Axons crossing the midline!

[Haj]

I know one way that directs axons across the midline (Slit, comm, robo etc) but need to explain another way. Im thinking along the lines of chemotractic guidance and chemotrpic factors and I think i understand them, just not how that makes some growth cones direct the axons across the midlineand some not?

[Clanad]

Which tract; anterior or posterior commissure?

[Haj]

I don't know, the question doesn't say.

Im thinking of using chemotractic guidance and chemotropic factors as a way to explain it as well as the slit, comm, rob explantion. Does this sound ok?

[Clanad]

S. Yoshikawa, et al, states ;

In nervous systems with bilateral symmetry, many neurons project axons across the midline to the opposite side. In each segment of the Drosophila embryonic nervous system, axons that display this projection pattern choose one of two distinct tracts: the anterior or posterior commissure. Commissure choice is controlled by Derailed, an atypical receptor tyrosine kinase expressed on axons projecting in the anterior commissure.
The growth cones of developing neurons are guided to their targets by attractive and repulsive cues in the extracellular environment. Specific receptors on the growth cones recognize these cues and transduce signals that ultimately lead to changes in direction of growth. The best understood of these cues and their axonal receptors are involved in guidance of the large number of axons that project across the midline to the opposite side of the central nervous system (CNS). Distinct groups of cells at the midline divide the two halves of the CNS and have a critical role in axon guidance. These cells, termed midline glia in Drosophila, secrete diffusible factors, the Netrins, capable of attracting contralaterally projecting axons. They also secrete a repellent factor Slit, which together with its receptor Roundabout (Robo) and an intracellular sorting factor that modulates the delivery of Robo to the cell surface, controls whether or not axons will cross the midline.

The authors demonstrate that Derailed keeps these axons out of the posterior commissure by acting as a receptor for Wnt5, a member of the Wnt family of secreted signalling molecules. The results reveal an unexpected role in axon guidance for a Wnt family member, and demonstrate that the Derailed receptor is an essential component of Wnt signalling in these guidance events. (Emphasis mine) Source: Science Week 2003

[Haj]

Wow, thanks for finding that for me. I actually understood all of that (it's a miracle!!) and it will help no end, thanks so much.