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The image shows a Drosophila embryo 2 hr after fertilization, with nuclei at the surface fluorescently labeled for Bicoid protein (blue), Hunchback protein (green), and DNA (red). Using two-photon microscopy these embryos were imaged to quantitatively characterize the dynamics and precision of how morphogen molecules communicate positional information to individual nuclei. In this example, the shallow Bicoid gradient generates a sharp Hunchback boundary (enlarged in the background), partitioning the embryo in half. This input/output relationship is quantitatively represented in the foreground (yellow), where each dot specifies the Bicoid concentration (horizontal axis) and Hunchback concentration (vertical axis) measured in a single nucleus. The results indicate that the precision with which the embryo interprets and locates this boundary is very high, approaching limits set by simple physical principles.