Tunneling Nanotubes and the Eye: Intercellular Communication and Implications for Ocular Health and Disease

Figure 1

Morphogen transport and the drunken sailor analogy. (a) The transport of morphogens from a source establishes a gradient in the target field. (b–h) Five major morphogen transport models are illustrated using the drunken sailor analogy, in which drunken sailors move by random walks from a ship into a city. In this analogy, morphogen molecules are represented by sailors and cells are represented by buildings. (b) In the case of free diffusion, sailors (green dots) leave the ship (blue oval) and disperse into the city (white square). Inset: sailors take steps of the indicated fixed size, and the direction of each step is random. This “random walk” describes the diffusive behavior of molecules in solution. (c) In the tortuosity-mediated hindered diffusion model, buildings (gray) act as obstacles that sailors must move around, thus increasing the tortuosity of the environment. (d) In the case of diffusion that is hindered by tortuosity and transient binding, the sailors stop in pubs (negative diffusion regulators, yellow) located at the periphery of buildings. Note that, in contrast to effects from tortuosity alone, sailors congregate at the periphery of buildings, and there are relatively few freely moving sailors. (e, f) The shuttling model does not require a localized source of sailors. Instead, sailors are initially present mostly in pubs (negative diffusion regulators, yellow) and uniformly distributed in the city (e). Police officers (positive diffusion regulators, red) disperse from a source on the right side, pick up sailors from pubs, and escort them through the city by preventing further pub visits (f). When police officers disappear (not shown), sailors can reenter the pubs. Over time, this results in the concentration of sailors on the left. (g) In the transcytosis model, the sailors travel through the buildings. (h) During directed transport-mediated by cytonemes, the sailors travel through subway tunnels (orange), which deposit the sailors in buildings (reproduced from Muller et al. (2013) with no alterations under the Creative Commons Attribution 4.0 International license (http://creativecommons.org/licenses/by/4.0/)) [1].

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