A gradient of diffusible substance in a monolayer of cultured cells.

Monolayers of hypoxanthine phosphoribosyl transferase-deficient and their corresponding wildtype cells have been placed adjacent to each other with a newly described method. Autoradiographs from such preparation after incubation with 3H-hypoxanthine allow the direct visualization of gradients of incorporated radioactivity at the border between the two cell types. The gradients can be described by an exponential function, and the amount of radioactivity incorporated decreases to less than 1% at a distance of 1 mm from the wild-type cells. A possible mechanism to convert exponential gradients to linear ones over a certain concentration range is discussed.