in the second case, instead, old cells reorganize, differentiate, and regain reproductive properties. In any case, the organ reproduced through such processes does not recapitulate the embryonic mechanism by which it was first formed. A particular case of regeneration is heteromorphosis, whereby another organ is regenerated in place of the one removed; a typical example is that of the prawn (Palemon), which regenerates an antenna in place of a removed eye.
As has been said, the higher one ascends the zoological scale, the more limited regenerative processes become; this is to be related to the antagonistic condition between differentiation and regeneration, differentiation being always greater the higher one ascends the zoological scale.
In mammals and in humans, such processes are reduced to physiological regeneration affecting labile tissues, such as epithelia, some glands, and blood, while traumatic regeneration is encountered only in bone tissue, involving the periosteum, connective tissues in general, and epithelia through scarring processes. As for nervous tissue, there is no regeneration of cells, though fibers may regenerate, especially those with a peripheral course in nerves. In the central nervous system, neuroglial regeneration occurs.
The regenerative processes of some animals and of many plants link this process to reproduction and specifically to vegetative reproduction. Such are the cases of division or fission in protozoa, annelids, many coelenterates, etc. This may be a natural process by which some forms reproduce, in addition to sexual reproduction (q.v.), or it may occur traumatically and accidentally. Thus, if a planarian is cut in two, one part regenerates the head and the other the caudal part, and two complete individuals are “regulated.” Another case of regeneration is reconstitution, described by Wilson in sponges: if a sponge is passed through a sieve, thereby isolating the various cells, and these are collected on a support, they reaggregate, mostly dying, but those that survive, endowed with multiple potencies, reconstitute the new animal body.