Inhibition of bone formation during space flight.

Parameters of bone formation and resorption were measured in rats orbited for 19.5 days aboard the Soviet Cosmos 782 biological satellite. The most striking effects were on bone formation. During flight, rats formed significantly less periosteal bone than did control rats on the ground. An arrest line at both the periosteum and the endosteum of flight animals suggest that a complete cessation of bone growth occurred. During a 26-day postflight period, the defect in bone formation was corrected. No significant changes in bone resorption were observed.

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.

Alterations in calcium homeostasis and bone during actual and simulated space flight.

The weightlessness experienced in space produces alterations in calcium homeostasis. Gemini, Apollo, and Skylab astronauts exhibited a negative calcium balance due primarily to hypercalciuria. In addition, the bone mineral density of the calcaneus declined by approximately 4% in Skylab crew members after 84 d of orbital flight. The negative calcium balance and loss of calcaneal bone mineral in normal adults subjected to prolonged bed rest was comparable to that observed in space. The pathogenesis of bone loss during space flight and bed rest is not well understood due to the lack of histomorphometric data. It is also uncertain whether osteoporotic changes in astronauts are corrected postflight. The observed bone loss would be reversible and of no long-term consequence if the only abnormality was an increased remodeling rate. However, altered bone cell activity would probably result in irreversible bone loss with the premature development of senile osteoporosis many years after space flight. The main skeletal defect in growing rats placed in orbit aboard Soviet Cosmos biosatellites appears to be diminished bone formation. Bone resorption was not elevated during weightlessness. Although cortical bone returned to normal postflight, the decline in trabecular bone mass was somewhat persistent. These studies established that the modeling of a growing skeleton was altered in a weightless environment, but do not necessarily imply that a remodeling imbalance occurs in adults during space flight. However, various forms of simulated space flight inhibited bone formation during both skeletal modeling and the remodeling of adult bone.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of phosphate depletion on bone.

Phosphate depletion causes significant changes in the composition of the cell population in bone and the metabolic activities of these cells. The data presented indicate that a vitamin D metabolite has a significant role in producing the increase in osteoclast number associated with phosphate depletion. The increased resorptive activity and number of osteoclasts leads to a marked increase in the rate of bone resorption resulting in the liberation of calcium phosphate, while the decrease in the rates of the processes involved in bone formation (matrix production, osteoid maturation, and mineralization) reduces the amount of phosphate which is removed from the circulation. Thus, all of the effects of phosphate depletion on bone are consistent with the interpretation that bone acts as a reservoir of phosphate and is used to maintain soft tissue and serum phosphate levels at the expense of bone.

Osteoblast histogenesis in periodontal ligament and tibial metaphysis during simulated weightlessness.

According to nuclear size, fibroblast-like cells adjacent to bone surfaces in the periodontal ligament (PDL) and tibial primary spongiosa (PS) were classified as less differentiated progenitors and committed osteoprogenitors (A/A'), nonosteogenic cells (B), or preosteoblasts (C/D). The ratio of A/A' to C/D cells reflects osteogenic status of bone lining tissue. When 83-day-old rats were subjected to simulated weightlessness (S-W) for 17 d and examined for changes in osteoblast histogenesis, PDL and PS cell populations increased in A/A' cells (p less than 0.01; less than 0.05) but decreased in C/D cells (p less than 0.01; less than 0.05) compared to controls. These data indicate that the nuclear volume method, originally developed in PDL, can also be used to assess osteoblast histogenesis in PS of long bones, and that simulated weightlessness in the present experimental context interferes with osteoblast histogenesis. Since the surfaces of both weightbearing (PS) and nonweightbearing (PDL) bones were affected, systemic factors appear important in the gravity-related mechanism of osteoblast histogenesis. Although unloading of the tibia and cephalad fluid shifts occur during S-W, the data attained in this experiment could also be explained by stress and/or cessation of growth in the S-W rats.

Use of impedance plethysmography to continually monitor bone marrow blood flow.

Methods for measuring bone-blood flow are often time-consuming, tedious, single-point measurements which require sacrifice of the animal. An impedance plethysmographic technique is described which can be used to quantify temporal bone marrow blood flow changes. Results obtained with the impedance technique compare favorably with the data from simultaneously administered microspheres. Injection of sympathomimetic drugs produced measurable responses: isoproterenol caused a significant increase in bone marrow blood flow within 1 min and levarterenol decreased bone marrow blood flow. Data obtained with impedance plethysmography suggest that the technique is feasible for multiple measurements on the same animal and that the technique can be used to study acute or chronic changes in bone marrow blood flow following various experimental treatments.

Simulating certain aspects of hypogravity: effects on bone maturation in the non-weight bearing skeleton.

This study reports the effects of simulation of certain aspects of hypogravity (via partial skeletal unloading) on the growth and maturation of the non-weight bearing mandibles of 41-d and 1-yr-old rats. Partial skeletal unloading was effected by elevating the hindquarters (PULEH), and this simulation was controlled with normally loaded animals fed either ad libitum or the average amount of food consumed by the the experimental group (group-mean fed). The chemical status of the mandibles after 10 d or 14 d PULEH closely resembled that of control rats. The younger PULEH rats and their group-mean fed controls demonstrated a trend toward impaired maturation of mineral and matrix moieties; yet the concentrations of calcium (Ca) and phosphorus (P) expressed as a ratio to collagen hydroxyproline content were normally distributed within a density gradient profile which separates the mineral and matrix moieties into various age-dependent fractions. These data demonstrate that 10 d or 14 d PULEH in young or old rats, respectively, is not sufficient to elicit the maturation deficit observed in the mandibles of rats flown for 18.5 d in the Soviet Biosatellite Cosmos-1129. Unless the duration of PULEH is critical, the cephalad fluid shift which is common to PULEH and spaceflight animals cannot be solely responsible for the flight-induced maturation deficit. Because the mandibles of the PULEH rats remain antigravity-postured, the results emphasize the importance of gravity unloading to the impairment of mandibular bone matrix/mineral maturation during spaceflight. Decreased gravity and, hence, gravity unloading cannot be mimicked in ground-based models of hypokinesia.

Inhibition of cortical and trabecular bone formation in the long bones of immobilized monkeys.

The acute effects of immobilization on cortical and trabecular bone formation were studied in juvenile male rhesus monkeys (Macaca mulatta). Four animals were immobilized for two weeks by application of total body casts. Two control monkeys were housed in separate metabolic cages under similar environmental and dietary conditions. Tetracycline derivatives were administered on three separate occasions to label sites of bone formation. The tetracycline-labeling frequency and mineral apposition rate of osteons and trabecular bone surfaces in the humerus and femur were determined. The inhibition of bone formation induced by immobilization was more pronounced in trabecular bone. Immobilized monkeys exhibited a moderate, but statistically nonsignificant, reduction in the percentage of osteons forming bone. Conversely, the dramatic decline in the percentage of trabecular surfaces undergoing bone formation in immobilized monkeys was found to be highly significant. The diminished rate of mineral apposition in osteons suggested that osteoblastic activity was impaired in cortical bone during immobilization. The mineral apposition rate in trabecular bone could not be determined reliably due to minimal tetracycline deposition, which indicated that osteoblastic activity and/or recruitment almost ceased in the metaphyseal tissue of immobilized monkeys.

Effect of spaceflight on periosteal bone formation in rats.

Male Wistar rats were placed in orbit for 18.5 days aboard the Soviet COSMOS 1129 biological satellite. Tetracycline was administered before and after spaceflight to label areas of bone formation. An inhibition of periosteal bone formation occurred during spaceflight in the tibial and humeral diaphyses, but this defect was corrected during the postflight period. The increased extent of arrest lines at these skeletal sites suggested that periosteal bone formation may have even ceased during spaceflight. The rib exhibited a small but nonsignificant decrease in periosteal bone formation. Endosteal bone resorption was not affected markedly by spaceflight conditions. The observed inhibition of periosteal bone formation may be a result of mechanical unloading, but endocrine factors cannot be ruled out.

Recovery of the rat skeleton from the adverse effects of simulated weightlessness.

A rat model that involves total mechanical unloading of the hind limbs has been used to simulate some aspects of weightlessness. In a previous study, the presence of marked skeletal abnormalities in the proximal tibial and humeral metaphyses of unloaded rats was detected. In the current study, the rats were removed from the model after a 2-week suspension period and allowed to recover for 2 weeks in individual metabolic cages before sacrifice. Tetracycline derivatives were administered on three separate occasions to evaluate radial and longitudinal bone growth. Periosteal bone formation in the tibial diaphysis of suspended animals was significantly depressed during the suspension period but approached control levels during the recovery period. Periosteal bone formation in the humeral diaphysis was not affected by simulated weightlessness. In the proximal tibial and humeral metaphyses, longitudinal bone growth, the amount of mineralized tissue, and the fat content of the bone marrow, all of which were previously determined to be abnormal after 2 weeks of simulated weightlessness, returned to control levels during the recovery period. The previously observed decline in the osteoblast population and increased numbers of osteoclasts adjacent to the growth plate in suspended animals also proved to be reversible at both skeletal sites. This study demonstrates that the skeleton of growing rats is capable of a rapid recovery from the adverse effects of simulated weightlessness.

Skeletal abnormalities in rats induced by simulated weightlessness.

A hypokinetic model has been developed which attempts to simulate the weightlessness experienced during space flight. Male rats (approximately 130 g) were suspended from the model with a head-down tilt for a two-week period. Total mechanical unloading of the hind limbs and partial unloading of the fore limbs occurred. In comparison to pair-fed control rats, the skeletal alterations in the proximal tibial and humeral metaphyses of suspended rats were determined to be a diminished rate of longitudinal bone growth, a reduced mass of mineralized tissue, and an accumulation of marrow fat. Also, suspended rats exhibited decreased numbers of osteoblasts and increased numbers of osteoclasts immediately adjacent to the growth plate-metaphyseal junction at both skeletal sites. Although the reduction in mineralized tissue and the fat accumulation were more marked in the tibia, the skeletal changes in the proximal tibial and humeral metaphyses were generally comparable. The observed abnormalities may be due to mechanical unloading and/or a hypersecretion of corticosteroids.

Proliferation and differentiation sequence of osteoblast histogenesis under physiological conditions in rat periodontal ligament.

To define the mechanism of osteoblast histogenesis, nuclear morphometry was utilized as a marker for precursor cell differentiation. One hour after 3H-thymidine injection, groups of 7-week-old rats were killed at hourly intervals over one complete 24-hr photoperiod (LD 12:12). S-phase and mitosis were assessed in autoradiographs of 3-micron sections of molar periodontal ligament (PDL) adjacent to a physiological bone-forming surface. Labeled nuclei were divided into four categories according to morphometry of nuclear size: A (40-79 micron3), B (80-119 micron3), C (120-169 micron3), and D (greater than or equal to 170 micron3) cells. C and D cells synthesize DNA during the light and divide in the following dark phase; the rhythm for A cells is the opposite. B cells demonstrated no preference and were subsequently determined to be nonosteogenic. Compared to A cells the S-phase photoperiod of C and D cells (combined) is approximately a one-to-one reciprocal relationship, suggesting two proliferating progenitors in series. Based on arrest points in the histogenesis sequence, five compartments are defined: 1) A cells, less differentiated, self-perpetuating precursors; 2) A' cells, committed osteoprogenitors; 3) C cells, G1 stage preosteoblasts; 4) D cells, G2 stage preosteoblasts; and 5) Ob cells, morphologically distinct osteoblasts. Minimal elapsed time for the A----A'----C----D----Ob sequence is about 60 hr (five alternating dark/light cycles). A stress/strain-mediated increase in nuclear volume (A'----C) is an important, rate-limiting step in osteoblast differentiation.

Effects of spaceflight on trabecular bone in rats.

Alterations in trabecular bone were observed in growing male Wistar rats after 18.5 days of orbital flight on the COSMOS 1129 biosatellite. Spaceflight induced a decreased mass of mineralized tissue and an increased fat content of the bone marrow in the proximal tibial and humeral metaphyses. The osteoblast population appeared to decline immediately adjacent to the growth cartilage-metaphyseal junction, but osteoclast numbers were unchanged. These results suggested that bone formation may have been inhibited during spaceflight, but resorption remained constant. With the exception of trabecular bone mass in the proximal tibia, the observed skeletal changes returned to normal during a 29-day postflight period.

Effect of a simulated weightlessness model on the production of rat interferon.

A rat model simulating some aspects of weightlessness was used to determine whether simulated weightlessness might alter interferon production. The optimum time for in-vivo induction of alpha/beta interferon (alpha/beta-IFN) by polyriboinosinic-polyribocytidylic acid was determined to be four hours in normal, mature rats. Rats suspended in the model for two weeks were injected with polyriboinosinic-polyribocytidylic acid and bled four hours later. A dramatic decrease (80%) in alpha/beta-IFN production was observed in those animals exposed to simulated weightlessness as compared to control rats. These data suggest that weightlessness may alter certain immunological functions.

Preosteoblast production 55 hours after a 12.5-day spaceflight on Cosmos 1887.

The influence of 12.5 days of spaceflight and a 55 h stressful recovery period (at 1 g) on fibroblastlike osteoblast precursor cells was assessed in the periodontal ligament (PDL) of rats that were 91 days old at launch. Nuclear morphometry was used as a marker for precursor cell differentiation in 3 microns sections cut in the midsagittal plane from the maxillary first molar. According to nuclear volume, cells were classified as preosteoblasts (C + D cells, greater than or equal to 120 microns 3) and less differentiated progenitor cells (A + A' cells, 40-79 microns 3). Compared with synchronous controls (simulated flight conditions), the 55 h postflight recovery period at 1 g resulted in a 40% decrease in the A + A' cell population, a 42% increase in the C + D cells, and a 39% increase in the number of PDL fibroblastlike cells near the bone surface. These results are consistent with a postflight osteogenic response in PDL. This recovery response occurred despite physiological stress in the flight animals that resulted in a highly significant (P less than or equal to 0.001) increase in adrenal weight. The data suggest that after spaceflight there is a strong and rapid recovery mechanism for osteoblast differentiation that is not suppressed by physiological stress.

Nuclear morphometric analysis of osteoblast precursor cells in periodontal ligament, SL-3 rats.

Five small (55 days old, 196 +/- 5 g) (mean +/- SE) and five large (83 days old, 382 +/- 4 g) Sprague-Dawley strain, specific pathogen-free rats were exposed to a 7-day spaceflight and 12-h postflight recovery period. As measured in 3-micron sections, periodontal ligament (PDL) fibroblastlike cells were classified according to nuclear size: A + A' (40-79), B (80-119), C (120-169), and D (greater than or equal to 170 microns 3). Since the histogenesis sequence is A----A'----C----D----osteoblast, the relative incidence of A + A' to C + D is an osteogenic index. No difference in A + A' or C + D cells in small rats may reflect partial recovery of preosteoblast formation (A----C) during the 12-h postflight period. Large flight rats demonstrated increased numbers of A + A', indicating an inhibition of preosteoblast formation (A----C). At least in the older group, a 7-day flight is adequate to reduce PDL osteogenic potential (inhibition in PDL osteoblast differentiation and/or specific attrition of C + D cells) that does not recover by 12-h postflight.