The morphometric parameters obtained on fetal vertebrae and femurs evidenced a dense trabecular structure as compared to that of young adults. The histomorphometric analysis of femoral metaphysis and the 3D micro-CT analysis of vertebral bodies were consistent and showed a significant increase of trabecular bone volume BVTV with gestational age. Three-dimensional analysis of anisotropy of cancellous bone in vertebra and in femoral metaphysis did not exhibit the same behavior at these two bone sites. The isotropy of trabecular bone in vertebral bodies demonstrated a growth that expands radially, while the anisotropy of the femoral metaphysis was related to the growth of long bone which spreads out longitudinally. In the vertebral body, cancellous bone was characterized by an inner core made of trabeculae about 100 jm in thickness, and a peripheral layer with thinner trabeculae of about 9 jm. The peripheral region is supposed to correspond to newly formed bone struts which allows the vertebral body to expand radially. In the inner core, trabecular thickness is of the same order of magnitude than in adults.
In the femoral sample, the trabecular thickness provided by histo- morphometry, was around 95 jm from the 28th week of gestation. In the new data presented here, we found a slightly smaller trabecular thickness of about 82 jm in the samples of 20 and 29 weeks. The trabecular thickness given here was averaged on a whole 3D volume starting close to the growth plate where trabeculae were the thinnest. Thus, the differences are expected to come mainly from differences in the location of the analyzed regions. The evolution of the trabecular thickness with the distance to cartilage junction observed from 3D micro-CT data are in agreement with the previous observation made in histomorphometry. Three-dimensional analysis allowed to follow this evolution with a distance step of 10 jm (slice thickness), and the data were very well fitted by a linear regression. In the 29-week sample, where the height of cancellous bone region analyzed was 4.5 mm, the trabecular thickness varied between 44 and 100 jm. The values are thus in the same range than that given in (25). Note that it is probable that the trabecular thickness reaches a plateau around 100-150 jm at some level (about 10 mm from the growth plate). By following the same line than in (25) and making the hypothesis that the growth plate advances at a rate of 0.16 mm/day, the slope of the linear regression for a distance to the growth plate between 2 and 4.5 mm, allows to estimate the thickening rate of trabeculae. Our measurements yield a thickening rate of 15,03 x 0,16 ~ 2,4 jm/day on the 29-week sample, 4 jm/day on the 17-week sample and 2.4 jm /day on the 20-week sample, thus a mean value close to the 3 jm/day found in.
If our study is very limited in terms of the number of samples, the estimation of the slope in each sample is quite reliable since the distance step was 10 jm instead of the larger bandwidth (750 jm) due the method used in histomorphometry. 3D micro-CT also yielded new information on connectivity and geometry of bone structure. Connectivity was assessed via the Euler density which average on the 3 samples was -77.1/mm3 Note that this value is very high as compared to that reported in studies dealing with adult bone closest to -5/mm3. Thus, cancellous fetal bone appears to be highly connected. Connectivity density varies as the opposite of Euler density, and is increased either with the number of loops and/or with the perforations in plates. We hypothesize that this very high connectivity could be partly related to the high degree of vas- cularization of growing bone, plate perforation offering pathways for blood supply. To our knowledge, there are no comparative data in the literature concerning connectivity in bone development. Regarding the geometrical information on bone structure, a significant prevalence of rodlike trabeculae was found, which differed from the assumed usual platelike model of cancellous bone. This finding concerning the geometry of bone structure perfectly explains the differences in trabecular thickness estimated using the model assumption (Tb.Th) and the model independent method (Tb.Th*) as seen in Table 3. In this situation, the model-based trabecular thickness is typically underestimated. This new method could be used to follow the geometrical characteristics on cancellous bone at birth and in growing children, which are not fully elucidated. Despite morphological information, the quantitative nature of 3D SR micro-CT enabled to estimate the distribution of the degree of mineralization within the 3D bone sample. The mean degree of mineralization in the examined samples was 0.823 g/cm3, which is relatively low as compared to normal adults. In addition, the images themselves allows to identify newly formed versus more ancient bone, typically found in the medial line of trabecular structure, and reflecting the thickening of trabeculae by bone apposition. Can't afford your medication? Buy generic viagra online
SR micro-CT is a three-dimensional imaging technique available at various scales. The 10 jm scale is appropriate for the accurate quantification of bone microarchitecture, even in fetal bone. The added value of 3D imaging as compared to histomorphometry is the availability of model independent parameters, anisotropy, connectivity and geometry characteristics. The analysis of micro-CT images clearly showed that the typical parallel plate model was not appropriate at this stage of formation since the structure appeared made as a mixture of rods and plates, with a preponderance of rods. At a 2 jm scale, micro-CT images of femurs enable quantifying very thin struts as those that appeared in developing fetal vertebral bone. Other information such as for instance the analysis of lacunae would also be possible, but has still not be reported. The very good matching between micro-CT at 2 jm and histology demonstrates that for the analysis of mineralized tissue, micro-CT is as reliable as histology and even more since it avoids any deterioration due to slicing. However micro-CT only reflects mineralized tissue, cartilage may be guessed but with very little contrast, and does not allow to differentiate woven or lamellar bone. We have reported a limited number of quantitative microarchitectural data obtained on fetal bones during their development. More studies are still required to get a better knowledge on the morphological mechanisms involved in bone growth.
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