Fgf8 morphogen gradient forms by a source-sink mechanism with freely diffusing molecules

Abstract

It is widely accepted that tissue differentiation and morphogenesis in multicellular organisms are regulated by tightly controlled concentration gradients of morphogens. How exactly these gradients are formed, however, remains unclear. Here we show that Fgf8 morphogen gradients in living zebrafish embryos are established and maintained by two essential factors: fast, free diffusion of single molecules away from the source through extracellular space, and a sink function of the receiving cells, regulated by receptor-mediated endocytosis. Evidence is provided by directly examining single molecules of Fgf8 in living tissue by fluorescence correlation spectroscopy, quantifying their local mobility and concentration with high precision. By changing the degree of uptake of Fgf8 into its target cells, we are able to alter the shape of the Fgf8 gradient. Our results demonstrate that a freely diffusing morphogen can set up concentration gradients in a complex multicellular tissue by a simple source-sink mechanism.

New Model for Understanding Mechanisms of Biological Signaling: Direct Transport via Cytonemes

“…The SDD mechanism predicts the exponentially decaying signaling profiles, which qualitatively agrees with many experimental observations. 4,5,[8][9][10] Despite common use of the SDD model and related approaches for many biological systems, there is a growing number of experimental observations, suggesting that the indirect delivery of the morphogens via diffusion might not be the only mechanism in the transportation of morphogenetic signals. 6,22,23,34 It has been argued that the complex environment of the embryo systems might prevent the passive free diffusion from establishing distinguishable morphogen gradients at different regions.…”

“…1,2,4 A large number of experimental and theoretical investigations on the formation and functioning of morphogen gradients has appeared in recent years. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]21,22 However, many aspects of biological signaling remain not well understood. 4,6 One of the most important problems of the biological development is to understand the mechanisms of the morphogen gradients formation.…”

Modeling and analysis of collective cell migration in an in vivo three-dimensional environment

“…Gradients of chemical signals are believed to guide moving cells to their targets (1,2). Classically, the formation of such a gradient involves a "source" that continuously produces a signal that freely diffuses through extracellular space and a "sink" that actively eliminates it (3)(4)(5)(6). To study cellular responses, investigators have artificially created chemical gradients in vitro using gels (7,8), micropipettes (9), and microfluidic devices (10)(11)(12).…”

Local kinetics of morphogen gradients

“…Morphogen gradients can form by reaction-diffusion mechanisms, the simplest of which is the so-called source-sink mechanism, whereby a locally produced molecule is degraded as it diffuses through the tissue (12)(13)(14)(15). The source-sink model has been used to explain gradient formation in multiple developmental contexts, including the vertebrate neural tube, the embryonic precursor of the central nervous system.…”