Comparative analysis of cell replacement in hibernators

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Camp. B&hem. Ph@/. Vol. IOIA, No. I, pp. 11~18, 1992 Prinled in Great Britain

MINI REVIEW

COMPARATIVE

ANALYSIS OF CELL REPLACEMENT IN HIBERNATORS INNA I. KRUMAN

Institute

of Theoretical

and Experimental Biological Physics, Pushchino, 142292, U.S.S.R. (Received

U.S.S.R.,

Academy

of Sciences,

3 April 199 1)

Abstract-l. Cell renewal in hibernators undergoes seasonal rhythm independent of the hibernation state. 2. We propose that seasonal depression of cell renewal in tissues of hibernators is caused by seasonal involution of thymus in these animals. 3. The latter is known to be involved in the control of cell proliferation. 4. The state of hibernation per se has also an effect on cellular proliferation. 5. It induces the block of cells in the permitotic phase. It is suggested that the blockage of cells in renewing tissues of hibernators under natural deep hypothermia throughout a period of torpidity represents the adaptive reaction of the organism.

depression was observed even in hibernators maintained at room temperature in an active state throughout several years (Pengelly and Asmundson, 1974; Pengelley and Fisher, 1961).

INTRODUCTION It is known that hibernation is a unique biological fo1.m of animal adaptation to stressful conditions. D. tring hibernation many functions are depressed. In thus state, when the body temperature reaches nearly 0 13, the animals can survive without food and water. TI ey are more resistant to toxic substances than mnhibernators (Kalabuchov, 1985). The response of thl: hibernating animals to various extrinsic influences sus:h as neoplastic growth under chemical carcinogens and inoculation of tumorogenic cells (Patterson c[ al., 1957; Ohashi, 1977; Kemper and Ruben, 1982; Rllben, 1982), radiation injury (Barr and Mussachia, 15’67; Jaroslow et al., 1976) is almost completely dc’pressed. It seems likely that cell growth and repl#icement in hibernators are reduced or stopped dl ring this period. However there have been a few reDorts showing that proliferative processes do occur in hibernating animals. For example, Thompson c,t al. (I 962) reported that regeneration during hibern: tion is slowed down but not stopped. It was found that maturation of erythrocytes is greatly slowed down but not stopped (Brock, 1960). When hiberni tion begins the testes appear to be fully involuted. Irlvolution occurs before hibernation begins. Usually xpermatogenic recrudescence begins shortly after the mid-hibernal interval (January to early March) and cc,ntinues at a gradually accelerating pace through tkle remaining weeks of dormancy (Kayser, 1961; u”imsatt, 1969). In other words, spermatogenesis continues, though slowly, throughout hibernation. T’lus, the processes of renewing and growth are not stopped during hibernation. It has been recently shown that endogenous factors seem to play a major role in control of biological reactivity of hibernators. Continuous depression of almost all processes occurs well before the animals enter hibernation and independently of it. The same

SEASONAL VARIATIONS OF CELL REPLACEMENT IN HIBERNATORS

The whole endocrine system is depressed during hibernation. Polyglandular involution is typical for hibernation and possibly is necessary for this process but independent of hibernating state (Wang, 1982). Depression of endocrine functions is most remarkable in the hypothalamo-hypophyseal-adrenal system (Ilyasova, 1985). The functions of the endocrine system in hibernators are markedly depressed but not stopped during hibernation. It was shown, for example, that hibernation was abolished after functional thyroidectomy with “‘J, while injection of thyroxine restored the normal course of hibernation (Wang, 1982). Hibernation was found to cease in adrenalectomized animals however, they regained the ability to hibernate after injection of cortical hormones (Vidovic and Popovic, 1953, 1954; Popovic, 1960). It is known that corticosteroids play a great role in metabolic processes in animals. These hormones are more important for hibernators due to their stimulatory effect on the enzymes involved in gluconeogenesis (Burlington and Klain, 1967; Twente and Twente, 1967; Daudova and Usatenko, 1970; Galster and Morrison, 1970, 1975; Burlington, 1972; Steele, 1975) and lypolysis (South and House, 1967; Burlington, 1972; Fain and Czech, 1975). One of the typical features of hibernation is the maintainance of the balance of different physiological functions. It is believed that a major role in doing so is played by the central nervous system. The brain 11

12

INNA I. KRUMAN

monoamines are involved in the control of the body temperature of animals including hibernators (Feldberg and Myers, 1964; Glass and Wang, 1978a,b). The brain monoamines are also involved in the control of hypothalamic and hypophyseal hormone secretion (Smelik, 1977; Moore and Bloom, 1979) and through it in related processes, probably causing their seasonal changes. The question arises as to whether the cell proliferation, just as other processes in true hibernators, undergoes seasonal variations or whether the depression of cellular proliferation during hibernation is caused by suppression of biological reactivity at low body temperature. Some of the available data suggest that there are seasonal variations of cell replacement in hibernators which are independent of the hibernating state. For example DNA-synthesis is found to show a gradual decrease in the intestinal crypt cells of ground squirrel (Citellus undulutus) in September when the majority of these animals are active (Kruman et al., 1988). The level of DNA-synthesizing cells progressively decreases with a minimum in November-January, During the time of preparation for arousal there is the reactivation of proliferative processes. In renewing tissues of both torpid and active dormice (Glis glis) during hibernating period, the fraction of DNA-synthesizing cells is not different but the mean specific activity of DNA is 24 times higher in active animals than in hibernating ones (Adelstein et al., 1967). The levels of DNAsynthesizing cells in intestine of both torpid and active ground squirrels (Citellus erythrogenus) during hibernating period are not different either (Kruman et al., 1986). The mitotic index in stomach epithelia of ground squirrel (Citellus erythrogenus) begins to rise during late hibernation season (Vinogradova, 1986). It is shown that as the hibernation proceeds the stomach mucosa becomes gradually thinner (Vinogradova and Stepanenko, 1971). The proliferative activity in the cultures of lymph nodes taken from the active and hibernating ground squirrels in November is lower than that in summer (Alekseeva and Unker, 1970). It should be also noted that some hibernators (ground squirrels and woodchucks) come into full sexual activity during hibernation. It is associated with the rise of cellular proliferation in testes at this period (Wimsatt, 1969). All the above presented data clearly demonstrate that variations in the level of cells entering the mitotic cycle in renewing tissues of true hibernators are seasonal. This seasonal dynamics of proliferative processes in hibernators indicates that they are subjected to endogenous control. What is this control like? THE ROLE OF THE THYMUS AND T-LYMPHOCYTES IN CONTROL OF CELL PROLIFERATION

There are a number of studies showing that the thymus is involved in the control of cell proliferation. For example, it is known that the neoplastic growth under chemical and virus cancerogens is impossible in athymic nude mice. Tumors in these mice are found much more seldom compared to normal ones. The transfer of thymus to athymic nude mice leads to restoration of the sensitivity to cancerogens (Makinodan, 1977). The suppression of the mitotic activity in different renewing tissues is observed some

weeks after thymectomy (Mamontov and Kremly, 1983). The aging accompanied by involution of the thymus leads to depression of proliferative activity in all tissues (Dontsov, 1986). For instance, the intestine mucosa of rats and mice shows some signs of atrophy with aging (Baker et al., 1963; De Laey, 1966; Andrew, 1972). The period of renewing of intestine epithelia is 1.5 times higher in young than in old persons (Eder, 1966; Lesher and Sacher, 1968). There is evidence indicating that the intestine mucosa becomes thinner with age (Valenkevich, 1984). Thymectomy in young animals leads to a delay of their growth and maturation and in new-born animals to death caused by dystrophic changes of all renewed tissues rather than by infectious agents (Dontsov, 1986). Clinical observations suggest that tumors grow more slowly in aged subjects (Fidler, 197.5; Poste and Fidler, 1980; Ershler et al., 1984). Inoculated melanoma cells grow more slowly in old as compared to young mice. However, thymectomy in young mice reduced the tumor growth (Tsuda et al., 1987). It is known that T-lymphocytes are able to control the cell proliferation of different cell types. For example, T-lymphocytes derived from mice with regenerated liver can induce proliferation of hepatocytes in normal recipients (Babaeva, 1972; Babaeva et al., 1982). The parathyroid hormone whose action is specific to osteoclasts having no receptors to this hormone mediates its effect by the T-lymphocytes (Yamamoto et al., 1983). It is well known that the growth control of stem cells of the bone marrow is due to T-lymphocytes (Petrov and Seslavina, 1977). Lymphocytes are shown to have receptors to somatotropic hormone. Their level rises throughout the postnatal growth (Rosenfeld et al., 1977). There is a close correlation between tumor growth of melanoma cells inoculated in mice and the number and functional competence of their T-cells (Tsuda et al., 1987). T-lymphocytes also affect the cell growth in citro (Granger et al., 1973; Lozzio and Lozzio, 1973). Thus, taken together, these data show that the thymus is involved in the integral control of cell proliferation in animals. The ability of T-lymphocytes to control proliferation of different cell types in vivo and in vitro has been interpreted as indicating that these cells are direct carriers of control information able to move easily in the internal environment (Dontsov, 1986). It is of particular interest that the thymus of hibernators undergoes seasonal anatomo-histological involution that may be considered a natural spontaneous seasonal thymectomy (Galletti and Cavallari, 1972; Alekseeva and Unker, 1977). The involution of the thymus in these animals begins prior to hibernation. At the end of August the mast and plasma cells are decreased in number and the Hassal bodies are more numerous. At the beginning of September there are only small islets of thymus within the brown adipose tissue but these islets retain significant numbers of Hassal bodies during the whole month. There are few lymphocytes. Mitoses are lacking. At the end of Septemberthe thymus is almost fully epithelial. No lymphocytes and Hassal bodies are present. In this time the structure of thymus is completely subverted,

Cell replacement in hibernators indicating a complete or nearly complete supression of its function. This state of thymus is typical not only for hibernating but active animals almost throughout hibernation. At the end of this period in the spring the thymus shows an obvious tendency toward the reconstruction of the parenchyma. At awakening time and thereafter the structure of this organ is completely restored. The great depression of thymus function and the absence of cellular mitosis in it during hibernation cause significant decrease of helnatic T-lymphocytes (Kayser, 1961; Unker et al., 1975; Kalabuchov, 1985) which may be compared wil h the development of dramatic lymphopenia in children with congenital under-development of th:rmus (Drzhevetskaya, 1983). It is found that as the bouts (periods of torpor) of hibernation become lottger, the number of hematic leucocytes during hiljernation decreases showing an increase at the enll of hibernation when the bouts become shorter (I-nker et al., 1975). At awakenings the number of lel.cocytes rapidly rises mainly due to the appearance of lymphocytes from the spleen (Unker et al., 1975). Seasonal anatomo-functional involution of the thm/mus in hibernators should not be interpreted as being related to the general adaptation syndrome. AII~ manifestation seen in this organ in the general adaptation syndrome is secondary, while in the hiberns tors the thymus appears profoundly involuted pr or to entering hibernation and independently of it (Cialletti and Cavallari, 1972; Alekseeva and Unker, 1477). Seasonal involution of the thymus of hibernztors differs from accidental due to the appearance in the thymus of numerous plasma cells. It should be nc ted that there are no conditions for production of pl#isma cells in the normal thymus (Alekseeva and U lker, 1977). In addition, there is a thymus-adrenal in .erplay. Thus, a rapid diminution in the volume of thz thymus is seen with the increase of glucocorticoid sel:retion under some stress. Injection of the extractll In of thymus (or its transplantation) into the animal le ids to a supression of the adrenal cortex. If no al!e-related involution of thymus in animal occurs, UIIder-development of the adrenal cortex function takes place, resulting in a decrease of the resistance tc stress-induced conditions (Drzhevetskaya, 1983). Involution of thymus and adrenal in hibernators occurs almost simultaneously and both events ale independent of the hibernation state (Galletti alid Cavallari, 1972; Alekseeva and Unker, 1977; Il’lasova, 1985). Thus, hibernators are characterized b:‘, the seasonal involution of thymus and the seasonal dt,pression of proliferative processes owing to diminulion of the number of cells entering the mitotic cycle. Bl)th processes are independent of the hibernation st lte. It must be reemphasized that the thymus is not tfle only immune organ but the one directly conct’rned with control of cell growth. The seasonal reduction of proliferative activity in hibernators has been interpreted as the effect of seasonal involution 01‘ thymus in these animals. THE EFFECT CONDITIONS

OF THE HIBERNATION STATE-INDUCED ON CELL RENEWAL IN HIBERNATORS

Cell growth in hibernators is the subject of more than seasonal changes. The hibernation state per se

13

accompanied by dropping of core temperature and depression of the metabolic activity affects the cell cycle. Moreover, hibernation is not a continuous process and is always interrupted by periods of arousal when animals become normothermic. During bouts of hibernation, mitoses are completely (Mayer and Bernick, 1958; Mayer, 1960) or nearly completely absent (Suomalainen and Oja, 1967; Unker and Alekseeva, 1974; Vinogradova 1982), but the absence of mitotic figures in the tissues of an animal in hibernation state does not necessarily mean that all events of the cycle stop during bouts of hibernation. Adelstein et nl. (1967) have shown that the cells in renewing tissues of the dormouse (Glis g/is) incorporate injected tritiated thymidine during torpor though the rate of incorporation is much lower than in active animals. In other words, the cells of the animal continue to enter the mitotic cycle and are progressing from G, to S during the bout of hibernation. No labeled mitoses are seen in the hibernating animal but they appear 2-3 hr after the beginning of arousal. As the mitotic index rises just before the appearance of labeled mitoses after arousal, it seems likely that the cells do not enter the mitosis and the block is at Gz. Two to three hours after arousal there is the peak of mitosis caused by premitotic blockage. Unfortunately, direct registration of DNA-synthesizing activity by conventional method is not possible for most species of hibernators. The dormouse (Glis gfis) is one of a few species of hibernators whose cells can incorporate tritiated thymidine and other labeled DNA-precursors (Adelstein et al., 1967; Koleva et al., 1980; Vinogradova, 1982). However, most results obtained by indirect methods (analysis of mitotic activity and flow cytometry) in different tissues of some species of hibernators are in agreement with the findings of Adelstein et al. (1967). Thus, fluorishing of mitoses and marked rise of mitotic index 2-3 hr after initiation of arousal (the temperature reaches 34-36’C) may be the result only of a preceded blockage at Gz or late S but not at G,. A much greater time period is needed for the cells blocked in G, to enter mitosis. The results of Jaroslow et al. (1976) are not in agreement with those obtained by other authors. It was found that the number of mitotic figures per crypt is lowest 3-9 hr after initiation of arousal, and the count does not reach the level of active control animals until 18 hr after arousal has begun. Because of the long delay between arousal and the peak of mitosis, it is concluded that the cells are blocked in G, There are data indicating the accumulation of the cells in some renewing tissues of the ground squirrel Citellus suslicus, with a low rate of cell renewal in one G,/G, phase of the cell cycle during hibernation, i.e. the complete absence of Sand G,-cells (Kolaeva et al. 1980). However, the studies of the tissue with a high rate of cell renewal (epithelia of intestine in ground squirrel, Citellus undulatus) using flow cytometry have shown that the level of DNA-synthesizing cells during hibernation is greatly reduced but not absent (Kruman et al., 1986). Thus, there are S-cells in the epithelia of intestine of torpid animal, which is impossible with a complete blockage in G,. Probably, marked seasonal reduction of the level of cells entering the mitotic cycle during hibernation may complicate the detection of S-cells in

14

INNA

I.

tissues with a low rate of ceil renewal, where the initial level of these cells is low. It makes the tissues with a low rate of cell renewal not su~~ientty adequate for studying the dynamics of proliferative activity in hibernators. Thus, the results obtained by direct method (incorporation of labeled DNA-percursors) and most data obtained by indirect methods indicate that the entry into the mitotic cycle as well as the DNA-synthesis continue in the renewing tissues of hibernators during a bout of hibernation. The mitotic cycle is not completed. There is a blockage in the premitotic phase. It should be noted that a large drop of core temperature of hibernators almost to O’C during hibernation does not lead to a complete cessation of DNA (Gordon et af., 1987) and protein synthesis (Derij and Stark, 1985). Thus, low core temperature during torpor can hardly be interpreted as an insurmountable barrier for progression of cells through the mitotic cycle, In view of this it seems relevant to consider the effect of h~pothe~ia on the cell growth in homoiothe~ic animals. THE EFFECT OF HYPOTHERMIA ON THE CELLS OF HOMOiOTH~RMtC ANIMALS IJ~ VZVO AND Zi\’ VITRO

The hypothermia response of different mammalian cells in citra is cell cycle phase-dependent. Hypothermia leads to a reduction in the rate of progression through all the periods of interphase, It has the greatest effect on the progression through mitosis. The spindle and/or centriolar components in metaphase ceils are sensitive to cold treatment (Alov, 1982). Whether or not this effect on the cells is reversible depends upon the degree of cooling and the length of exposure. The exposure to suboptimal temperatures (20-24°C) for short periods of time (some hours) produces reversible changes in mitotic cells (Rao and Engelberg, 1968; Boltovskaya, 1977). The long-term exposure to low temperature (0 to +-2°C) causes irreversible changes (Hampel and Levan, 1964; Rao and Engelberg, 1966; Nagasawa and Dewey, 1972), which can be associated with the destruction of spindle microtubuies (Alov, 1982). After cold treatment the number of pathological mitoses rises (Portugalov et al., 1968). The hypothermia response of proliferating mammalian cefis it? &a is distinct from that & t:itro. For exampb, 13 hr hypothe~ia (t 5-I 7°C) produces arrest in G, of the duodenal epithelium cells in newborn rats (Lebedeva and Savarzin, 1966). A similar cold-induced block is produced in cornea1 epithelium and in bone marrow of adult rats (Simon et al., 1979; Timoshin et al., 1979). The void-induced G,-block was found also in epidermis of pads and ear tips in mice. These tissues are greatly subjected to environmental temperature effects (Gelfant, 1962; Adelstein er aL, 1967). There is some evidence indicating that the antimitotic effect observed in mammals in response to hypothermia as a stress-induced condition represents an adaptive reaction (Epifanova, 1965; Timoshin, 1983). The antimitotic effect in homoiothermic animals may be the result of different stress-induced conditions (Epifanova, 1965; Timoshin and Afekseenko, 1974; Timoshin, 1983). It is known that the adrenal hormones released into circu-

KRUMAN

lation under stress are responsible for the antimitotic effect (Alov, 1964; Epifanova, 1965). For example, in the case of adrenale~tomi2ed rats, stress-induced conditions, although having no effect on a delay in GZ, caused an increase in the frequency of abnormal mitotic cells (Ryabucha, 1958; Timoshin et al., 1979; Timoshin, 1980) as observed in the cold treatment of cultured cells. The attempts to induce the antimitotic effect by stress in newborn animals until an appointed stage of their maturation were unsuccessful (Suvorova, 1955, 1956b; Lebedeva and Savarzin, 1966; Valvas, 1976). It may imply that the hypothalam~hypophyseal-adrenal system in newborn homoiothermic animals, until an appointed stage of their development, has a low sensitivity to stress conditions due to under-development of this system at early stages of ontogeny (Koryakina, 1987). No C&-blockage is present in tissues of poikilotherms under long-term exposure to the low temperature (+ 3’C) (3arber and Callan, 1943; Suvorova, 1955; Suvorova, 1956b). It may indicate that the antimitotic effect induced by stress conditions appears on appointed stages of ontogeny and phylogenesis. The hypothermia and probably other stress conditions lead in z+tro to interruption of normal mitosis progression and different irreversible destructions of mitotic apparatus, which in turn induces chromosome mutations and irregular distribution of chromatin between daughter cells. This is one of the basic nle~han~srns of aneuploidy and a~&umulation of genetic heterogeneity in cell populations (Kasantseva, 198 t )_The stress effect on the poputation of proliferating cells in 2%~)mediated by the whole organism leads to an antimitotic effect which is probably a part of the adaptive syndrome. It prevents the cells from reaching mitosis during stress-induced canditions and thus interruption of normal mitosis. The excessive stress-induced effect ~h~pothermia) (Timoshin, 1983) or moderate stress-induced effect on adrenalectomized animals (Ryabucha 1958; Timoshin st al., 1979; Timoshin, 1980) result in the deadaptation due to the absence of antimitotic effect and the appearance of abnormal mitotic cells. This is further evidence that the antimitotic effect can be regarded as an adaptive reaction. Most of the available studies on adrenal cortex in hibernators are based on histological observations (Ilyasova, 1385). But what are the conditions of adrenal cortex hormones secretion during hibernation‘? Aldosterone is secreted in hibernators at the low body temperature during bouts of hibernation (Kastner et cd., 1978). There is probably no secretion of glucocorticoids in torpid animals. It is quite possible that these hormones are produced during spontaneous arousals (Ilyasova and Kolaeva, 1986). The high level of glucocorticoids known from some studies (Gustafsan and Belt, 1981) seems to be due to diminished tissue utilization during bouts of hibernation (Khabibov and Krass. 1976). There have been a few data on the concentration of corticosteroids in hibernators and they are contradictory. Thus. to our knowledge they may not provide the strong evidence that glucocorticoids are involved in an antimitotic effect seen in hibernators. However, it should be reemphasized that hibernation cannot occur in adrenalectomized hibernators (Vidovic and Popovic,

15

Cell replacement in hibernators Block

Endogenous seasonal control

v

I

G1

S

,

G2

,M,

4

The rate of progressing through the cell cycle

*

Control related to the hibernation state

-I

I Block

Fig 1453, 1954) and injection of cortical hormones into adrenalectomized animal restores the ability to hibernate (Popovic, 1960). This may imply that corticosteroids play a most important role in the changes accompanying the entry into the torpor, including antimitotic effect. It would be reasonable to suppose that the block in Gz is not induced in hibernators ruder natural hypothermia during a bout of hibernation as well as under artificial hypothermia in h(*moiothermic animals by direct temperature effects OIL proliferating cells, but is rather the result of independence of cell regulation. This seems to represent an adaptive reaction. The removal of stressin,duced conditions both in the homoiothermic arimal and hibernator during arousal leads to the overcoming of premitotic blockage and entry into mitosis. However, a low number of mitotic cells are present in the renewing tissues of hibernators during bouts of hibernation (Mayer, 1960; Suomalainen and Ota, 1967; Unker and Alekseeva, 1974). These mitoses are abnormal (Saetersdal, 1963; Adelstein et al., 1967; Suomalainen and Oja, 1967; Vinogradova, 1!%6). It is known that metaphase blockage and production of abnormal mitoses are responses to h,rpothermia dividing cells (Alov, 1982). The metaphase blockage is observed in poikilotherms at Icw temperature during torpor (Barber and Callan, 1!‘43). Premitotic blockage of cells as the hypothermia response is not typical for poikilotherms (Barber arid Callan, 1943; Suvorova, 1955; Suvorova, 1!)56a,b). If intermitotic blockage in poikilotherms was accompanied by the appearance of abnormal mitoses, it would not lead to a marked production of genetic damage caused by this blockage since, in p(>ikilotherms, in contrast to mammalian hibernators, torpor is not interrupted for a long time. It has been shown for blood-forming tissues of a frog (Rana temporaria) the life time of an erythrocyte in an amphibian is much longer than the period of their hibernation. Therefore these animals need no special mechanism for preservation of erythrocyte population during the winter. The erythrocyte renewal occurs during 3-4 summer months (Gorishina, 1986). In contrast, the hibernators can maintain tissue re-

newal during hibernation due to periodic arousals accompanied by core warming. The mitotic cells do not accumulate during hibernation (Adelstein et al., 1967; Suomalainen and Oja, 1967). Thus, the low number of mitotic cells observed in cell-renewing tissues of hibernating animals may be interpreted as indicating that the intermitotic block occurs just during entry of animal into the hibernating state. Unfortunately, there are few data on the level of mitoses as a part of cells entered mitotic cycle in hibernators. Even if all these mitoses are abnormal it is quite possible that their level will not exceed that of normal tissues in nonhibernators. CONTROL OF CELL REPLACEMENT HIBERNATORS

IN

Control of cell replacement in hibernators is summarized in Fig. 1. As shown in the figure, the proliferative processes in hibernators are subject to seasonal control that is independent of the hibernation state per se, body temperature, metabolic activity, etc. The growth control is endogenous. It occurs due to the rise or reduction of the level of cells entering the mitotic cycle. This level falls from a maximum in the summer through to an intermediate autumn level and to a minimum during deep hibernation. It increases again through the intermediate value during the time of reactivation to its peak in the summer. The seasonal depression of the proliferative activity is probably closely associated with the seasonal involution of the thymus. Cell renewal in hibernators is controlled also by the factors concerned with the hibernation state per se. The progression of cells through the mitotic cycle during the bout of hibernation does not stop, but its rate is markedly reduced. The cells are blocked in the premitotic phase and accumulate at this phase throughout a bout. We suggest that the block in Gz under natural deep hypothermia throughout a bout of hibernation as well as under artificial hypothermia and other stress-induced conditions in homoiothermic animals, is the result of independence of cell regulation and represents the adaptive reaction. As in

16

INNAI. KRUMAN

homoiotherms, the removal of stress-induced conditions in hibernators during arousal leads to overcoming

of premitotic

blockage

and entry

into mitosis.

REFERENCES

Adelstein S. J., Lyman C. P. and O’Brien R. C. (1967) Cell proliferation kinetics in the tongue and intestinal epithelia of hibernating dormice, G/is g&s. fn mammalian Hibernation 111 (Edited by Fisher K. C., Dawe A. R., Lyman C. P., Schiinbaum E. and South F. E. ), pp. 3988408. Oliver and Boyd, Edinburgh. Alekseeva G. V. and Unker V. M. (1970) A cytophysiological study of the tissue of the souslik lymph nodes in autumn-winter season Zh. Ubshch. Biol. 31, 361-364. Alekseeva G. V. and Unker V. M. (1977) Seasonal histostructural variations of thymus in ground squirrel (Citeilus erythrogenus). Isv. Sib. Otd. Acad. Nauk SSSR. Ser. Biol. 2, 1255129. Alov I. A. (1964) Articles about Physiology ofMitotic Cell Division, Medicine, Moscow. Alow I. A. (1982) M~hanisms of divided cell reaction on hypothermia. Usp. Sonr. B&l. 93, 6472. Andrew U. (1972) Ageing histological changes in intestinal epithelia of amphibians and mammals. Proc. IV-Zntern. Congr. Gerontology, pp. 399. Kiev. Babaeva A. G. (1972) immunological Mechanisms ofControl of Regeneralion Processes. Medicine, Moscow. Babaeva A. G.. Nesterenko V. G. and Udina N. V. 11982) Increased capacity of lymphocytes of mice after repeated liver resection to transfer “regenerative information”. Bull. exp. Biol. Med. 93, 98-99. Baker S. J., Mathan V. J. and Cherian V. (1963) The nature of the villi in the small intestine of the rat. Lancet 1, 860-86 1. Barber H. N. and Callan H. G. (1943) The effect of cold and colchicine on mitosis in the newt. Proc. R. Sot. Ser. B. 131, 258-271. Barr R. E. and Mussachia X. J. (1967) Radiation senitivity of the hibernating ground squirrel, Citellus tridecemlineal~~.~.Proc. Sot. exp. Biol. Med. 124, 120441207. Boltovskaya M. N. (1977) Eheect of suboptimal temperature on the mitotic regimen. Bull. exp. Biol. Med. 83,216-218. Brock M. A. (1960) Production and life span of erythrocytes during hibernation in the golden hamster. Am. Physiol. 198, IiSl- 1186. Burlington R. F. (1972) Recent advances in intermediatory metabolism of hibernating mammals. In Hibernation, Hypothermia Perspectives and Challenges (Edited by South F. E., Hannon J. P., Willis J. R., Pengelley E. T. and Alpert N. R.), pp. 3-.lS. Elsevier, New York. Burlington R. F. and Klain G. J. (1967) Gluconeogenesis during hibernation and arousal from hibernation. Conlp. Biochem. Physiol. 22, 701.-708. Dauova G. M. and Usatenko M. C. (1970) Activity of gluconeogenesis liver enzymes in ground squirrel (Citellus suslicus). Zh. Evol. Biochem. Physiol. 6, 35-41. De Laey P. (1966) Development of the intestinal digestion mechanism of starch as a function of age in rats. Nature 212,

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