Evaluation of 9.4-T MR microimaging in assessing normal and defective fetal bone development: comparison of MR imaging and histological findings.

We evaluated 9.4-T magnetic resonance (MR) microimaging in assessing normal and defective bone development in mouse embryos. For this purpose, we performed 9.4-T MR microimaging on developing bones in normal embryos, and also in Runx2/Cbfa1-/- embryos with severely defective bone development. MR images were compared with the histological and histochemical features of these fetal bones. MR microimaging delineate successfully the normal long bone development in embryos. The T1- and T2-weighted MR microimaging demonstrated chondrocyte maturation in different regions of growing cartilage, such as epiphysis, physis, hypertrophic cartilage, and zone of provisional calcification. These developmental changes were detectable in as early as E14.5 embryos. The MR microimaging clearly demonstrated defective bone development in Runx2/Cbfa1-/- embryos. The femur from E18.5 homozygous Runx2/Cbfa1-/- embryos lacked MR signal intensity patterns including the hypertrophic cartilage, which are characteristic of the bone from the age-matched Runx2/Cbfa1+/+ embryos. Interestingly, however, the tibia from the same mutants was associated with MR signal patterns indicative of hypertrophic cartilage but not of the primary spongiosa and ossifying perichondrium, suggesting that bone development is differently regulated in these two long bones. On the other hand, the bones from heterozygous Runx2/Cbfa1+/- embryos exhibited an MR phenotype intermediate between the Runx2/Cbfa1+/+ and Runx2/Cbfa1-/- embryos; the primary spongiosa and ossifying perichondrium formation occurred normally even in the absence of preceding organized maturation of chondrocytes, a phenotype that was not detected by histological examinations. We concluded that MR microimaging is useful in assessing the bone development.