REGULATION (in BIOLOGY). — This term denotes one of the fundamental phenomena of living beings, whereby an organism tends to “regulate” any anatomical and physiological modifications, so as to restore itself to its initial integrity.
The term was introduced into embryology (Driesch) to indicate the property of embryonic parts (isolated blastomeres) that develop not into a part of an individual, but into complete individuals. Thus, the first cells of a germ, which when united would have produced a single unit, when isolated regulate themselves, and each produces, in addition to the part it would normally have produced, the others as well, thus giving rise to a complete individual. Experimentally, this phenomenon has been produced in a large number of animals, with, for example, sixteen individuals being obtained from a single egg in medusae (Zoia). The r. of the blastomeres of a single egg also occurs spontaneously in many animals and in humans. Such is the case with human monozygotic twins, perfectly identical and, naturally, of the same sex. But there are cases in which the phenomenon is the normal rule of reproduction: among mammals, one example is the tatusia, an armadillo, which normally gives birth to as many as 12 mono-ovular embryos (polyembryony), and among insects, certain hymenopterans hold the record, producing thousands of individuals from a single egg (Litomastix). For the problems that may arise in relation to the human species vedi ANIMAZIONE.
It has thus been experimentally demonstrated that r. also occurs in the reverse direction: that is, if two eggs are fused (experiments in the newt), a single individual is formed: a single individual that may have two fathers and two mothers!
R., which in the early stages of development may be complete (there are eggs in which this does not occur because of the early differentiation of the egg parts: mosaic eggs), becomes, with development, restricted to regenerative phenomena alone; and these are normally more extensive at the lower levels of the scale of living beings and become limited at the higher levels, being reduced in mammals and humans to localized regenerative possibilities and the phenomena of cicatrization. In plants, the possibility of reproducing individuals from parts of the body (cuttings, slips, etc.) exists, 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 this 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 that 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 severing the latter into several parts, as may be practiced in an earthworm. Here regeneration is the process that enables the various parts to recover the missing parts.
In vertebrates, regeneration does not enable the adult to reconstitute several individuals from one individual, but it does permit the repair of complex organs such as the tail and limbs of newts, the tail of lizards, etc. This regenerative process, which permits the r. of the impaired individual, becomes increasingly limited as one ascends the zoological scale; thus, in humans, regeneration is only partial (connective glands, bone tissue, etc.), while cicatrization predominates. Although the latter is, on the one hand, also a regenerative phenomenon, on the other it is a phenomenon that opposes regeneration. Thus, if the edges of a newt’s wound following amputation of the tail are sutured so as to promote cicatrization, regeneration of the tail no longer occurs. Regulatory phenomena also occur in the physiological sphere, when, once morphological conditions have been altered in such a way as to prevent normal function, physiological activity is rebalanced through the activation of compensatory processes.
The phenomenon of r. seems to correspond perfectly to a finalist principle that leads to anatomical and functional harmony within the organic unit. Nevertheless, many authors, basing themselves on facts that appear to contradict this principle, have sought to exclude finality from r. The examples are numerous; it is enough to consider bicephalic formations and conjoined twins, more or less anatomically united, as well as the phenomena of hyperregeneration. In both double monstrosities and hyperregenerations, r. has certainly not led to equilibrium. But is this sufficient to exclude a finalist principle from r.? It seems not, because in these cases there is a reason for the failure to attain equilibrium: through r., the organism always tends toward this equilibrium, but if material conditions do not permit it, the process cannot be fully expressed. Thus, in cases of double formations, it is known that they are due to the unequal 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 the tendency toward r. is present, it is quite modest; this is related to the high degree of cytological differentiation, which limits the reproductive and pluripotential capacities of the cells.
It has been said that r. is not a phenomenon restricted to living beings: regulatory phenomena would also occur in the inanimate world. Thus, a broken crystal, immersed in the mother liquor, eventually repairs the break. In reality, however, this is a profoundly different process: in the crystal, molecules from the mother liquor are superimposed, depositing themselves in an orderly fashion upon the ordered molecules of the crystal, in a phenomenon of juxtaposition that may be regarded as similar to the orderly settling of marbles in a small box; in the regeneration of living beings, the part is formed through intrinsic activity according to an intrinsically determined plan.