Difference drives division

Morphogens that spread from a point or line source are useful for patterning and for defining the outer bounds of a tissue. How to convert such a gradient into uniform cellular growth is, however, far from obvious. Can a pro-growth morphogen avoid encouraging excessive growth near its source and inadequate growth further away?Dragana Rogulja and Kenneth Irvine (Rutgers, Piscataway, NJ) now provide one possible solution for cells that will become fly wings. The cells, they find, make their division decision in response to a gradient rather than absolute concentrations of the Decapentaplegic (DPP) morphogen.

The Rutgers group expressed or repressed the DPP pathway in small clones. In both cases they saw new cell division both within the clone and in neighboring areas. This nonautonomous growth had not been evident in previous experiments, probably because the earlier experiments used expression systems with a long lag time. The new experiments used drug-inducible expression to get tighter temporal control.

The researchers believe that cells measure the difference between their own DPP expression level and that of their neighbor, and go through cell division only if there is a difference. This growth effect may be able to spread for several cell diameters both back into the DPP-expressing clone and out into the area expressing lower levels of DPP. Intercell comparisons could be made, for example, by cells expressing receptors that bind homophilically to their partners in a neighboring cell, but signal if there are no more binding partners available.

Further from the DPP source, the gradient effect drops away, probably because in wild-type tissue this area would experience extremely shallow gradients of DPP. The cells in this area seem to depend more on the absolute values of DPP for their instructions.

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