Histogenesis and dependent glutamine synthetase inducibility in embryonic neural retina. Irreversible inhibition of differentiation by 5-bromodeoxyuridine

Young, mitotically active neural retinas from 7-day chick embryos were cultured with 5-bromodeoxyuridine (BrdU) for 8 hr or more. After this treatment, they failed to differentiate beyond the stage at which they were explanted; there was no histogenesis or increase in glutamine synthetase (GS) inducibility in intact tissues or in aggregates of dissociated cells. Normally GS can be induced in the retina with hydrocortisone as the cells cease to be mitotically active and begin showing histological organization after day 7. This inhibition by BrdU was irreversible even in the presence of excess thymidine. Overall incorporation of ¹⁴C-amino acids into protein was only slightly inhibited, and the ability of cells from treated tissue to aggregate and sort out from nonneural cell types was unaffected. Control cultures without BrdU showed considerable histogenesis and a parallel increase in enzyme inducibility. Postmitotic 10-day retinas appeared to be unaffected by BrdU. The incorporation rates of both tritiated BrdU and thymidine (dT) into DNA were 14× higher in 7- than in 10-day retinas. Simultaneous addition of excess unlabeled dT with either of the labeled nucleosides reduced their incorporation and reduced the inhibitory action of BrdU on differentiation.It is concluded that BrdU irreversibly inhibits the differentiation of retina cell surface properties involved in histogenesis and dependent cytodifferentiation without affecting already differentiated properties of the cell surface. The results support the hypothesis that histogenesis is directed by genes affecting specific cell surface properties.