Size Matters

Science's STKE  20 Jul 2004:
Vol. 2004, Issue 242, pp. tw264-TW264
DOI: 10.1126/stke.2422004TW264

Developmental size control is very poorly understood, especially in vertebrates. Why do different body parts attain particular relative and absolute sizes and then stop growing? In the case of limbs, continued growth and patterning of the embryonic bud depends on the maintenance of a positive feedback loop between Sonic hedgehog and fibroblast growth factors. Scherz et al. now describe the timing mechanism that terminates this feedback loop and hence determines the length of the digits and the number of phalanges they contain. Over time, an expansion occurs in the number of cells in the posterior limb bud that cannot support the intermediate steps of the feedback loop downstream of Sonic hedgehog. When the size of this nonresponsive zone grows broader than the range of Sonic hedgehog diffusion, the loop is broken.

P. J. Scherz, B. D. Harfe, A. P. McMahon, C. J. Tabin, The limb bud Shh-Fgf feedback loop is terminated by expansion of former ZPA cells. Science 305, 396-399 (2004). [Abstract][Full Text]