Fig. 7. A balance of summation of the total encountered ephrin and adjustment of sensitivity to the local ephrin concentration may lead to growth cone stop. (A) Schematic of the total amount of encountered ephrin in the gradient (red dots in the rectangle on the left) and local encountered ephrin (red dots in the rectangle on the right). (B) When the local ephrin concentration is plotted against the total encountered ephrin-covered area at the stop point for all patterns, a linear correlation becomes apparent. Blue data points are derived from patterns printed with 8 µg/ml, red data points from patterns printed with 4 µg/ml ephrin printing ink. The relative slope of the gradient (1, 2/3, 1/2, 1/3, 1/6) or the thickness of the stripes in the non-graded patterns (1.8, 0.6, 0.3 µm) is noted next to the data point. Data points for gradients with the relative slope 1 (marked with stars) deviate noticeable from the linear smoothing function. For explanation, see text. (C) The correlation shown in B can be explained by a model in which the repulsive ephrin signal is summed up over time by accumulation of a slowly degrading signaling molecule X, which positively affects growth cone stop, retraction and/or collapse. Temporally delayed (t), a second, rapidly degrading component Y leads to an adjustment of the sensitivity to the local ephrin concentration, counteracting the output of X. The activity of X is postulated to be proportional to the total encountered ephrin, whereas the activity of Y is proportional to the local ephrin concentration. The growth cone stops when both activities are proportionable.