Low-dose, ionizing radiation and age-related changes in skeletal microarchitecture

Abstract

Osteoporosis can profoundly affect the aged as a consequence of progressive bone loss; high-dose ionizing radiation can cause similar changes, although less is known about lower doses (≤100 cGy). We hypothesized that exposure to relatively low doses of gamma radiation accelerates structural changes characteristic of skeletal aging. Mice (C57BL/6J-10 wk old, male) were irradiated (total body; 0-sham, 1, 10 or 100 cGy (137)Cs) and tissues harvested on the day of irradiation, 1 or 4 months later. Microcomputed tomography was used to quantify microarchitecture of high turnover, cancellous bone. Irradiation at 100 cGy caused transient microarchitectural changes over one month that were only evident at longer times in controls (4 months). Ex vivo bone cell differentiation from the marrow was unaffected by gamma radiation. In conclusion, acute ionizing gamma irradiation at 100 cGy (but not at 1 cGy or 10 cGy) exacerbated microarchitectural changes normally found during progressive, postpubertal aging prior to the onset of age-related osteoporosis.

Figure 1

Mouse body mass as a function of age and radiation exposure. Experiment to determine whether a single dose of ¹³⁷Cs gamma rays causes age-related changes in skeletal structure and bone cell differentiation. Male C57BL/6 mice (10 weeks of age, n = 8/group) were irradiated at 1, 10, and 100 cGy or were sham-irradiated (0 cGy) on day 0 and tissues harvested 1 month (inset) or 4 months after irradiation. Mean ± SD. ^#Denotes P < 0.05 for changes with age (linear regression) for the respective cohorts (1 and 4 months after irradiation).

Figure 2

Cancellous microarchitecture of the proximal tibial metaphysis as a function of age and radiation exposure at the time of irradiation (basal, 0 months) and 1 and 4 months after irradiation. During aging, sham-irradiated controls show relative bone volume (BV/TV) remaining constant (a), increases in tissue density (b) and trabecular thickness (Tb.Th*) (c), and declines in trabecular number (Tb.N*) (d) and connectivity density (Conn.D) (e). The ratio of trabecular rods to plates, the structure model index (SMI), remains constant with age (f). The maximum eigenvalue (|H₂|) of the fabric tensor increases with age (g), with the degree of anisotropy (h) remaining constant. At 1 month after irradiation, 100 cGy reduced Tb.N* by 20% (d), Conn.D by 36% (e) compared to age-matched controls. Irradiation at 100 cGy, but not at 1 cGy or 10 cGy, altered cancellous structure within 1 month in a direction and magnitude similar to 4 months of normal aging, with no further modulation between 1 and 4 months after irradiation. Mean ± SD. *Denotes P < 0.05 for changes with IR dose (ANOVA and Tukey-Kramer posthoc compared to age-matched sham controls). ^#Denotes P < 0.05 for changes with age (linear regression).

Figure 3

Bone-cell differentiation potential as a function of age and radiation exposure. Hindlimb bone marrow was flushed and cultured under osteoblastogenic or osteoclastogenic conditions to assess differentiation potential of the marrow. Between 1 and 4 months of aging (white columns), osteoblastogenesis (a), measured as the area of calcified matrix per well, and osteoclastogenesis (b), measured as number of TRAP-positive, multinuclear (≥3/cell) cells per area, remained constant in sham-control mice. Irradiation, at any dose (gray or black columns), did not affect the differentiation potential of bone cells in the marrow at 1 (not shown) or 4 months post-IR (a, b). Images of alizarin red-stained mineralized nodules (c) and TRAP-stained osteoclasts (d).