REGULATION (IN BIOLOGY). - This term designates one of the fundamental phenomena of living beings, whereby an organism tends to "regulate" any anatomical and physiological modifications, so as to return to its initial integrity.
The term was introduced into embryology (Driesch) to indicate the property of embryonic parts (isolated blastomeres) which develop, rather than into a part of an individual, into complete individuals. Thus the first cells of a germ which, brought together, would have given a single unit, when isolated regulate themselves, and each one produces, in addition to the part it would normally have produced, also the others, so as to give rise to a complete individual. Experimentally this phenomenon has been produced in a great number of animals, succeeding in obtaining, for example, in jellyfish, sixteen individuals from a single egg (Zoia). The r. of the blastomeres of a single egg also occurs spontaneously in many animals and in man. Such is the case of human monochorial twins, perfectly identical and, naturally, of the same sex. But there are cases in which the phenomenon is the normal rule of reproduction: one example among mammals is the tatusia, an armadillo, which normally gives birth to as many as 12 monovular embryos (polyembryony), and among insects certain hymenopterans which hold the record with the production of thousands of individuals from a single egg (Litomastix). For the problems that may arise in relation to the human species see the entry ANIMAZIONE.
Thus it has been experimentally demonstrated that r. also takes place in the reverse direction: that is, if two eggs are fused (experiments on the triton) a single individual is formed: a single individual that can have two fathers and two mothers!
The r. which in the early stages of development can be complete (there are eggs in which this does not occur because of the early differentiation of the ovular parts: mosaic eggs) becomes, with development, limited to regenerative phenomena alone; and these are as a rule greater in the lower rungs of the scale of living beings and become limited in the higher ones, being reduced in mammals and in man to localized regenerative possibilities and to the phenomena of scarring. In plants the possibility of reproducing individuals from parts of the body (cuttings, rooted slips, etc.) is possible, but in plants there is no strict individuality and every bud has the potential value of an individual. In lower animals (coelenterates, worms, etc.) there is often the possibility of vegetative reproduction, but it differs from the plant reproduction just mentioned because it is not related to pre-existing units such as the buds of plants. The formation of buds is an active mode of reproduction which demonstrates the totipotency of groups of cells even in the body of the adult and differentiated animal. Of particular interest is the reproduction of several individuals from a single individual by means of cutting the latter into several parts, as can be practiced on an earthworm. Here regeneration is the process that allows the various parts to reacquire the missing parts.
In vertebrates, regeneration does not allow, in the adult, the reconstitution of several individuals from one individual, but it does allow the repair even of complex organs such as the tail and limbs of tritons, the tail of lizards, etc. This regenerative process which allows the r. of the maimed individual becomes ever more limited as one ascends the zoological scale, and thus in man regeneration is only partial (connective glands, bone tissue, etc.) while scarring prevails, which, if on the one hand it is also a regenerative phenomenon, on the other is a phenomenon that opposes regeneration. Thus, if on a triton whose tail has been amputated the flaps are sewn together, so as to favor scarring, the regeneration of the tail no longer occurs. Regulatory phenomena also occur in the physiological field, when, once the morphological conditions that would not permit a normal function are altered, physiological activity is rebalanced through the setting in motion of vicarious processes.
The phenomenon of r. seems to respond perfectly to a finalist principle that presupposes the anatomical and functional harmony of the organic unit. However, many authors, basing themselves on facts that seem to contradict this principle, have sought to exclude the finality of r. The examples are many; suffice it to think of bicephalic formations and of Siamese twins, more or less anatomically united, and of the phenomena of hyperregeneration. Both in double monstrosities and in hyperregenerations, r. has certainly not led to an equilibrium. But is this enough to exclude a finalist principle in r.? It seems not, because in these cases there is the reason for the equilibrium not attained: the organism always tends through r. toward this equilibrium, but if material conditions do not permit it, this cannot be brought about. Thus in the cases of double formations it is known that this is due to the impartial separation of the blastomeres; in hyperregenerations, such as, for example, the multiple regeneration of limbs, it is known that experimental or accidental factors have caused a subdivision of the regenerative blastema. Thus in the higher forms, although there is a tendency toward r., it appears quite modest, but this in relation to the high degree of cytological differentiation which limits the reproductive and pluripotential possibilities of the cells.
It has been said that r. is not a phenomenon limited to living beings: even in the inanimate world there would be regulatory phenomena. Thus a broken crystal, immersed in the mother solution, ends up repairing the wound. But in reality this is a profoundly different process: in the crystal it is a matter of the superposition of molecules which from the mother solution deposit themselves in order upon the ordered molecules of the crystal, with a phenomenon of juxtaposition that can be considered similar to the orderly settling of little balls in a small box; in the regeneration of living beings the part is formed through intrinsic activity according to an intrinsically determined plan.
Bibliography2 works
Alberto Stefanelli