Involution

INVOLUTION (from Latin involutio - rolling up), a biological and pathological term used to denote phenomena of reverse development in cellular elements, tissues, organs or their parts, as well as entire organisms, i.e., to denote regressive changes in their entirety. In such a broad sense, the term was introduced into science by D. Barfurth. Other authors use this term in a narrower sense, meaning processes of reverse development characteristic of old age - senile involution. Instead of the term involution, the term "reduction" (E. Schultz) or "bioreduction" (Milman) is also used. Phenomena of involution are observed in both simple and complex organisms; in the latter - throughout the entire life cycle, but more often during the development period, both embryonic and post-embryonic, and then during old age as a constant and characteristic phenomenon. Such ontogenetic involution is closely connected with phylogenetic involution, reverse development in the process of evolution of organic forms (regressive evolution), leading to degradation of the entire organism (as observed, for example, in cases of parasitism) or individual organs (disappearance of organs, rudimentary organs). Involution can also arise under the influence of changed living conditions or as a result of experimental effects (for example, during regeneration), and in the final result can lead not only to the death of the biosystem but also to its rejuvenation. The processes by which involution occurs are diverse and complex, being partly physiological and partly pathological in nature. Involution in simple unicellular organisms is most often observed in cultures of infusoria (Butschli, Balbiani, Maupas, Hertwig, Calkins). After a series of divisions in the culture, a period of depression sets in, when infusoria divide less frequently, decrease in size, lose their cilia; in some species the protoplasm becomes granular, in others - vacuolated. Some individuals die at this time, the remaining ones begin to conjugate with each other, as a result of which rejuvenation occurs. Depressions can be avoided by constantly providing infusoria with fresh nutrient material (Woodruff, Metal'nikov), consequently involution in this case can be attributed to the influence of the external environment. Similar changes (loss of peristome, cilia) are observed in various infusoria before their encystment, which also occurs under unfavorable external conditions. Phenomena of physiological involution in the form of destruction of part of the nuclear apparatus always accompany the sexual process of simple organisms, in particular conjugation of infusoria, during which of the 4 products of division of the micronucleus, 3 disappear, and the macronucleus is destroyed and completely resorbed. Involution of complex organisms during ontogenesis concerns individual cells, tissues, organs or their parts and is encountered more often than is generally thought. "The existence alongside each other of formations that are fading away and developing further characterizes the entire course of life" (Mehnert). Involution is already noted among the sexual elements: in all vertebrates, degenerative changes are found in the ovaries at various stages of follicle development; in mammals this phenomenon is known under the name of follicular atresia and has been described by many authors, starting with Flemming. The egg cell dies in phenomena of chromatolysis, sometimes after preliminary division; the follicular epithelium first proliferates, then disappears, the theca undergoes hyaline degeneration. Individual sperm cells also undergo involution; in certain seasons involution captures the entire testis (animals with winter hibernation and periodic sexual activity). Involution at the early stages of development has been described many times in vertebrates and invertebrates; these include the shedding of the covering layer in the blastoderm of the rabbit (Rauber's layer), degenerative changes in the trophoblast of the human embryo, degeneration of large nuclei (megospheres, merocytes) at the edges of the blastoderm in eggs with partial cleavage (selachians, reptiles), parts of the endoderm in insects, etc. Extensive degenerative changes are described in the embryonic membranes, organs that have only a temporary existence and disappear with birth, to which belong the yolk sac, allantois, chorion, in mammals - the placenta, about which there is extensive literature concerning changes associated with nutrition of the fetus (Bonnet, Strahl). There also exist formations that appear in the embryo and then disappear without leaving a trace; such is the notochord of fish, tailless amphibians, birds. But even those organs that undergo further development exhibit various involution phenomena during the embryonic period, usually connected with their restructuring. The skeleton undergoes tremendous changes. The chorda, which is laid in all vertebrates from the base of the skull to the end of the tail, already at early stages in the anterior and posterior segments atrophies; subsequently, as the vertebrae develop, it disappears, remaining only in the intervertebral discs. Cartilage, of which the greater part of the embryonic skeleton consists, is then replaced by bone, and the bone itself undergoes further restructuring: bony trabeculae are resorbed and replaced by new ones. In this process, numerous changes occur in the number and arrangement of skeletal parts: shortening of the posterior part of the skull, fusion of vertebrae, bones, limbs. In the vascular system, regressive changes appear very early: the primary yolk circulation quickly disappears; in the aorta and aortic arches, fusions and obliteration of individual branches occur, ending after birth (ductus Botalli); the same happens in the venous system, where cardinal veins disappear. In the nervous system, changes of an involutionary nature have been repeatedly noted: reduction of the olfactory lobe, shortening of the occipital region, associated with reduction of the roots and disappearance of ganglia. The shortening of the caudal end of the spinal cord in mammals has been studied in detail, as a result of which the filum terminale is formed (Zietzschmann). Involution has also been observed in the muscular system: disappearance of individual fibers at early stages, regression of a number of muscles. Of the internal organs, the genitourinary system shows the greatest changes, where in higher vertebrates the head kidney, primary (Wolffian body) and definitive kidneys are successively laid, the first disappearing completely, the second to a large extent. Müllerian ducts in males and Wolffian in females leave behind small areas in the form of rudimentary organs. Interesting are the changes in the development of the ovary, where the first generations of Pflüger's tubes are resorbed without a trace and only the last one gives rise to follicles. - In those cases where development is associated with metamorphosis, at the moment of transition of the larva to the adult individual, entire organs disappear. In insects with complete metamorphosis, in the pupal stage the entire organism is rebuilt anew: muscles, intestine, nervous system are destroyed. In the tadpole, the tail is resorbed and there is a significant shortening of the intestinal canal, sometimes by half. Involutionary changes in the formed organism subside, but never completely cease, since the function of a number of organs (coverings, glands, blood) is associated with physiological death of cellular elements; in addition, regressive changes early begin in various parts, continuously passing into senile involution. Involution of the duodenal glands has been described in adult cats, in humans - involution of the epiglottis; regressive changes in the vascular system in the form of sclerotic changes in the walls of arteries begin relatively early. Phenomena of senile physiological involution (which cannot always be separated from pathological processes) consist in the decrease in weight of all organs, loss of elasticity and drying of connective tissue, disappearance of pigment in the hair; subsequently to this is added the progressive disappearance of cellular elements: epithelial and nervous, whose place is taken by connective tissue. To what limits all these changes can go is difficult to say, since natural death is rarely encountered. An example of physiological involution is also the atrophy of the thyroid gland, occurring at the end of the growth period of the organism. Involution is also called the return of the uterus to its normal volume after childbirth (postpartum involution of the uterus), of the mammary glands after the nursing period.--Pathological involution is usually considered to be those cases when, for example, physiological involution occurs prematurely; in particular, premature onset of manifestations of senile involution (so-called progeria), too early involution of the thyroid gland, etc. The term involution is also used in pathology sometimes to denote the reverse development of various pathological tissue proliferations (involution of granulations, tumors), For some processes of obliteration of the lumen in hollow organs, for example, in the vermiform appendix. Phylogenetic involution consists in the disappearance of organs or their parts in the process of evolution; formations that have become unsuitable, non-functioning are removed, without which the development of new, more suitable organs would be difficult. According to Weismann, an organism that had retained all the organs of its ancestors would be a monster.

The regression of various organs is observed in all classes of complex organisms, including humans; according to Wiedersheim's research, humans have more than 90 regressing organs and only about 15 progressing organs. Organs whose involution has reached such a degree that they are no longer able to function are commonly called rudimentary; but it should be kept in mind that some authors (Bonnet), based on the original meaning of the word rudimentum (first trial, beginning), call rudimentary organs that are not yet fully formed in the process of evolution (e.g., the horns of fossil deer, the shells of reptiles and birds, the tongues of fish, etc.). Therefore, it is necessary to distinguish between cataplastic-regressing and anaplastic-developing organs (the terminology of Naeser). Examples of rudimentary cataplastic organs can be: the unpaired parietal eye of some lizards, the reduced eyes of cave animals, the remains of the pelvic bone in whales, the remains of limbs in some snakes, the right ovary of birds, the left lung of snakes, the human vermiform appendix, wisdom teeth, etc. The history of development shows that regressing organs are laid down later and vary greatly in size (Mehnert); as for the functional value of even small remnants, it must be denied with great caution (Peter); evidence can be the vermiform appendix, views on which to this day cannot be established. A close connection must exist between phylogenetic and ontogenetic involution, since the former can arise only on the basis of the latter. According to E. Schulz's hypothesis, the matter begins with the fact that in the process of ontogenesis, a given organ develops from beginning to end, and then the final stages of its development undergo reverse development, as a result of which it seems to stop at an earlier stage; over time, these final stages simply fall out, and then a shortening of development (Menert's abbreviation) is obtained, which becomes more and more intense and leads to the complete involution of the organ. An example can be the shortening of the tail, observed in many animals and humans: the tail is laid down much longer and atrophies in the embryonic period. Involution due to changes in living conditions can most easily be caused by starvation. During starvation, a decrease in all organs is observed - to a greater or lesser degree. E. Schulz's experiments on starving hydras and planarians showed that in addition to the decrease in the size of the organism, dedifferentiation of cellular elements is observed, as a result of which the whole body or individual parts (atrium genitale of planarians) turn into an undifferentiated cell mass. This kind of reduction makes it possible to speak of rejuvenation and to assert the reversibility of development. Similar results have been obtained in experiments on sponges with starvation or removal of calcium salts: clusters of embryonic cells remain, which can give rise to a new organism. Numerous experiments on the regeneration of various animals show that it is always accompanied by the involution of certain areas; especially indicative are Driesch's experiments on the ascidian Clavellina: the cut gill box undergoes reverse development and turns into a cell mass. The mechanism of involution. Experimental involution confirms the long-expressed position that the main cause of physiological involution is a violation of nutrition, and above all, the insufficient inflow of nutritional material. Tadpoles by the time of metamorphosis stop eating, pupae experience complete starvation. According to Milman, old age represents a natural experiment of incomplete starvation; recent research adds to this the deposition of slags (Ca, cholesterol) in tissues as a result of metabolic changes. Another basic cause is the absence of function, functional atrophy as one of the moments of "functional adaptation" by W. Roux. Excess, non-functioning bone bars are resorbed; the same is the origin of windows in the omentum or tissue of heart valves (R. Beneke). The histological changes accompanying involution are extremely diverse: simple atrophy, protein, fatty, watery degeneration, impregnation with lime salts, hyalinosis. Mechnikov "attributed the main importance in the processes of metamorphosis and old age to phagocytosis; later, the dissolution of cells, lyocytosis (see Histolysis) began to be put in the first place."