Epithelial branching: the power of self-loathing.

Branching morphogenesis of epithelia is an important mechanism in mammalian development. The last decade has seen the identification of many signalling pathways and intracellular mechanisms that control epithelial branching. Tissue-level mechanisms that space new branches out have, however, remained an unsolved problem. A recent publication by Nelson et al.1 suggests--if extrapolation from their novel and abstract culture system is valid--that branches may be spaced out by a system of mutual inhibition based on diffusion of TGFbeta. Such a system would allow a developing tree to arrange itself, without detailed genetic specification, by adaptive self-organization.