High-throughput micro-computed tomography imaging as a method to evaluate rat and rabbit fetal skeletal abnormalities for developmental toxicity studies

Fetal skeletal assessments are routinely conducted as a part of preclinical safety studies to support the development of novel therapeutic agents. Alizarin red staining with visual inspection of fetal skeletons is the gold standard in evaluating skeletons for the presence of developmental abnormalities. X-ray based micro-computed tomography (micro-CT) imaging has been used to evaluate small skeletal structures, both in vivo and ex vivo. Recent technological advances have reduced micro-CT image acquisition time making this technology practical for routine fetal skeletal evaluations. Herein we report on the use of micro-CT imaging as a method to perform high-throughput assessment of fetal skeletons. Micro-CT imaging of rat and rabbit fetal skeletons was conducted under a variety of conditions, including, in vivo, contrast-enhanced in vivo, and ex vivo. To increase throughput, micro-CT imaging was employed using custom designed polystyrene foam fetal holders to image entire litters of ex vivo fetuses. After micro-CT imaging, fetuses were routinely stained with alizarin red to compare micro-CT imaging results with traditional alizarin red staining. Fetal skeletons could be visualized using in vivo micro-CT imaging; however, due to crowding, specific identification of individual fetuses was deemed not practical. Administration of a routine contrast agent to pregnant females highlighted maternal vascular structures including the placenta, but unfortunately, did not cross the placenta and did not highlight any fetal soft tissue structures. Ex vivo fetal imaging provided the best image quality of fetal skeletons and allowed for specific fetal identification. The fetal holders allowed for micro-CT imaging of ~ 400 rat fetuses or ~ 140 rabbit fetuses per hour. Micro-CT image skeletal findings and alizarin red findings were comparable. The very few discrepancies between the two methods involved the smallest skeletal elements with minimal ossification. In conclusion, micro-CT ex vivo imaging can provide a reliable high-throughput method to assess fetal skeletal abnormalities for developmental toxicity studies.