[Histological and histomorphometric examination of tibiae of Mongolian gerbils Meriones unguiculatus of different age as possible objects in space experiments].

Histological and histomorphological examinations of tibiae taken from young (body mass = 27 g) and adult (body mass = 73 g) male Mongolian gerbils Meriones unguiculatus was fulfilled. Tibial bones of Mongolian gerbils have a hockey-stick form and are extremely thin. Morphologically the gerbil's tibial bone is very similar to that of rat. The cartilaginous growth plate in the epiphysis is divided into the areas of dominance, proliferation, and hypertrophy/calcification. In young gerbils, height of the cartilaginous growth plate is thrice as large as in adult animals. Immediately under the growth plate there is an area of primary spongiosis consisting of tightly packed thin bone trabeculae with a cartilage in the center. In adult gerbils, trabeculae in the primary spongiosis are less numerous as compared with young animals; they are thicker and more apart. In young gerbils, primary spongiosis is higher than in adult animals. The primary spongiosis region grades into ill-defined secondary spongiosis. Not numerous trabeculae of the secondary spongiosis are thicker comparing with trabeculae of the primary spongiosis, more separated from one another and fully ossified. In adult gerbils, secondary spongiosis is a bit larger than in young animals, mostly due to thickening of the bone trabeculae.

[Histological study of the visceral organs of Mongolian gerbil Meriones unguiculatus as a subject in spaceflight experiments].

Survey histological study of the heart, lung, liver, kidney, pancreas, adrenals, thymus, spleen, testicles of the Mongolian gerbil Meriones unguiculatus with a body mass of about 27 g showed their macro- and microscopic similarity with the organs of laboratory rats and mice notwithstanding some slight differences. For instance, the ascending knee of Hengle's loop in the gerbil kidney is much better developed and forms in whole a kind of a singular cortical fiber bordering the medulla. It is the well-developed parts of Hengle's ascending loop in gerbil that ensures a more complete water reabsorption decrease the quantity of urine and sharply reduce the amount of exogenous fluid vitally important for animals in arid areas. The Mongolian gerbil is distinguished by large adrenals and small corticosteroid-sensitive thymus and spleen suggesting high sensitivity of this animal to stresses. Spleen abundance of both mature and immature megacariosities--thrombogoniums--explains the rapid coagulability as compared with rats and mice.

[Role of insufficient hormone production in development of osteopenia in consequence of physical loads defficiency].

Experiments with rats deprived of support loading on hind limbs by tail-suspension showed that injection of hormones participating in bone metabolism impeded the development of tibial spongy osteopenia; also, normalization of longitudinal bone growth was observed in several cases. Investigations were aimed to evaluate the effectiveness of growth hormone, sex hormones, thyreoid hormones, calcitonin, CNS stimulating ephedrine and strychnine, and graded support loads. The best results were obtained after injection of calcitonin combined with sinestrol (synthetic analog of estradiol) and graded support loads which acted as initiators and amplifiers of hormonal effects. The combination of calcitonin, sinestrol and support loads prevented osteopenia and provided the normal bone growth in length during rat's suspension.

[Dynamics of the structural recovery of the rat's tibia from suspension-produced osteopenia].

The qualitative histomorphometric techniques were used to study the dynamics of tibia structural recovery from osteopenia in 30 female rats previously suspended by tail for 30 days. The suspension was found to have caused osteopenia in the tibial proximal metaphysis that was fully gone only after 60 day; in other words, readaptation took twice as much time as the osteopenia development. This fact should be taken into consideration by designers of rehabilitative methods.

[Action of estradiol, calcitonin and weight-loading on the development of osteopenia in tail-suspended female rats].

Experiments with tail-suspended female rats demonstrated that injection of sinestrol (synthesized estradiol) and myocalcic (sermon calcitonin) on a background of graded weight bearing (suspension interruption for 2 hrs. a day) prevented osteopenia in the spongy metaphyses of the shin-bone deprived of loading.

Morphological studies of bone and tendon.

The Soviet biosatellite COSMOS 2044 carried adult rats on a spaceflight that lasted 13.8 days and was intended to repeat animal studies carried out on COSMOS 1887. Skeletal tissue and tendon from animals flown on COSMOS 2044 were studied by light and electron microscopy, histochemistry, and morphometric techniques. Studies were confined to the bone cells and vasculature from the weight-bearing tibias. Results indicated that vascular changes at the periosteal and subperiosteal region of the tibia were not apparent by light microscopy or histochemistry. However, electron microscopy indicated that vascular inclusions were present in bone samples from the flight animals. A unique combination of microscopy and histochemical techniques indicated that the endosteal osteoblasts from this same mid-diaphyseal region demonstrated a slight (but not statistically significant) reduction in bone cell activity. Electron-microscopic studies of the tendons from metatarsal bones showed a collagen fibril disorganization as a result of spaceflight. Thus changes described for COSMOS 1887 were present in COSMOS 2044, but the changes ascribed to spaceflight were not as evident.

Preosteoblast production in COSMOS 2044 rats: short-term recovery of osteogenic potential.

The influence of a 13.8-day spaceflight and approximately 8.5-11 h of recovery at 1 g on fibroblast-like osteoblast precursor cells was assessed in the periodontal ligament of rat maxillary first molars. Preosteoblasts (C + D cells), less differentiated progenitor cells (A + A' cells), and nonosteogenic fibroblast-like cells (B cells) were identified by nuclear volume analysis (i.e., A + A' = 40-79 microns 3; B = 80-119 microns 3; C + D greater than or equal to 120 microns 3). No differences were observed among flight (F), synchronous (SC), vivarium, and basal control groups in the A + A' (F: 28.0 +/- 3.7 vs. SC: 27.4 +/- 2.2), B (F: 33.1 +/- 1.4 vs. SC: 32.4 +/- 2.4), or C + D (F: 38.4 +/- 4.5 vs. SC: 39.2 +/- 1.6) cell compartments (mean +/- SE, n = 5). Compared with previous spaceflight experiments, the present data are consistent with a postflight response to replenish preosteoblasts and restore periodontal ligament osteogenic potential. These data emphasize the need to 1) unequivocally determine the flight effect by killing the animals in-flight and 2) further assess the postflight recovery phenomenon.

Effects of microgravity on the composition of the intervertebral disk.

The lumbar annuli of rats flown on COSMOS 2044 were compared with those of three control groups and a tail-suspension experimental model. The wet and dry weights of the annuli were significantly smaller (P less than 0.05) in the flight group than in three control groups. The collagen-to-proteoglycan ratio was significantly greater (P less than 0.001) in the flight group than in the three control groups, but there were no detectable changes in the relative proportions of type I and II collagen or in the number of pyridinoline cross-links. When the annuli were immersed in water for 2 h, more proteoglycans (P less than 0.001) leached from the annuli of flown rats than from the tissue of control animals, suggesting abnormal or smaller proteoglycans. Safranin-O indicated a normal spatial distribution of the proteoglycans within the annulus. Tail suspension did not affect the size of the annuli, but more proteoglycans (P less than 0.05) leached from the tissue of suspended animals than from the normal annuli. The reasons for smaller disks and the abnormal ratio between the fibrous collagenous network and the proteoglycan gel in the flight group are unknown at this time. It is, however, probable that these changes may affect the biomechanical functions of the annulus, although they may be temporary and totally reversible if injuries are avoided in the interim period.

Adaptations of young adult rat cortical bone to 14 days of spaceflight.

To determine whether mature humeral cortical bone would be modified significantly by an acute exposure to weightlessness, adult rats (110 days old) were subjected to 14 days of microgravity on the COSMOS 2044 biosatellite. There were no significant changes in peak force, stiffness, energy to failure, and displacement at failure in the flight rats compared with ground-based controls. Concentrations and contents of hydroxyproline, calcium, and mature stable hydroxylysylpyridinoline and lysylpyridinoline collagen cross-links remained unchanged after spaceflight. Bone lengths, cortical and endosteal areas, and regional thicknesses showed no significant differences between flight animals and ground controls. Our findings suggest that responsiveness of cortical bone to microgravity is less pronounced in adult rats than in previous spaceflight experiments in which young growing animals were used. We hypothesize that 14 days of spaceflight may not be sufficient to impact the biochemical and biomechanical properties of cortical bone in the mature rat skeleton.

Effect of a 22-day space flight on the lymphoid organs of rats.

The effect of a 22-d space flight on the lymphoid organs of rats has been investigated. It has been shown that the flight leads to hypoplasia of lymphoid organs, the spleen and thymus weight decreasing very noticeably. Histological, morphometric, and cytological examinations have demonstrated that hypoplasia of the spleen occurs due to a reduction of the number of lymphocytes and erythroid cells and hypoplasia of the thymus and lymph nodes due to a decrease of the lymphocyte number. Changes found in the lymphoid organs of the flight rats are reversible and the structure of lymphoid organs recovers 27 d postflight. Pathogenetic mechanisms of the changes developing in lymphoid organs of rats under the influence of space flight are discussed. The animal data are compared with the results of postflight medical examinations of astronauts.

Discussion of the combined effect of weightlessness and ionizing radiation on the mammalian body: morphological data.

The combined effect of weightlessness and ionizing radiation, from the Cs137 source at 800 rads for 24 h, on the animal body was studied. The morphological examination of organs and tissues of rats flown aboard the biosatellite Cosmos-690, kept in the ground-based simulation experiment, and kept in the vivarium, indicated prevalence of radiation-induced changes in both experimental groups of rats. An exposure of animals to space flight factors did not produce a substantial aggravation of radiation-induced effects. This is indicated by the lack of significant differences in the weight of testes, thymus, and spleen of flight and simulation rats. However, this exposure affected adversely the development of reparative processes in the hemopoietic tissue of bone marrow. Inflight irradiation aggravated weightlessness-induced changes. A combined effect of weightlessness and irradiation did not result in the summation of the effects exerted on skeletal muscles by either factor alone.

Biochemical and morphological stress-reactions in humans and animals in microgravity.

This article is a literary review focused on the problem of the stress-effect of microgravity. Based on the all-round analysis of data from manned missions and space experiments with rats it is concluded that microgravity as a permanent factor of space flight does not produce an intense chronic stress in either humans or animals. On the other hand, microgravity is responsible for deconditioning of a number of vital systems and of the organism as a whole. On return to Earth, the deconditioned bodies of humans and animals exaggerate the usual terrestrial loads due to gravity forces and respond by acute gravitational stress.

[The use of large doses of retabolil and support loads for prevention of osteopenia in male rats during suspension]. / Ispol'zovanie bol'shikh doz retabolila i opornykh nagruzok dlia profilaktiki osteopenii u krys-samtsov pri vyveshivanii.

The histological and histomorphometric techniques were used to evaluate effects of anabolic steroid rhetabolil and graded support loads on the osteopenia progress in suspended rats. Thirty-day head-down tail suspension was found to inhibit body mass gain, induce a moderate stress reaction, dampen the longitudinal bone growth and provoke osteopenia in the spongy metaphysis of rats' tibiae. Rhetabolil injection at a total dose of 10 mg per 100 g of the body mass (a large dose) prevented tibial osteopenia, and yet it hampered bone growth in length. Daily 3-hr. support loads also inhibited osteopenia in the suspended rats, especially when combined with the rhetabolil injections. The unfavorable side-effect of the dose was virtually complete thymus aplasia in all animals. Hence, large rhetabolil doses as a means against osteopenia in the inadequately loaded locomotor system should be given with great care and require further investigations.

[Comparative histomorphometric studies of shin bones of male and female rats following tail suspension]. / Sravnitel'noe gistomorfometricheskoe issledovanie bol'shebertsovykh kostei u samtsov i samok krys pri vyveshivanii za khvost v antiortostaticheskom polozhenii.

In a 30-d experiment with tail-suspension of male and female rats histomorphometric studies were performed to compare impacts of deficient weight loading on the tibial spongy tissue. The 30-d suspension was found to slow down the body and adrenal mass gain irrespective of sex; however, this effect was more pronounced in male rats suggesting a greater stress to males as compared to females. Also, suspension caused tibial osteopenia in either of sexes. Decreases in the tibial spongy mass in suspended male and female rats were similar to control. At the same time, inhibition of the longitudinal bone growth attested by diminished height of the cartilaginous growth plate was more noticeable in males as compared with females. We attribute this observation to the reduction in testosterone production and a more violent stress reaction.

[Effect of ephedrine and support loads on development of osteopenia and growth of shin bones in suspended rats]. / Vliianie éfedrina i opornykh nagruzok na razvitie osteopenii i rost bol'shebertsovykh kostei u krys pri vyveshivanii.

Effects of graded support loads and stimulators of the central nervous system (ephedrine and strychnine) on growth of bones and the volume of spongy bone tissue in the proximal metaphyses of the shin bones were studied in the 14-day experiment with rats suspended by tail in the orthostatic position. It is indicated that the support loads in combination with ephedrine injection prevent the suspension-induced inhibition of bone growth and development of osteopenia of spongiosa of metaphyses. The preventive effect of support loads and ephedrine relates with their stimulating influence on the functional activity of the STH-cells of adenohypophysis producing the growth hormone.

[The healing of bone fractures in rats during the readaptation after a 14-day head-down suspension]. / Zazhivlenie perelomov kostei u krys v periode readaptatsii posle 14-sutochnogo vyveshivaniia.

Histological, histochemical and histomorphometric methods have been used to study an effect of a 14-day head-down suspension on healing the fractured fibular diaphysis in the rats traumatized immediately after suspension. The study of the fibulae on Days 11 and 17 following an operative fracture indicated that both in control and experimental rats the ends of fractured bones were tightly interconnected by the calluses. A comparative study of the calluses and their structural components as well as detailed investigation of histological structure of calluses on the 11th and 17th post-traumatic days did not reveal significant differences between control and experimental animals. Furthermore, the rate of cartilage replacement by spongy bone tissue in calluses of experimental animals was even somewhat higher than that in control rats. Thus the data obtained give ground to believe that in a readaptation period after 14-day suspension resulting in the osteoporotic changes of weight-bearing bones a repair process in the bones is rapidly normalized and in some parameters exceeds it in control animals.

[Adaptation to weightlessness and stress]. / Adaptatsiia k nevesomosti i stress.

The literature on the problem of stress-inducing effect of weightlessness is reviewed. On the basis of scrupulous analysis of the data accumulated in the rat flight experiments aboard the Cosmos biosatellites and the US Space Life Sciences Laboratory (SLS-2) it is concluded that although the ever-acting factor of space mission, microgravity does not cause any intensive chronic stress to animals. Nonetheless, it contributes to the development of deconditioning of a number of vitally important systems and the body as a whole. On return to Earth, the usual terrestrial loads due to the gravity effects are perceived by the deconditioned animals as extraordinary and they respond by severe gravitational stress.

[Healing fibular bone fracture in rats during extended suspension ]. / Zazhivlenie pereloma malobertsovoi kosti u krys pri dlitel'nom vyveshivanii.

Healing of peroneal bone fracture was studied histologically and histomorphologically in rats exposed to a 30-d tail-suspension. To fracture bones, anesthetized rats were operated on day 15 of suspension. Hind limb deprivation of the support loading for a period of 30 days minimizes healing processes in fractured peroneal bones as was evidenced by undersized callus and lowered strength of bone consolidation. Mass-metric and histomorphometric investigations showed that, as compared with the control, on day 15 of regeneration the callous mass and volume in the suspended rats were by 34% and 35% less, respectively; the newly formed spongy bone, the fibrous-cartilaginous tissue and the cartilage were by 44%, 70% and 86% smaller in size, respectively. Nonetheless, healing dynamics in the suspended rats was not disturbed and the calluses were on the same phase of development as in the control, which is attested by similarity of the histological picture. Based on the comparative retrospective analysis of fracture healing in rats in 15- and 30-d experiments, the extended (30-d) suspension did not further impede the post-traumatic bone regeneration. It is suggested that minimization of bone fracture healing during the 15-d suspension was conditioned not only by inhibition of the growth hormone production but also by an acute stress reaction in the early period of adaptation.

[Age-related characteristics of osteopenia in rats caused by deficient support loads on hind limbs]. / Vozrastnye osobennosti razvitiia osteopenii u krys pri defitsite nagruzok na zadnie konechnosti.

Progress of osteopenia in 40-, 100-, and 220-d old rats who had been suspended by tails to release support loading on their hind limbs for 30 days was compared. Deficient support loading was shown to inhibit the longitudinal growth of tibia and produce osteopenia of the proximal metaphysis spongiosis in all groups of rats. Humerus of the rats whose front limbs were still partially loaded, suffered same changes as tibia though much less pronounced. Inhibition of bone growth in length and osteopenia in bone spongiosis reached maximum dimensions in 220-d old rats as a result of summation of kindred changes due to the load deprivation, and aging. Established was a correlation between suppression of the functional activity of STH-cells in adenohypophysis and progress of bone atrophy during suspension. Functional activity of thyroid calcitonin-producing cells was also reduced with the maximum decline in the 40-d old rats.

[Stimulation of bone fracture healing in rats under the support load deficit]. / Stimuliatsiia zazhivleniia perelomov kostei u krys pri defitsite opornykh nagruzok.

Experiment with rats the hind limbs of which were deprived of support loads (tail suspension) for two weeks following operational fracture of fibular bones evidenced inhibited callus formation and lowered strength of bone fragments consolidation. Injection of anabolic steroid retabolite to the traumatized animals together with limited support loading was favorable to the bone callus formation and consolidation of fragments. This measure also distinctly increased the functional activity of adenohypophysial somatotrophs (growth hormone producers) and thyroid C-cells (calcitonine producers). The direct dependence of bone fracture healing on the somatotropic function of the adenohypophysis suggests that the favorable effect of retabolite combined with the support loading is at least partly achieved through the somatorophic function of the adenohypophysis.