The formation of the feather pattern in chick skin after a proportion of cells have been killed by X-irradiation

The formation of periodic patterns is of fundamental importance in embryoni c development. One of the simplest and most frequently observed patterns is the maintenance of a minimum distance between neighbouring elements, for e xample between teeth, hair, feathers, digits etc. Theoretical models descri bing these phenomena have been proposed for feather patterning. However, th ere has been no detailed quantitative analysis of the relationship between cell population density and feather spacing. To define the relation between these quantities and specifically to test the prediction of a mathematical model, we have examined the formation of the feather pattern after varying proportions of the dermal cells have been killed by X-irradiation. It is k nown that the development of a feather primordium is normally associated wi th an increase in cell population density in the dermis. Using X-ray irradi ation of the skin in vivo and in vitro, we show that the relation between c ell population density and spacing of feather primordia indicates the impor tance of a threshold number of cells for feather patterning. Moreover, ther e is a prima facie case for supposing that X-rays act on feather spacing sy stem, reducing the ability of dermal cells to prevent spreading of the patt ern. Thus, X-irradiation may have a secondary effect on the spacing of prim ordia rather than, or as well as, affecting the mechanisms that determine t heir primary positions.