Branching mechanism and organ-scale self-organization.

<p>(<b>A</b>) The branching mechanism. Consider any prominence on the epithelium (here displayed in red). This bud increases the local gradient of concentration and thus the local flux of FGF10 received by epithelial cells. It will thus grow faster and be amplified. This instability mechanism is balanced by surface tension, which prevents thin branches to form. (<b>B</b>) The mechanism of self-organization. When two branches get too close, the local gradient of concentration in the interstitial mesenchyme tends to zero (red circle). Growth thus tends to zero and prevents branches from any collision. This mechanism, at the organ scale, leads to the self-avoiding bronchial tree.