Single cells can sense their position in a morphogen gradient

Xenopus blastula cells show a morphogen-like response to activin by express ing different genes according to the concentration of activin to which they are exposed, To understand how cells recognize their position in a concent ration gradient, it is essential to know whether each cell responds individ ually to activin concentration. An alternative idea, proposed by previous w ork, is that cells need to interact with their neighbours to generate a con centration-related response. To distinguish between these ideas, we have cu ltured blastula cells under conditions which provide different degrees of c ontact with other cells, allowing nil to maximum communication with their n eighbours, The cultures include cells attached to fibronectin and cells res ting unattached on an agarose base. The cultures also include cells that ha ve no contact with any cell except their clonal progeny, cells that have la teral contact to neighbouring cells, and cells that are completely envelope d by other cells in a reaggregate, We have used RNase protection and in sit u hybridization to assay the expression of the activin-responsive Xenopus g enes Xbra, Xgsc, Xeomes, Xapod, Xchordin, Mix1, Xlim1 and Cerberus. We find no difference in gene expression between cells attached to fibronectin and those unattached on agarose, Most importantly, we find that cells respond to activin in a concentration-related way irrespective of their degree of c ontact with other cells. Therefore interaction among cells is not required for the interpretation of morphogen concentration, at least in the case of the early genes studied here. We conclude that isolated blastula cells can sense and respond individually to activin by expressing genes in a concentr ation-dependent way.