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.

We evaluated the effects of therapy with a small dose (0.5 Iug/day) of 1,25-dihydroxyvitamin D3 in 12 previously untreated patients with postmenopausal osteoporosis. Combined radiocalcium kinetic and balance studies showed that at base line, net calcium absorption (mean ± SEM) was low (7 + 3%), calcium balance was negative (-59 ± 22 mg/day), and bone resorption rate (297 ± 40 mg/day) exceeded bone formation rate (239 ± 36 mg/day). After short-term therapy (6-8 months), calcium absorption was normal (27 ± 3%, P < 0.001 for difference from base line), calcium balance had improved (+2 + 26 mg/day, P < 0.05), and bone resorption rate (195 ± 35 mg/day, P < 0.01) had decreased more than bone formation rate (197 ± 26 mg/day, NS). Compared with base-line values, after long-term therapy (2 years) the increased level of calcium absorption was maintained (27 ± 3%, P < 0.001), but calcium retention declined to a level intermediate (-27 ± 24 mg/day, NS) between base-line and shortterm treatment values, and both bone resorption rate (294 ± 33 mg/day, NS) and bone formation rate (267 + 34 mg/day, NS) increased. Urinary hydroxyproline excretion was lower than before treatment (26.3 ± 2.0 mg/day) after both short-term (21.1 ± 1.6 mg/day, P < 0.001) and long-term (22.0 ± 1.8 mg/day, P < 0.01) treatments. Trabecular bone volume was 11.3 ± 1.0% at base line and increased to 16.0 ± 1.2% (P < 0.01) after long-term treatment. By contrast, the eight patients studied before and after 6-8 months of placebo treatment had no significant change in any variable.Many investigators have found that a substantial proportion of patients with postmenopausal osteoporosis have impaired intestinal calcium absorption as assessed either by metabolic balance or by absorption ofradiocalcium (1-6). We have previously reported (6) that patients with postmenopausal osteoporosis have a significant decrease in mean serum 1,25-dihydroxyvitamin D [1,25(OH)2D], the major hormonal factor controlling calcium absorption. Some (7-9) but not all (10) investigators also have found that serum 1,25(OH)2D was low normal or low in patients with osteoporosis. Another group (11) reported that, when patients with osteoporosis were stratified on the basis of calcium absorption, those with calcium malabsorption had serum 1,25(OH)2D levels that were significantly lower than normal. In that same study, the women with osteoporosis had no change in serum 1,25(OH)2D levels in response to a low-calcium diet, whereas the normal control subjects had significant increases.It is not known whether the impaired calcium absorption contributes to the negative calcium balance characteristic of osteoporosis. We have previously reported that treatment with a small dose of 1,25(OH)2D3 for 7 days normalizes calcium absorption in patients with this disorder (6). The current study was undertaken to determine whether this result could be maintained with continued treatment and, if so, whether it would have a beneficial effect on abnormal bone and calcium metabolism characteristic of untrea...

No abstract

No abstract

The population of periodontal ligament (PDL) fibroblasts examined in this study may include osteogenic progenitor cells. PDL fibroblast and osteoblast kinetics in the periodontal ligament of the rat were measured following orthodontic stimulation of bone formation. Both single and multiple injections of tritiated thymidine (3H‐TdR) were used. In single injection experiments, the peak percentage of PDL fibroblasts labeled with 3H‐TdR is 15% at 22 hr post‐stimulation. In multiple injection experiments, the total percentage of fibroblasts in the PDL which respond by synthesizing DNA is 50%. 3H‐TdR‐Iabeled osteoblasts appear at the same rate as, but with a time delay after, the labeled fibroblasts. Following stimulation, the most likely source of osteoblasts at the bone‐forming site is not only fibroblasts which make DNA, divide, then differentiate, but also fibroblasts which either are differentiated to osteoblasts without DNA synthesis and cell division, or are released from G2 block by the orthodontic stimulation.