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Charles Manning Child, professor of biology in the University of Chicago, has given the most comprehensive statement of his problem to date from the standpoint of his science, although, as we shall see, much has been done since.174His most interesting and important contribution for our purpose is his refutation of the older view that life is always a progressive process and that true rejuvenescence does not occur. Of course, in higher animals the progressive features are predominant and development ends in death. But the above generalizationdoes not take due account of what occurs in lower organisms, while even in man and other mammals the different tissues do not undergo senescence either alike or synchronously. Some, for example, cells of the epidermis remain relatively young till and after the death of the individual. In other tissues such replacement of old, differentiated, or dead cells by younger ones occurs more or less extensively and tissue regeneration following injury occurs more or less in all tissues save only the nervous system. Such regeneration retards the aging of the tissue or organ as a whole. Minot thought that in such cases regeneration arises from cells or parts of cells that have never undergone differentiation, so that even in such cases development is progressive and not regressive. Even if he is right in maintaining that fibrillar substance cannot regenerate, it must be noted that new fibrillar substance does arise in continuity with the old, while isolated cells apparently do not produce it. Child maintains that there is differentiation in such cases and that these regenerating cells have returned to a kind of activity characteristic of the early stages of embryonic development; that is, that cellscanassume an activity characteristic of an earlier stage. “Even in the outgrowth of new nerve fibers from the central stump of a cut nerve there is return to a process of growth and development which is normally characteristic of an earlier stage of development.” Thus regression and differentiation do occur in most tissues of man and higher animals, although cells of one tissue can never produce those of another.
Again, after hibernation regeneration is often extensive. The large proportion of young cells in the body in such cases renders the animal as a whole appreciably younger than at the beginning of hibernation, so that the periodic cycle of activity and hibernation is much like an age cycle. This rejuvenescence may begin duringthe hibernation, when the animal is living on its own substance. Again, we see periodic changes that resemble the age cycle in glands. In the pancreas cell, for example, the loading of the cell is both morphologically and physiologically similar to senescence, and the discharge, to rejuvenescence. In this case the change occurs in individual cells without cell reproduction. Even the cells of the nervous system throughout mature life possess no appreciable capacity for differentiation and regeneration beyond the power to regenerate fibers arising from them. Child believes that the effect of a change in mental occupation or of a vacation may afford “some slight degree of rejuvenescence of the nerve cells.” Verworn, he tells us, distinguishes between fatigue due to accumulations that check metabolism and exhaustion due to lack of oxygen, both of which may cause senility in nerve cells. “Thus exhaustion resembles senility as death from asphyxiation resembles death from old age.” Recovery from exhaustion is not the same sort of change as rejuvenescence except as it involves increase in the rate of oxidization. But fatigue and recovery constitute a cycle resembling closely the age cycle.
Studies of starvation suggest the same thing. Various experiments have shown that in the later but premortal stage of starvation there is a certain activation of vital processes, including heat production, and it is possible that this has some significance for regeneration. Higher animals are apparently unable to use their own tissues as a source of nutrition to any such extent as the lower forms can do, and this is probably connected with a higher physiological stability of the tissue components. The body weight often does, however, increase and become greater after starvation than it was before, so that a fasting period is followed by an increase in vigor and body weight and hence the wide belief in its therapeutic value. On the other hand, the injurious effects of over-nutritionin man are supposed to be due to the accumulation of food or to intoxication, but it is possible that overnutrition actually increases the rate of senescence by augmenting in the cellular substratum not only the decomposition of food but other substances that decrease the rate of metabolism. There are certainly many instances of longevity in man on a low diet. Again, after certain bacterial diseases, for example, typhoid, the body weight often becomes greater and vigor increases. While low diet often does good, it may, on the other hand, aggravate many diseases. Frogs and salamanders may live a long time without food and undergo great reduction, and starvation sometimes has a directly rejuvenating effect. The animals grow much more rapidly afterward and use a larger percentage of nutrition in growth and attain a larger size than those continuously fed.
Death of cells apparently from old age occurs at every stage of development and many cells do not die when the individual does, for he does so only because some tissue or organ that is essential reaches the point of death. Some have thought glands are primarily responsible for it; but others, whose view Child adopts, hold that it is the nervous system, especially its cephalic part, that dies first in man. In various insects and, for example, the salamander, death occurs almost at once after the exclusion of the sexual products, but this is exhaustion. In most, the length of life of the individual is determined by that of the shortest-lived essential organ or of the tissue that is least capable of regression and rejuvenation and the development of which, therefore, remains most continuously progressive. In cold-blooded animals where the rate of metabolism is dependent on external temperature, senescence can be reduced by cold, and in certain lower invertebrates by the simple method of underfeeding. When cells lose the capacity to divide,they differentiate, grow old, and sooner or later die, although death everywhere is the result of final progressive development if this process goes far enough and is not interrupted by regression caused by the need of repair, reproduction, or lack of food. Death is due, thus, to increased physiological stability of the substratum of the organism or to an increasing degree of differentiation that this general stability makes possible. And as individuation increases, death becomes more and more inevitable. Rubner calculated the total energy requirements in calories for doubling the body weight after birth and the requirements per kilogram in body weight for the whole period of life, for a number of domestic animals. His totals for all, except man, showed close agreement, and hence he concludes that the amounts of energy required are the same in all species except for man, who has a far greater amount of energy, that is, a smaller percentage of the energy of food is consumed in growth and maintenance of body weight and more in activity than in other animals. Very likely domestic animals expend less energy than their wild congeners but it is certainly difficult to correlate these results with Minot’s criteria of age as measured by the decrease of growth.
Child concludes that senescence is more continuous in man than in the lower forms. His long evolution has given a physiological stability to the protoplasmic substratum and a high degree of individuation results from this. But the central nervous system, being least capable of progressive change, always dies first, so that the length of man’s life is that of his nervous system and physiological death and senescence inhere in its fortunes. In the lower forms the death point may never be attained under normal conditions because of the low stability of the substratum and the consequent decrease of individuation that permits the frequent occurrenceof a high degree of rejuvenation. But in the higher forms of life the capacity for the latter is limited by greater stability; and this, again, has been acquired through a process of evolution lasting through so many millennia that we must certainly “admit that this task [man’s rejuvenation] may prove to be one of considerable difficulty.”
Thus, according to Child, whose views are the most philosophical and insightful in the field of biology up to date for our purposes, senescence and rejuvenescence are both going on all the time in all cells and organs and are not special processes. In most cells and in most lower organisms dedifferentiation and despecialization of structure and function, which we may term in general regressive tendencies, are always less pronounced than progressive impulsions, while the latter predominate still more in the higher forms of life. It is “quite impossible to account for the course of evolution and particularly for many so-called adaptations in organisms without the inheritance of such acquired characters, but since thousands or ten thousands of generations may be necessary in many cases for inheritance of this kind to become appreciable, it is not strange that experimental evidence upon this point is still conflicting” (p. 463). Germ plasm is not something apart from or uninfluenced by all that goes on in its immediate environment within the body. Regression and dedifferentiation involve reconstitution and always approximate reproduction. To state the matter roughly, all processes involved both in growing old and in growing young might conceivably be arranged on a kind of Porphery ladder with agamic forms of indefinite reproduction, as illustrated in unicellular organisms or in germ plasm at the lower orsummum gensend, and the most differentiated cells that have progressively lost the power of reproducing the whole organism, regenerating lost parts, power to grow, divide, andnourish themselves, at the top of the ladder, representing theinfimaspecies. On such a ladder, development, differentiation, and individuation is progress up, and all rejuvenating activities are descent toward the most generalized function of perpetual self-reproduction. This conception is in very suggestive harmony with the analogous psychoanalytic law of restitution to mental health by reversion to a more primitive state of psychic development, for all these methods might be called rejuvenation cures.
Physiological integration, with its increasing stability of the structural substratum, makes senescence cumulative as we go up the scale of evolution, so that it is ever less balanced or offset by rejuvenation, reproduction, or other regressive changes, as is the case with simple organisms whose life cycle consists merely of brief alternating phases of progression and regression, for the large protozoan cell about to divide is old compared with the two smaller daughter cells formed from it. Senescence is retardation and rejuvenescence is the acceleration that works by transforming, readapting, and even sloughing off old and useless structures. It will take long to modify the course of evolutionary processes that are the result of millions of years of alternating progressive and regressive changes, but not only the phenomena of rejuvenescence but “sports” and saltatory mutation, to say nothing of the findings of recent experiments showing how life and even activities of somatic cells separated from the body and given a more favorable environment may be indefinitely prolonged, point toward a vast reservoir of vitality. Thus we come to a new appreciation of the incalculable energy behind all the phenomena of animate existence and the hope is irrepressible that somehow, although we have as yet no idea how or when, we may abate or inhibit the forcesthat check or repress it and man may emerge into a fuller and even a longer life.
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Jacques Loeb, of the Rockefeller Institute, has devoted himself for many years, with a rare combination of great learning and originality, to problems directly or indirectly bearing upon old age and death.175As his studies of tropism show, he is prone to mechanical and chemical interpretations; and since science has more or less eliminated smallpox, typhoid, yellow fever, malaria, rabies, diphtheria, meningitis, etc., the citizens of scientific nations will sometime, he thinks, be guaranteed a pretty fair probability of a much longer duration of life than they now enjoy. If we define life as the sum of all those forces that resist death, which means disintegration, the latter is comparable to digestion, which transforms meat into soluble products by two ferments, pepsin in the stomach and trypsin in the intestine. These ferments break up the mass into molecules small enough to be absorbed by the blood, and both of them exist not merely in digestive organs but probably in all living cells. They do not destroy our body, perhaps because the coöperation of both is required to do so and this is possible only at a certain degree of acidity, which cannot be reached in the living body because respiration is constantly removing acid. Death thus really comes when respiration ceases.
Of course there is another cause of disintegration, namely, microörganisms from the air and in the intestines. During life the cells are protected by a normal membrane that is destroyed in death and then the action of the microörganisms can superpose itself upon thatof digestion. Thus in man death is stopping the breath and this may be done by poison, disease, etc. The problem is whether there is any natural death, for if not we ought to be able to prolong life indefinitely. But we cannot experiment on man because neither the intestines nor respiratory tract can be kept free from microbes. A Russian, Bogdanow, solved this problem for the fly, putting its fresh eggs into bichlorid of mercury, which a few survived, with no microörganisms on the outside. These eggs were then developed on sterilized meat in sterile flasks and Guyemot raised 80 generations of fruit flies thus. Loeb himself and Northrop raised 87 generations. Their dead bodies were transferred to culture media such as are used for the growth of bacteria and more were produced thus for years. Hence fruit flies freed from infection and well fed would not entirely escape death and probably higher organisms would thus die from internal causes were external ones excluded. Eggs, for example, those of starfish, ripen and disintegrate very rapidly if not fertilized by the process of autolysis, which acts only after the egg is ripe. The fertilized egg, however, does not degenerate in the presence of oxygen but dies in its absence, so that we might say that the fertilized egg is a strict aërobe and the unfertilized, an anaërobe. The entrance of the spermatozoön saves the life of the egg.
Is natural death due to the gradual production in the body of harmful toxins or to the gradual destruction of substances required to keep up youthful vigor? If the latter, the natural duration of life would be the time necessary to complete a series of chemical reactions that would produce enough of the toxins to kill. Now, the period necessary to complete a chemical reaction diminishes rapidly when the temperature is raised, and increases when it is lowered. This time is doubled or trebled when the temperature is lowered by 10° C. Theinfluence of temperature on the rate of these processes seems typical. If the duration of life, then, is the time required for the completion of certain chemical reactions in the body, we should expect it to be doubled or trebled when we lower the temperature. We can test this only where, as in our flies, infection is avoided. Northrop put their fresh eggs on sterilized yeast at a temperature of 0.2° C., and the higher temperatures selected were 5°, 10°, and 25°. All the flies died at nearly the same time when kept in the same temperature. The total average duration of life was 2½ days at 30° C., when nearly all of them died. At 10° C. it was 177 days. Thus heat accelerates all chemical action, and here we have the duration of life increased from 200 to 300 per cent. In man the body temperature is constant, for example, 35.5° C. whether in the tropics or the Arctic regions. If we could reduce our temperature, we might live as long as Methuselah. If we could keep the body temperature at 7.5° C. and follow the above ratio, we should live about 27 times 70 or about 1,900 years. Thus the duration of life seems to be the time required for the completion of a chemical reaction or a series of them. The latter may be the gradual accumulation of harmful products or the destruction of substances required for sustaining youth. Not only are unicellular organisms immortal and the life of all their successive generations a continuum, but a bit of cancer tumor can be transplanted to other individuals and there grow larger, and a bit from this second individual transferred to a third, and so on indefinitely; so that the same cancer cell continues to live on in successive transplantations throughout many individual lives. It has thus outlived many times the natural life of the mouse. Indeed, it seems to be able to live on indefinitely and Carrel has shown that this is true of other normal cells. Thus death may not be at all inherent in the individualcell but only be the fate of more complicated organisms in which the different types of structure depend on each other. Certain cells are able to produce substances that slowly become harmful to some vital organ or center and its collapse brings death to the whole.
In man there is no sharp limit between youth and maturity unless it be marked by puberty, but in lower forms of life it is demarcated by a metamorphosis. The tadpole, for example, becomes a frog in the third or fourth month of its life and this process can be accelerated by feeding the creature with thyroid, no matter from what animal. Gudernatsch was able to make frogs no larger than a fly. Allen showed that the tadpole with the thyroid removed can never become a frog, although it may live long and continue to grow larger than the usual tadpole; but if such aged tadpoles are fed with thyroid they promptly become frogs. Salamanders metamorphose by merely throwing off the gills and changing the skin and tail, and the Mexican axoloti maintains the tadpole form through life; but even it, when fed with thyroid, promptly metamorphoses. Schwingle induced metamorphosis in tadpoles by feeding them with a trace of inorganic iodine. Thus the duration of the tadpole stage seems to be the time required to secure a certain compound containing iodine. Insects hatched as maggots will become chrysalides and then flies, but if thyroid is fed to the maggot it accelerates the metamorphosis, although we do not know whether it is due to the accumulation or formation of definite compounds.
Loeb sought to determine whether the duration of the maggot in the larval stage could be due to temperature and he found that this had effects similar to those described above. The larval period lasted 5.8 days at 25° C. and 17.8 days at 15°. The total duration of life was 38.5 days at 25° and 123.96 at 15°, both ratios being1 to 3. Thus the influence of temperature upon the larval period was like that which it exerted on adult life. The same effect he found in salamanders, all of which suggested to him the conclusion that the duration of life and of the larval period is really the time required for the completion of certain chemical reactions. The cessation of respiration, which means death, and alterations in circulation, which mean metamorphosis or the death of youth, are critical periods and perhaps both points are reached when a certain toxin is formed in sufficient quantity or when a necessary substance is destroyed or reduced. Thus a shortened youth can, in amphibians, be prolonged by modifying the temperature or offering the specific substance that causes metamorphosis, namely, iodine or thyroid. There is no end to the substances capable of hastening death; shall we ever find one that can prolong life?176
Pearl’s experiments on the fruit fly177show that where long- and short-lived strains are mixed, the first generation they produce is longer-lived than either parentand that for subsequent generations Mendelian laws hold even for longevity, so that there is increased vigor in the hybrid generation due to the mingling of germ plasms that are different. As to bacterial invasion, the stability and resistance of the organism is also a factor, but by rearing insects kept free from all such invasion it appears that “bacteria play but an essentially accidental rôle in determining the length of the span of life in comparison with the influence of heredity.” Pearl criticizes the conclusion of statisticians like Hersch that poverty shortens human life, despite the fact that this is perhaps the most potent single environmental factor affecting civilized man to-day. But we have no real evidence that if the conditions between the rich and poor were reversed the death rate would also be reversed. The influence of high temperature, which is known to accelerate all the metabolic processes, does not interfere with the predominant influence of heredity because it only accelerates life processes exactly in the same way that it accelerates chemical activities and the same is more or less true of the influence of the secretions of the endocrine glands.
Pearl concludes178that it has already been demonstrated that cells from nearly every part of the metazoan soma are potentially immortal, even in the case of tumors by transplantation, though of course not yet for such exceedingly specialized structures as hair and nails. Under artificial conditions cells from nearly all organs can be made to long outlive the body from which they are taken, just as grafts from apple trees may be passed on indefinitely to successive generations. Thus death is not a necessary inherent consequent of life in even somatic cells but “potential longevity inheres in most of the different kinds of cells for the metazoan body exceptthose which are extremely differentiated for peculiar functions.” The special conditions under which this occurs are often very complex and differ greatly for different tissues and animals, and we shall probably know far more later of the chemico-physical conditions necessary to insure continuous life, for these studies are new, having begun barely twenty years ago. The reason that all these essential tissues are not actually immortal in multicellular animals is that the individual parts do not find in the body the conditions necessary for their continued existence, each part being dependent upon other parts. This view differs from Minot’s that there is a specific inherent lethal process going on within the cells themselves that causes senescence. Pearl concludes “that these visible cytological changes are expressive of effects, not causes, and that they are the effects of the organization of the body as a whole as a system of mutually dependent parts and not a specific inherent and inevitable cellular process. Cells in culturein vitrodo not grow old. We see none of the characteristic senescent changes in them.” Thus it may be inferred that when cells show characteristic senescent changes it is because they are “reflecting in their morphology and physiology a consequence of their mutually dependent association in the body as a whole and not any necessary progressive process inherent in themselves. Thus senescence is an attribute of the multicellular body as a whole consequent upon its scheme of morphologic and dynamic organization.” The lethal process, thus, does not originate in the cells themselves. “In short, senescence is not a primary attribute of the physiological economy of cells as such.”
It has long been known, as we have seen, that unicellular organisms could go on dividing indefinitely and that germ plasm had a potential mundane immortality; but no one had suspected that highly organized and differentiatedsomatic cells, which had lost the power of producing the whole individual and could only produce cells of their own special tissue, had this power. Recent experiments, however, indicate that under certain highly elaborated conditions they, too, can be made to live and even grow indefinitely and that this growth can not only be observed but measured under the microscope. Many attempts had been made by many individuals to grow tissues artificially to see their development, their functions, and decay, in both health and disease. This can now be done by taking pieces of living tissue from the body, for science has never produced a single living cell, and placing it in artificial media made out of blood plasma especially prepared, for nutrition for such a bit of tissue deprived of access to the normal circulation of the blood is the prime condition for such growth.179Indeed, until Carrel, who had long been interested in the regenerative processes of scars, succeeded in actually causing cells of the connective tissue to grow after being deprived of the circulation of the blood, this was supposed to be impossible. Leo Loeb had already produced artificial growth within and without the body as early as 1907, and in such processes that utilized the body fluid it was found that the same course was followed as in nature, so that the processes in such culture media approximated those that followed grafting. In 1907 Harrison gave details of such a process thatseemed convincing, although he worked only on cold-blooded animals, cultivating nerve fibers from the central system of the frog. Carrel extended this method to warm-blooded creatures and mammals, studying especially the laws of regeneration of tissues after surgical wounds.
The method of these remarkable achievements, now often repeated, is to put tiny bits of living tissue in a plasma of blood serum that will coagulate. The blood must be deprived of its cells by the centrifugal process and must generally be taken from the animal for which the tissue is to be cultivated or, at any rate, generally from the same species, although this is not without exceptions, for chicken tissue has been grown in the blood of human beings, dogs, and rabbits; morbid tissue, perhaps, like cancer, being most indifferent. The tissue is taken from an etherized subject, with every possible precaution against bacteria, chilling, or drying, and so liable is it to be killed by exposure to air that it is best dissected in serum. Both plasma and tissue are kept in cold storage and the time during which it can be thus kept varies very greatly with different animals. The bit of tissue must be very small because only the outer edge can get the nourishment when deprived of the normal blood circulation, for when the piece of tissue is large, all but the periphery dies. To see these changes of form, small bits of tissue are grown on the inside of a coverglass of a microscope slide that has been overlain with a prepared plasma, sealed with paraffin and put into an electric incubator provided with a microscope. The period before growth begins varies but when it occurs, the microscope shows the direct division of the nuclei and the growth taking the form either of layers or of radiating chains, depending on whether epithelial or connective tissue is being developed. Each tissue, whether normal or morbid, develops very precisely tissueof its own kind, and sometimes as, for example, with cancerous tissue, the growth is so rapid that it can be observed with the naked eye. This, of course, opens an immense field of observation and experiment, for example, immunity, protection against antibodies, redintegration, regulation of growth of the whole or parts, and perhaps especially rejuvenation and senility, to say nothing of the character and the influence of the secretions from all the glands. The trouble at first was that the artificial growth was so short-lived; but by changing the medium often and by frequent washing away of the waste products in a salt solution, it was found that the life and growth of these isolated bits of tissue could be very greatly prolonged. It seemed that the process of decay was due to the inability of tissues to eliminate waste products. So in 1912 Carrel’s problem was whether these effects could be overcome.
To solve this problem bits of the heart and blood vessels of a chick embryo were grown. These growths were immersed in salt solution for a few minutes and then placed in the new plasma and it was soon found that thus the tissue could be made to live on indefinitely. Growth is more rapid the earlier the stage of it and it soon declines; hence the advantage of using tissue from embryos. But by subjecting these artificial growths to washings it was found that they were many times greater at the end than at the commencement of the month, showing that they do not grow old at all. Thus C. Pozzi says:
The pulsations of a bit of heart which had diminished in number and intensity or ceased could be revived to a normal state by washing and passage through a new solution. In a secondary culture two fragments of heart, separated by a free space, beat strongly and regularly, the larger fragment 92, the smaller 120 times a minute. For three days the number and intensity of pulsations of the two parts varied slightly. On the fourth theydiminished considerably in intensity, the large fragment beating 40, the smaller 90 times. When the culture was washed and placed in a new medium, the pulsations again became strong, the larger one 20, the smaller one 60 times a minute. At the same time, the fragments grew rapidly, and in eight hours they were united and formed a mass of which all the parts beat synchronously.
The pulsations of a bit of heart which had diminished in number and intensity or ceased could be revived to a normal state by washing and passage through a new solution. In a secondary culture two fragments of heart, separated by a free space, beat strongly and regularly, the larger fragment 92, the smaller 120 times a minute. For three days the number and intensity of pulsations of the two parts varied slightly. On the fourth theydiminished considerably in intensity, the large fragment beating 40, the smaller 90 times. When the culture was washed and placed in a new medium, the pulsations again became strong, the larger one 20, the smaller one 60 times a minute. At the same time, the fragments grew rapidly, and in eight hours they were united and formed a mass of which all the parts beat synchronously.
Pozzi again says:
On January 17 the fragment of a chicken heart embryo was placed in plasma. It grew readily on a thick crown of conjunctive cells. In three days the pulsations, which were regular and strong at the beginning, grew feeble and ceased completely, and this state continued for more than a month. On the 29th of February, the culture, which had been subjected to fourteen passages, was dissected and the central film placed in a new medium. After the fifteenth passage it contracted rhythmically, with pulsations as strong and frequent as on January 17,viz., from 120 to 130 per minute. During March and April this fragment of a heart continued to beat from 60 to 120 times per minute. As the growth of the conjunctive tissue became more active, it was necessary, before each passage, to extirpate the new connective tissue formed around the muscle. On April 17 the fragment beat 92 times, agitating all the mass of the tissue and the neighboring parts of the middle of the culture. On May 1 the pulsations were feeble and they were given their thirty-fifth passage. In the manipulation the muscular tissue was stretched and torn so that the contractions ceased.
On January 17 the fragment of a chicken heart embryo was placed in plasma. It grew readily on a thick crown of conjunctive cells. In three days the pulsations, which were regular and strong at the beginning, grew feeble and ceased completely, and this state continued for more than a month. On the 29th of February, the culture, which had been subjected to fourteen passages, was dissected and the central film placed in a new medium. After the fifteenth passage it contracted rhythmically, with pulsations as strong and frequent as on January 17,viz., from 120 to 130 per minute. During March and April this fragment of a heart continued to beat from 60 to 120 times per minute. As the growth of the conjunctive tissue became more active, it was necessary, before each passage, to extirpate the new connective tissue formed around the muscle. On April 17 the fragment beat 92 times, agitating all the mass of the tissue and the neighboring parts of the middle of the culture. On May 1 the pulsations were feeble and they were given their thirty-fifth passage. In the manipulation the muscular tissue was stretched and torn so that the contractions ceased.
Thus experiment seems to establish the fact that even connective tissue, composed of not the most highly developed but of vigorous though low-level cells, is immortal. Senility and death result because in normal conditions the blood does not succeed in removing waste products. Could science only wash them away in a living organism, life might be indefinitely prolonged. It is these connective tissues that give support to the textures that compose the body and that chiefly make up bone, cartilage, ligaments, and the lymph network, the cells ofwhich are endowed with special properties of growth and play a great rôle in rejuvenating injured tissue. All this work, in a sense, started from Claude Bernard’s principle that the life of an organism is dependent on the interaction of its cells and the medium in which they grow. Thus, to understand the process by which the body develops and why it must yield to decay and death, we must inquire into the cause of the loss of character of these interactions; and this was impossible until tissue could be grown outside the body so that the processes might thus be brought within the range of the microscope and all its conditions under control. Carrel’s first effort, thus, was directed toward the way in which the medium affected the life of the cell and in constituting this medium of plasma from the blood of dogs and chickens he found that the older the animal from which the blood was taken, the less rapidly and extensively the tissues grew in it. In the blood of a relatively old animal the increase became so slight as to be practicallynil. These comparative experiments were made, Grandcourt tells us, with the blood of animals from five months to five years of age, and there was enormously greater activity on the part of the blood of growing animals. Thus it would seem that when an animal attains its size and stops growing, its blood undergoes progressive changes till it lacks, more and more, the dynamic power of youth. So the problem was whether the plasma could be given the force of youth so far as its action on growing cells was concerned and this was accomplished by mixing it with juices extracted from the embryo. Experiments, too, were made with a strain of connective tissue cells that had been kept in artificial life for more than sixteen months. It was divided into two parts, one of which was grown on adult plasma and the other in a mixture of two parts, one of plasma and the other of embryonic juice. In two daysthe ring of tissue around the second part was three times as great as that around the first. Some of these tissues, passed through a salt solution 130 times, doubled their area in forty-eight hours. Another, washed 57 times, increased in volume fifteen times in ten days, etc. These rapid growths, however, could not be duplicated in normal plasma which was then further modified. Thus the different media have a pretty constant effect upon the rate of growth. Carrel says: “The special rapidity of the growth of the tissue depends so much on the composition of the medium that it may become possible to use as a reagent of the dynamic value of the humors of the organism a strain of cells adjusted to lifein utero.” If human connective tissue could be preserved in the condition of permanent life as the connective tissue cells of a chicken are preserved, the value of the plasma of an individual might be approximated by the cultivation in it of a group of these cells and by the observation of the rate of their multiplication. Such observations do suggest some indication of certain values of the blood of an organism and may give us some clue to old age.
Thus in the course of development the activity of the tissue is apt to vary in the body as a whole and in its parts. It therefore became a question whether each particular condition was permanent or whether the dynamics of the cell changes through the action of the medium upon it. To determine this, several bits of tissue, each having its own dynamic power, were cultivated in media exactly alike and differences in the character of the growth were noted. Then the influence of the medium began to tell. Measurements of the changes undergone on the part, in turn of a fast- and slow-growing tissue, showed that the former had lowered its activity one-half in forty-eight hours, while the latter had multiplied its activity by six. This process continueduntil the level of uniformity was reached, when the conditions of growth remained equal in all cases. Thus it appears that though, in the beginning, certain substances that the tissues had accumulated had the effect of accelerating or retarding its activity in the medium, yet in time the latter overcame these conditions and growth was brought under the laws of its own special mechanism. Thus the sum of the investigations on the influence of the medium on cells is that it may not only change the dynamic possibilities of the tissue but the character of the change may be regulated by a carefully considered modification of the medium (Grandcourt).
All this work involves the theory that the cells make such demands upon the nutrition supplied by the medium that they deplete it and then become indirect means of introducing into the life process a chemically destructive activity (catabolism). The result is a gradual slowing down of cell growth, which is progressive aging and death. A very analogous course was that followed in the earlier artificial cultivations. The tissues lived a short span of days and then died. But the process of degeneration could be obviated by salt solutions and other processes so that tissues now growin vitrofor a year and a half and may continue to multiply faster than those of the embryo. Thus for such tissues senility does not exist and the question naturally arises whether we can ever hope to accomplish anything of this sort inside the body.
Carrel in 1914 reported a strain of connective tissue that had undergone 358 passages and had then reached the twenty-eighth month of its lifein vitro. It was detached from the heart of a chick embryo seven days of age, which pulsated for 104 days and gave rise to a large number of connective tissue cells. These multiplied actively for the first two years, a great many cultureshaving been derived from this strain every week. The fragment of the tissue usually doubled in forty-eight hours, though rapidity of growth was subject to fluctuations. One striking result is seen by comparing the amount of tissue produced by a given culture in forty-eight hours this year with that produced in the same tissue by the same strain of cells a year before. This shows that the activity of the strain had increased, although this might, of course, be due to improvement of technic or possibly to a progressive adaptation to lifein vitro. Carrel says: “Thus it is conclusively shown that the proliferating power of the strain has in no wise diminished. During the third year of independent life, the connective tissue shows greater activity than at the beginning of the period and is no longer subject to the influence of time. If we exclude accident, the connective tissue cells, like infusoria, may proliferate indefinitely.” In the latest report at hand one of these cultures had been kept alive and growing thus for seven and a half years.
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The original and indefatigable American-Frenchman, C. E. Brown-Séquard (1817–1894) who in 1878 succeeded Claude Bernard in the chair of experimental medicine in theCollège de France, was one of the first experimental physiologists to study the functions of glands and to realize the importance of their secretions. After investigating the suprarenals in animals as early as 1869 and finding that their removal always caused death, he returned to this subject twenty years later to investigate the testicular fluids which, discharged into the blood, “exalted the power of the nervous system and kept up the vital energies.” He even injected the fluids extracted from the testes of animals into his own system hypodermically, with results that he thought distinctly beneficial to himself and says that he “at the age ofseventy recovered the force and energy of youth, with manifestations unknown for a number of years.” He thus believed that he had discovered a new therapeutic agent of great rejuvenating power. Berthelot says, “The subject required delicate manipulation, not only because of the extraordinary precautions required for this kind of investigation but of charlatanism, always ready to possess itself of new curative procedures. He did not protest against the abuses by which his name was used to cover industrial enterprises.” He persisted in his idea, and he, more than anyone else, should be called the founder of opotherapy or treatment by extracts from organs. His name will always have a prominent place in the history of endocrinology or the science that deals with the glands that secrete inwardly, a subject that already has a vast and rapidly growing literature, with an essentially new body of facts and insights and, at its present stage of development, yet far more precious hopes and expectations of great discoveries just ahead.
Some of the many commercial products of testicular juices, so very difficult to prepare in a form that can be preserved, were for many years widely used and the best known of these, Pohl’s spermine preparations, are still more or less in demand. But despite Brown-Séquard’s enthusiastic belief in his age-deferring cure, it lapsed from general attention, partly because the initial expectations were too high, until a very few years ago when the problems it had suggested were approached in a new way by a few investigators whose results have not only a high value in themselves but give promise of yet more important and definite subsequent discoveries—and that despite the conservatism and criticism that all efforts to deal scientifically and fundamentally with human sex problems always encounter.
Professor Eugene Steinach, who founded a laboratory of comparative physiology at Prague and was later made director of the biological institute at Vienna, continued to work there until his institute, for which Roux and others have solicited contributions from men of science, to have it opened again, was closed by the war. He began to publish his epoch-making results in 1910. In spring frogs brought to his laboratory he found 8 per cent impotent and also that testicular injection from normal frogs seemed to restore or intensify the embracement impulse and the strength of the forelegs.180The effect lasted, however, only a few days. Nevertheless he suggests that in borderline cases it might permanently restore fertility. The same process in castrated frogs showed the same effect, only in much less degree, and the injection of substance from the cerebro-spinal centers of these activities seemed to have a certain but very slight effect upon the sex nature.
When ovaries and testes were transferred in guinea pigs a few days old, he found, in general, that through the influences of the hormones from these glands the character of each sex underwent “slow but radical transformation over toward the other.”181In the one case the male organ atrophied and the breasts were developed, with a disposition to nurse, the hair became finer, the method of growth was transformed into that of the other sex; and the converse occurred when the transplantation was in the reverse direction. The change was thus both morphological and functional and Steinach believes that there is a distinct antagonism of the sex hormones due to transplantation of a heterological gland and that this is not due to biochemical differences ofblood but to a distinct antagonism between male and female hormones, which have a sex specificity that is the main factor in directing growth. He distinguishes between the specific sex influence and the antagonism that brings about heterological sex signs, which favor the development of other pubertal glands and control growth, even to the dimensions of the skeleton, both stimulating and inhibiting it. The transplantation can be so effected that the glands of both sexes, in a sense, inhibit each other, so that something like experimental hermaphroditism can be caused. These changes last sometimes through life and occasionally there may be periodic milk secretions in males. Each element checks and may throw the other out of function.
In a later article182Steinach published results of experiments upon the exchange of sex glands in other animals between the different sexes and found that the female masculated by being given the testes of her brother followed more or less his development rather than her own, almost equaling him in growth, weight, and robustness. This Steinach calls hyper-masculinization and a degree of this follows the development of the glands after transplantation, which the microscope showed was attended by real intussusception. He also showed hyper-feminization, so that we have a change of the ovaries into hypertrophic but analogous pubertal glands, with corresponding change of traits, dependent upon the degree of success or completeness of the operation. Thus he thinks, too, we can explain somatic and psychic precocity by the hypertrophy of these glands. In another article183the author emphasizes the great variability in the development of sex, both as to size oforgans and their functions in different individuals and believes that besides environment, heredity, race, etc., climate has a great deal to do with it. He finds that in warm countries the advent of sex maturity is somewhat earlier in all its aspects, although there is some suggestion that these accelerations may be connected with the development of other secondary traits. Experiments made with animals in artificial climates point to the same result and changes in this direction are observed in animals accustomed to cold that are transported to warm climates.
Interesting as these experiments on the interchange of primary and secondary sexual qualities are, they were, for Steinach, only preliminary to what chiefly concerns us here, namely, his studies of rejuvenation184and his problem was to see whether by his operations he could shed light upon the problem of whether age is a condition we are defenseless against, like an incurable disease, or senescence can, at least within certain modest limits, be influenced. He says his experiments have decided in favor of the latter alternative. He had first to determine whether orthoplastic, homoplastic, or a combination of both methods was the best. The former was chosen because it was quickest and easiest and independent of earlier implantation material, especially with men. And so, with his colleague, Lichtenstern, various operations were performed, of which three type cases are as follows: