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 c rest cells (NCC) is associated with a high cell density, resulting fro m an active cell proliferation. It is also known that treatment with r etinoic acid (RA) causes a dose-dependent inhibition of proliferation of some cell types, and that administration of RA during the early sta ges 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 e ffect exerted by RA on the proliferative behaviour of NCC in vivo and in vitro. Homogenates of RA-treated chick embryos showed a low [H-3]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 val ues of [H-3]dT incorporation than neural tube cells (P<0.0001). Autora diographs of RA-treated chick embryos showed a significantly lower [H- 3]dT incorporation in NCC at the prosencephalic and mesencephalic leve ls, as well as in the neural tube cells at the prosencephalic, mesence phalic and rhombencephalic levers, than in control chick embryos (P<0. 0001). NCC cultures treated with 1 or 10 mu M RA had a significantly l ower LI than in cultures treated with 0.1 mu M 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 w as significantly longer in the NCC of RA-treated embryos (similar to 4 0 h) than in controls (similar to 25 h). The length of the cell cycle phases of NCC was similar in both experimental conditions, except for GI phase, which was significantly longer in the RA-treated group than in controls. These results show that RA blocks DNA synthesis and lengt hens the proliferative behaviour of NCC both in early chick embryos an d in vitro, effects that could modify the morphogenetic patterns of NC C distribution through a decreased cell population.