Micro-computed tomographic evaluation of fetal skeletal changes induced by all-trans-retinoic acid in rats and rabbits.

BACKGROUND Our laboratory has been conducting positive control studies to evaluate the utility of micro-computed tomography (micro-CT) for qualitative evaluation of fetal skeletal morphology. All-trans-retinoic acid (atRA) was used to produce a different spectrum of defects compared to our previous studies with boric acid and hydroxyurea. METHODS Groups of five mated Crl:CD(SD) female rats each were administered vehicle or atRA (2.5-50 mg/kg) on GD 10, and groups of four mated Dutch Belted rabbits each were dosed with vehicle or atRA (6.25-25 mg/kg) on GD 9. Cesarean sections were performed on GD 21 and 28, respectively. Following external examination the viscera were removed and fetuses scanned in a micro-CT imaging system. Fetuses were subsequently stained with alizarin red. Skeletal morphology was evaluated by each method without the knowledge of treatment group. Total bone mineral content (BMC) of each fetus was quantitated using the micro-CT images. RESULTS In rats there were dose-related increases in the incidence of extra lumbar vertebra and non-dose-related increases in supernumerary ribs at all dose levels. There were decreases in mean number of ossified sacrocaudal vertebra at ≥ 5 mg/kg, and increases in skull bone malformations at ≥ 10 mg/kg. Rabbits were less sensitive on a mg/kg basis since skeletal malformations and a decrease in mean number of ossified sacrocaudal vertebra were observed only in the 25-mg/kg group. Micro-CT evaluation detected essentially the same incidence of skeletal abnormalities as seen in alizarin red-stained rat and rabbit fetuses. BMC analysis showed a trend toward slight decreases in atRA-treated rats, but no notable changes in rabbits. CONCLUSIONS These results add support to our previous work that demonstrates that micro-CT imaging can effectively assess rat and rabbit fetal skeletal morphology.