During embryonic development, gradients of morphogens and signaling molecules help to define how development proceeds. Scholpp and Brand examined how the gradient of a member of the fibroblast growth factor family, Fgf8, is generated and maintained in the nascent neuroectoderm of living zebrafish embryos. By looking at fluorescently tagged Fgf8 as it spread from its site of origin through target tissue, the authors obtained evidence for a restrictive clearance mechanism in which the factor is cleared from the immediate environment around target cells by endocytosis and subsequent degradation. When endocytosis was blocked, Fgf8 accumulated extracellularly and activated gene expression in more distant target cells, whereas activating endocytosis had the opposite affect, restricting the effective range of Fgf8.
Belenkaya et al. looked at the movement of another growth factor-related morphogen, Drosophila Decapentaplegic (Dpp), during anteroposterior patterning of the wing. In this system, movement of the growth factor was restricted by binding to extracellular proteoglycans rather than by endocytosis, leading again to a gradient of morphogen response. — SMH
Curr. Biol. 14, 1834 (2004); Cell 119, 231 (2004).