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The Robo proteins are critical regulators of midline crossing across species. In Drosophila embryos, Robo1 and Robo2 are required to keep ipsilaterally projecting axons from inappropriately crossing the midline, and to prevent contralateral axons from remaining stuck at the midline. Robo3, while it also binds Slit, does not appear to play a ...
Passes toy to other hand when offered a second object (referred to as "crossing the midline" – an important neurological development). Manages three to four objects by setting an object aside (on lap or floor) when presented with a new toy. Puts toys in mouth less often. Enjoys looking at picture books.
Comm promotes midline crossing by down-regulating Robo. A LPSY sorting signal motif has been shown to be required for Comm to sort Robo to endosomes, preventing it from accessing the surface of the growth cone. Thus, when Comm is expressed, axons are unaffected by the presence of Slit and are able to cross the midline. [27]
Comm expression turns off after the growth cone has crossed the midline; this permits Robo/Slit repulsion and prevents the growth cone from crossing the midline again. Vertebrates, on the other hand, do not possess a comm homolog ; instead they facilitate midline crossing through alternative splicing of Robo3 (aka.Rig-1).
This article is about the optic chiasm of vertebrates, which is the best known nerve chiasm, but not every chiasm denotes a crossing of the body midline (e.g., in some invertebrates, see Chiasm (anatomy)). A midline crossing of nerves inside the brain is called a decussation (see Definition of types of crossings).
Before crossing (ipsilaterally), the growth cone must navigate toward and be attracted to the midline. However, after crossing (contralaterally), the same growth cone must become repelled or lose attraction to the midline and reinterpret the environment to locate the correct target tissue.
Fig.2 - Cues from the midline lead the pioneer axon (bordeaux), while the follower neurons (blue and orange) trail it. Adapted from Bak M. and Fraser S.E. (2003) Other studies in the Drosophila central nervous system show that in the absence of either glia or pioneer neurons, longitudinal follower axons that would not normally cross the midline ...
The main function of Slit proteins is to act as midline repellents, preventing the crossing of longitudinal axons through the midline of the central nervous system of most bilaterian animal species, including mice, chickens, humans, insects, nematode worms and planarians. [3] It also prevents the recrossing of commissural axons.