Study Biology

Question 2

(Essay)

When Sperry and colleagues ablated the dorsal RGCs, the remaining ventral RGCs projected to the dorsal tectum. Conversely, when they ablated the ventral RGCs, the remaining dorsal RGCs projected to the ventral tectum.
A. What are some plausible models for dorsoventral patterning suggested by these results?
B. Ephrins and Ephs are distributed in dorsoventral gradients in both structures. 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?
C. A stripe assay was prepared in which stripes of artificially clustered EphB1 receptor extracellular domain were prepared on a polylysine coated coverslip. Based on your model, how would you expect the axons of dorsal retinal explants to behave in this assay?

Answer

A.Perhaps dorsal RGCs are attracted to the ventral tectum and/or repulsed by the dorsal tectum. Conversely, perhaps ventral RGCs are attracted to the dorsal tectum and/or repulsed by the ventral tectum.
B. According to these data, the RGCs expressing a high level of ephrin-B project to the part of the tectum with a low level of EphB. This suggests a mechanism of reverse signaling, with activation of ephrin-B by EphB mediating attraction of dorsal axons to the ventral tectum.
C. The dorsal axons should preferentially grow on the stripes containing EphB1.

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