Abstract
SummaryHow the site of axon emergence is specified during neural development is not understood. Previous studies disagree on the relative importance of intrinsic and extrinsic mechanisms. The axons of retinal ganglion cells (RGCs) emerge basally in vivo, yet because RGCs develop from polarized neuroepithelial cells within a polarized environment, disentangling intrinsic and extrinsic influences is a challenge. We use time-lapse imaging to demonstrate that Laminin acting directly on RGCs is necessary and sufficient to orient axon emergence in vivo. Laminin contact with the basal processes of newborn RGCs prevents the cells from entering a stochastic Stage 2 phase, directs the rapid accumulation of the early axonal marker Kif5c560-YFP, and leads to the formation of axonal growth cones. These results suggest that contact-mediated cues may be critical for the site of axon emergence and account for the differences in cellular behavior observed in vitro and in vivo.Video
Highlights
The extension of axons and dendrites from the cell body marks the dramatic morphological polarization of the typical neuron
The axons of retinal ganglion cells (RGCs) emerge basally in vivo, yet because RGCs develop from polarized neuroepithelial cells within a polarized environment, disentangling intrinsic and extrinsic influences is a challenge
We use time-lapse imaging to demonstrate that Laminin acting directly on RGCs is necessary and sufficient to orient axon emergence in vivo
Summary
The extension of axons and dendrites from the cell body marks the dramatic morphological polarization of the typical neuron. Morphological symmetry is broken in Stage 3, when one neurite grows much longer than the others and develops into the axon (Craig and Banker, 1994) This remarkable cellular behavior of neurons in tissue culture has allowed investigators to identify many factors involved in the extension of a single axon from a set of more or less equivalent neurites (Arimura and Kaibuchi, 2007; Barnes and Polleux, 2009; Tahirovic and Bradke, 2009), and to formulate a general model of cytoskeletal regulation, which lies at the heart of neuronal polarization. This reciprocal cAMP/cGMP regulation acts as a symmetry-breaking positive feedback loop, ensuring that only a single axon is formed (Shelly et al, 2010)
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