Studies on effects of 5-bromodeoxyuridine on the development of explanted early chick embryos.

Chick embryos were explanted at stages ranging from the definitive streak to one-somite and cultured for 19-21 h on thin albumen with or without 5-bromodeoxyuridine (BrdU) (2-12/ig/ml). BrdU primarily inhibited neurulation which was found to be stage dependent. If exposed to BrdU at the definitive streak stage, it resulted in an open brain region; if treatment was delayed until the head-process stage, it inhibited the closure of the midbrain and, to some extent, the hindbrain, but the forebrain closure was mostly unaffected. BrdU had no appreciable effect on later stages. It was also found that BrdU inevitably inhibited somite formation when exposed to 8 /*g/ml or higher concentrations. The blastodermal expansion, heart development, and blood island formation were usually not affected. Cytological changes were observed in adversely affected tissues, particularly the neural tissue. BrdU produced nuclear enlargement and clumped metaphase, but did not cause extensive necrosis. The grafting experiments showed that an increase in the concentration of BrdU from 4 to 12/Ag/ml progressively inhibited the self-differentiation of Hensen's node grafts. However, BrdU (8 /«g/ml) had no significant effect on the inducing capacity of node grafts nor the competence of the epiblast. Autoradiographic studies showed that the application of BrdU, at 6 /*g/ml or lower, caused no obvious variations in the incorporation of [H]uridine into chick embryonic cells. BrdU, at 10-12/tg/ml, selectively inhibited the uptake of [H]uridine in the neural tissue.