Effects of ultraviolet light on the activity of an avian limb positional signalling region

Cells from a region on the posterior margin of the developing avian limb bud (the polarizing region) can signal positional information to responding anterior cells. Polarizing activity may be assayed by disaggregation of the tissue into single cells, followed by reaggregation and grafting of cell pellet. This method enables treatments of the cells while in single-cell suspension. The effects of ultraviolet radiation (254 nm) were studied to determine the role of nucleic acids in polarizing activity. Ultraviolet radiation eliminated quail polarizing activity over the same range of doses as it reduced cell spreading (20–60 J/m2). In contrast to the published effects of γ-radiation in which polarizing activity is affected only at doses much higher than those that are cell lethal, ultraviolet apparently abolishes cell survival and polarizing activity at comparable doses. Compared to other biological systems, polarizing activity is quite sensitive to ultraviolet light: the D37 is 18 J/m2; the extrapolation number, n , is 3·6. From pomparison of the effects of ultraviolet radiation to those published for ionizing X- or γ-radiation, one may conclude that ultraviolet and γ-radiation abolish limb polarizing activity at equivalent nucleic acid dosages, not at equivalent cell lethal doses.