The micromass test: Is it subject to strain variation?

Strain difference in the teratogenic response in vivo is an important consideration in the design and interpretation of regulatory studies. The importance of strain difference in vitro has been examined using all trans-retinoic acid. Effects on development of the forelimb in vivo and on differentiation and survival of embryonic limb bud and midbrain cells in vitro were compared for three strains of rat, Allen & Hanbury Albino (AHA), Random Hooded (RH) and Alderley Park (AP). Transformed dose-response curves for the incidence of shortened forelimb in vivo were parallel. Comparison of ED(50) values (the concentration at which 50% of embryos respond with the index abnormality) allowed the strains to be ranked in order of susceptibility in vivo: AHA > RH = AP (ED(50) values 93, 124 and 123 mg/kg, respectively). In vitro, there was little difference in cytotoxicity or in effects on midbrain cell differentiation between strains. However, marked differences for effects on limb bud cell differentiation allowed the three strains to be ranked as follows: AHA > RH > AP (IC(50) values 0.008, 0.09 and 0.25 mug/ml, respectively). These findings suggest that for retinoic acid, the micromass test is subject to strain variation, although this had no effect on the prediction of teratogenic potential. Furthermore, the order of susceptibility of the three strains examined was the same in vivo and in vitro. Thus, target tissue sensitivity may play an important part in strain variation in the teratogenic response, although differences in vitro were much more marked than those seen in vivo.