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67% Mon 2:46 PM , e Window Help oSave ZO O ? ? ?Neuro HW 16 . Saved to SharePoint a ort Design Layout References Mailings Reviow View Q- Share Arial 16-Q3: Based on neural explant assays, does netrin act as a long- or short-range cue? Is it an attractive or repulsive cue? Explain your answer 16-Q4: Based on the competition model, what would be the expected result if the temporal axons could be ablated early in development (before they had grown into the tectum/SC)? 16-Q5: What did the expression of EphA3 in Islet2+ RGCs tell us about temporal axon targeting? 16-Q6: Ephrins and Ephs are distributed in dorsoventral gradients in the retina and the tectum, respectively. EphB1 is expressed in the tectum, in a low-dorsal to high-ventral gradient. Ephrin-B1 and ephrin-B2 are expressed in the retina in a high-dorsal to low- ventral gradient. What do these expression patterns suggest about the mechanism used for dorsoventral patterning of RGCs projecting to the tectum? 1 of2 273 woeds Foous MacBook ProExplanation / Answer
16-Q3:
Developing axons are guided to their targets in the nervous system by the combined actions of attractive and repulsive guidance cues. The cue identities and their contributions to axon guidance are explained here. A family of putative guidance cues for developing axons are netrins. Netrins are large (70 to 80 KDa) soluble proteins that show homology with laminin. Netrins were used to implicate circumferential axonal projections in vertebrates. Commissural axons began a circumferential trajectory to the floor plate at the ventral midline of the neural tube at the time of embryogenesis. A diffusible factor secreted by the floor plate cells promoted the outgrowth of commissural axons from the explants of dorsal spinal cord into the collagen matrices In Vitro and reoriented these axons within the neural epithelium. Netrin-1 expressed by the floor plate cells is a protein for mediating the outgrowth promoting activities ad reorienting activities of these cells while netrin-2 is expressed in the ventral 2/3rds of the spinal cord.
Netrin either induces attractive or repulsive signals based on the receptor types expressed and on the intracellular level of cAMP. It is not clear whether guidance cues are directional or temporal. But, they can be integrated spatially or temporally.
16-Q4:
No single pioneer neuron is necessary for the axon tract formation. The interaction between two pioneers is essential for the fascicle establishment and the pioneer neurons function synergistically to establish longitudinal axon tracts, to help in the fasciculation of follower neurons along with specific fascicles and to prevent axons from moving across the midline.
The ablation experiments have shown that retinal ganglion cell axons from incomplete retinas invade the whole tectum, which seems to be offering a competition between retinal ganglionic cell axons during map formation, the function of competition in retinocollicular mapping is not clear still.
16-Q-5:
The research has shown that recognition specificity during synaptic layer formation is formed through a temporally restricted axonal competence to respond to broadly expressed adhesion molecules. Developing neurons innervating the same target project into a different, birth-order-dependent sequence. Temporal identity appears to consist of crucial information in generating not only cell type diversity at the time of neuronal division but connection diversity of projecting neurons. EPh receptors were found to be mediating developmental events of the nervous system. Eph receptors have single kinase domain and an extracellular region consisting of Cys-rich domain and 2 fibronectin type III repeats.
16-Q-6:
EphrinB expression along the dorsoventral axis of retina suggests a role for these receptor proteins that also act as bidirectional signaling molecules in dorsoventral-mediolateral retinocollicular mapping. EphrinB2 tyrosine kinase activity and EphrinB1 intracellular signaling are the main mediators of ventral-temporal retinal ganglion cell axon retinocollicular mapping by interacting with EphrinB1 in the superior colliculus. The roles for the EphrinB2 intracellular domain in the retinocollicular mapping and both dorsal and ventral temporal retinal ganglion cell axons utilize reverse signaling for topographic mapping. The dorsoventral retinal ganglion cell axon termination is carried out by both forward and reverse signaling initiated by EPhB: ephrin-B interactions along the mediolateral axis of the superior colliculus.
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