Using live embryos to image the dynamics of the Bcd gradient, the team determined that the gradient was established within ∼1 hour after fertilization and that Bcd diffused through the cytoplasm of the syncytial embryo with a diffusion constant of 0.3 μm2 per second. But if one assumes that simple diffusion establishes the gradient, Bcd would never reach steady-state within the developmental timeframe. More work is needed to find other mechanisms that are at play.
This first look at a transcription factor's behavior in a live organism also revealed tightly regulated levels of nuclear Bcd between mitotic cycles. During four syncytial cycles, when nuclei multiply rapidly and get smaller, the Bcd concentration in a given nucleus returned at each interphase to within 10% of its starting concentration, holding the blueprint coordinates steady.
At the midpoint of the embryo—where Bcd levels are at the head–tail borderline—nuclei held ∼700 molecules of Bcd. A precision of 10% thus means that midpoint cells detected a difference of ∼70 molecules. The noise in Bcd readout (measured by its activation of the head gene hunchback) was also 10%, as was the reproducibility of the Bcd gradient from embryo to embryo.
The work argues that the cells along the embryo's anterior–posterior axis determine their position by a precise readout of their own Bcd concentration to either activate hunchback or not. And, the authors note, the readout may be even more exact, since the repeated 10% figure is “disturbingly close” to the noise introduced by their instrumentation.