In Brief

Abstract

Recent experiments demonstrate the molecular process by which an embryo begins to mark cells for a neural versus an epidermal fate. In the 1950s, the mathematician Alan Turing proposed the ‘morphogen model’ of development whereby a diffusible chemical signal (a ‘morphogen’) triggers certain differentiation processes. The morphogen is produced by one set of cells and removed by an adjacent set, leading to varying concentrations depending on distance from the source. Different morphogen concentrations could determine different cell fates. One potential morphogen is called Short Gastrulation (Sog) in fruit flies. However, until now, no one could demonstrate that Sog was present as a concentration gradient in vivo. A new sensitive fluorescence detection technique made it possible to observe the accumulation of Sog directly and showed that it is indeed graded. The researchers, led by Ethan Bier, also showed that an enzyme called Tolloid, which is produced in the epidermal region and which cleaves and inactivates Sog, might be responsible for this. ‘We observed that in the absence of Tolloid, Sog levels are greatly elevated in epidermal cells and accumulate steadily until the entire embryo contains nearly uniform amounts of Sog,’ said Bier. ‘These studies provide the final link demonstrating that Sog is a morphogen and explain how the graded distribution of Sog is created.’ (Srinivasan, S. [2002] Dev. Cell 2, 91–101) PL