Exogenous retinoic acid decreases in vivo and in vitro proliferative activity during the early migratory stage of neural crest cells.

We have previously demonstrated that directional migration of neural crest cells (NCC) is associated with a high cell density, resulting from an active cell proliferation. It is also known that treatment with retinoic acid (RA) causes a dose-dependent inhibition of proliferation of some cell types, and that administration of RA during the early stages of embryonic development, induces cranio-facial abnormal patterns corresponding to NCC derivatives. In view of these findings, it was of interest to determine if exogenous RA is a potential modulator of the mitotic rate of NCC, and to explore the hypothesis of an inhibitory effect exerted by RA on the proliferative behaviour of NCC in vivo and in vitro. Homogenates of RA-treated chick embryos showed a low [3H]dT incorporation, indicating a generalized diminution of DNA synthesis. The labelling index (LI = number of labelled cells/total number of cells) revealed that NCC from RA-treated and control embryos had higher values of [3H]dT incorporation than neural tube cells (P < 0.0001). Autoradiographs of RA-treated chick embryos showed a significantly lower [3H]dT incorporation in NCC at the prosencephalic and mesencephalic levels, as well as in the neural tube cells at the prosencephalic, mesencephalic and rhombencephalic levels, than in control chick embryos (P < 0.0001). NCC cultures treated with 1 or 10 microM RA had a significantly lower LI than in cultures treated with 0.1 microM RA or control cultures (P < 0.04). In chick embryos, the mitotic index of NCC was 0.026 for RA-treated and 0.033 for controls, while the duration of the cell cycle was significantly longer in the NCC of RA-treated embryos (approximately 40 h) than in controls (approximately 25 h). The length of the cell cycle phases of NCC was similar in both experimental conditions, except for G1 phase, which was significantly longer in the RA-treated group than in controls. These results show that RA blocks DNA synthesis and lengthens the proliferative behaviour of NCC both in early chick embryos and in vitro, effects that could modify the morphogenetic patterns of NCC distribution through a decreased cell population.