International Mammalian Genome Society


The 13th International Mouse Genome Conference
October 31-November 3, 1999

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E33 Comparison of Bone Mineral Content and Bone Mineral Density in C57BL/6J and C3H/HeJ Female Mice by pQCT (Stratec XCT 960M) and DEXA (PIXImus)

L. R. Donahue, C. J. Rosen, and W. G. Beamer. The Jackson Laboratory, 600 Main St., Bar Harbor, ME 04609

Recently it has become possible to measure bone mineral content (BMC) and bone mineral density (BMD) in mice with accuracy and precision. In our laboratory we are utilizing two different methodologies to quantify skeletal parameters, peripheral quantitative computerized tomography, pQCT (Stratec XCT 960M, Norland Medical Systems, Ft. Atkinson, WI) and dual-energy x-ray absorptiometry (PIXImus, Lunar, Madison WI). These instruments both provide measurements of skeletal phenotypes that are labeled BMC and BMD. Although absolute values for BMC, gathered by x-ray attenuation in both instruments, are directly comparable, those for BMD are not. BMD measured by pQCT is volumetric density, that is, mass divided by volume. BMD measured by the PIXImus, on the other hand is bone mineral content divided by bone area, often referred to as areal BMD. Consequently, although often used interchangeably, these density measurements describe different phenotypes.

We have previously validated use of the pQCT in mice (Bone, 1996, Beamer et al) and have reported that inbred strains differ in adult volumetric BMD. Here we have chosen C57BL/6J (B6) and C3H/HeJ (C3H) female mice that differ in pQCT femoral BMD by 44% to compare pQCT and PIXImus data. The precision of the PIXImus was established by measuring the same mouse five times with repositioning each time. The values for total body areal BMD (mg/cm2) were 0.64 for 4 of the measurements and 0.65 for the fifth. The accuracy of the PIXImus was tested by comparing BMC and BMD between four month old B6 and C3H females detected by pQCT and PIXImus as shown in the tables below.

PIXImus Bone Parameters C57BL/6J C3H/HeJ
  BMD (mg/cm2) BMC (mg) BMD (mg/cm2) BMC (mg)
Total Body (n=7,6) .63.002 54.6.023 .75.001 77.3.021
Isolated L4,L3,L2 (n=8,9) .85.002 13.0.002 1.07.001 18.0.001
Isolated Femur (n=7,8) .63.002 24.0.001 .74.001 27.0.001

 

pQCT Bone Parameters C57BL/6J C3H/HeJ
  BMD (mg/mm3) BMC (mg) BMD (mg/mm3) BMC (mg)
Isolated L5 (n=15,16) .278.003 5.426.106 .302.003 8.125.254
Isolated Femur (n=8,8) .488.002 18.4.6 .702.009 22.1.8

In addition we have compared BMC and BMD of 83 femurs from a BC mapping cross to determine the similarity between pQCT data and PIXImus data. Regression analysis for BMC and BMD are shown below.

graphs

We suggest that the differences in BMC result from imprecise detection of edges of bone in both systems, and that PIXImus provides more accurate measurement of whole body BMC or BMD than of isolated femur due to the difficulty in defining the area of a single bone. In conclusion, these data show that both methodologies measure valid phenotypes that can be used to assess skeletal attributes and changes; however, it should not be assumed that the actual values for BMD represent duplicate measurements.

 


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