Susceptible stages and abnormal morphogenesis in the developing mouse limb, analysed in organ culture after transplacental exposure to vitamin A (retinoic acid).

The development of form and morphology of cartilage in limbs from control and retinoic acid-treated DBA/2J mice were studied in organ culture. Somite number at explantation (SE) ranged from 29 to 56. All cartilage segments except digits developed in cultures of 29somite control limb-buds; digits developed well only from limb-buds of SE45 and older stages. Maternal treatment with retinoic acid specifically inhibited the growth of limbs rather than that of the whole embryo; the percentage reduction in limb length in culture ranged between 20 and 40 %, depending on the length of in utero exposure. The teratogenic effects of retinoic acid were expressed in culture even if the limb was explanted after only 1 h of in utero exposure. However, for a complete expression of the teratogenic effects, resulting in phocomelia, 24 h exposure was necessary. Limb-buds of 29 to 32-somite embryos were only mildly responsive to the teratogen. Embryos with 33-38 somites demonstrated the full extent of teratogenic effects. Embryos with more than 40 somites at the time of first exposure to retinoic acid escaped severe limb malformations except digital defects. Correlation between the developmental stage of the limb at the time of treatment and the final limb defect is attempted.