Ossified Bone Marrow Blood Vessels, Trabecular and Cortical Bone Properties and Hematological Parameters in 6‐ and 18‐month‐old Fischer 344 Rats

Abstract

BACKGROUNDWe previously discovered ossification of bone marrow blood vessels (BMBV) in 6‐ and 24‐month‐old F‐344 rats, whereby some BMBV had converted into bone (Prisby, 2014). Recently, we described the presence of ossified BMBV as early as 1 month in F‐344 rats, suggesting that this vascular pathology begins early in life (Guderian, ASBMR 2015). Since bone marrow is the site for blood cell production and blood cells access the peripheral circulation via blood vessels, we sought to assess whether BMBV ossification coincides with declines in red and white blood cell count. We examined BMBV ossification, trabecular and cortical bone properties, and hematological parameters as a function of advancing age.METHODSLeft femora and whole blood samples were collected from young (6 mon; n=5) and aged (18 mon; n=5) male Fischer‐344 rats. Bone microarchitecture (BV/TV, %; Tb.N,/mm2; Tb.Th, mm & Tb.Sp, mm) and trabecular bone density (g/cm3) in the distal femora and ossified vessel volume (OsVV) and ossified vessel density (g/cm3) in femoral shafts were assessed by mCT (Scanco Medical). Cortical bone parameters (cortical thickness, μm; cortical bone density, g/cm3 & cross sectional moment of inertia [CSMI, mm4]) were also compared between groups. SMI was compared between trabecular bone and ossified BMBV. Complete blood counts were determined with a Coulter ® Ac•T diffTM Analyzer (Beckman).RESULTSBody mass (p<0.05) increased with age. Trabecular BV/TV tended to be lower (p=0.056) in young (20.5±1.2%) vs. aged (25.0±1.5%) rats, resulting from decreased (p<0.05) Tb.Th (young, 68.84±2.14 mm vs. old, 89.12±2.65 mm). OsVV was similar between young (0.033±0.0120%) and aged (0.040±0.006) rats. SMI values for OsVV (young, 3.5±0.1 and old, 3.8±0.1) were indicative of cylindrical vs. the plate‐like shapes of trabecular bone (young, 1.8±0.1 and aged 0.8±0.1). Trabecular (young, 908.8±3.6 g/cm3 vs. aged, 935.2±3.6 g/cm3) and cortical (young, 1184.1±5.9 g/cm3 vs. aged, 1237.6±7.8 g/cm3) bone was denser with age (p<0.05). Trabecular bone density was similar to ossified vessel density (young, 882.1±18.9 g/cm3 vs. aged, 923.4±4.7 g/cm3). Cortical bone thickness (young, 724±3 μm vs. aged, 805±26 μm) and CSMI (young, 13.8±0.9 mm4 vs. aged, 22.2±1.8 mm4) were increased (p<0.05) with age. The complete blood count variables (e.g., WBC, RBC, Hgb, Hct, etc.) did not differ between groups.CONCLUSIONSOsVV and hematological parameters did not differ between 6‐ and 18‐month old rats. However, trabecular BV/TV, trabecular and cortical bone density, cortical bone thickness and CSMI were higher in 18‐month‐old animals.Support or Funding InformationSupported by National Institutes of General Medical Sciences (P20 GM103446; Project PI: R. Prisby)