The multiple decisions made by growth cones of RGCs as they navigate from the retina to the tectum inXenopus embryos

Abstract

Retinal ganglion cells (RGCs) of Xenopus laevis send axons along a stereospecific pathway from the retina to their target the optic tectum. Viewed from the point of the growth cone, this journey is reflected by discrete processes of axon initiation, axon outgrowth, navigation, target recognition, and innervation. These processes are characterised by distinct signalling mechanisms that trigger dynamic changes in growth cone morphology and behavior. Here we review work primarily from our laboratory, examining these events from a cellular and molecular perspective, focusing on the roles of FGFs, netrins, receptors, and intracellular effectors.

Bilateral retinofugal pathfinding impairments limit behavioral compensation in near-congenital one-eyed Xenopus laevis

“…However, these axons target the thalamus rather than the tectum, and our results suggest that other mechanisms are relevant for axonal crossing in this species, either of a chemical or mechanical nature 50,51 . Even though we are unable to verify whether individual fibers target the ipsilateral equivalent of their canonical contralateral target area, overall pathfinding to gross pretectal and tectal areas appears intact 52 .…”

Bilateral retinofugal pathfinding impairments limit behavioral compensation in near-congenital one-eyed Xenopus laevis

“…However, these axons target the thalamus rather than the tectum, and our results suggest that other mechanisms are relevant for axonal crossing in this species, either of a chemical or mechanical nature 50,51 . Even though we are unable to verify whether individual fibers target the ipsilateral equivalent of their canonical contralateral target area, overall pathfinding to gross pretectal and tectal areas appears intact 52 .…”

Bilateral Retinofugal Pathfinding Impairments Limit Behavioral Compensation in Near-Congenital One-Eyed Xenopus laevis

“…However, these axons target the thalamus rather than the tectum, and our results suggest that other mechanisms are relevant for axonal crossing in this species, either of a chemical or mechanical nature ( Koser et al, 2016 ; Murcia-Belmonte and Erskine, 2019 ). Even though we are unable to verify whether individual fibers target the ipsilateral equivalent of their canonical contralateral target area, overall pathfinding to gross pretectal and tectal areas appears intact ( Dingwell et al, 2000 ).…”

A Retino-retinal Projection Guided by Unc5c Emerged in Species with Retinal Waves

“…To assess a putative correlation between the expression of Unc5c in the retina and the existence of an R-R projection and its function in refinement, we analyzed Unc5c expression in the developing retinas of zebrafish and chickens. Expression of Unc5c was not detected in the developing zebrafish retina at the time that RGC axons grow to reach their targets (36–48 h postfertilization [hpf]; Figures 7C and 7D) [43]. Accordingly, monocular injections of DiI in zebrafish embryos did not reveal R-R projections (Figures 7E and 7E’).…”

Bilateral retinofugal pathfinding impairments limit behavioral compensation in near-congenital one-eyed Xenopus laevis

“…However, these axons target the thalamus rather than the tectum, and our results suggest that other mechanisms are relevant for axonal crossing in this species, either of a chemical or mechanical nature 50,51 . Even though we are unable to verify whether individual fibers target the ipsilateral equivalent of their canonical contralateral target area, overall pathfinding to gross pretectal and tectal areas appears intact 52 .…”

Bilateral Retinofugal Pathfinding Impairments Limit Behavioral Compensation in Near-Congenital One-Eyed Xenopus laevis

“…However, these axons target the thalamus rather than the tectum, and our results suggest that other mechanisms are relevant for axonal crossing in this species, either of a chemical or mechanical nature ( Koser et al, 2016 ; Murcia-Belmonte and Erskine, 2019 ). Even though we are unable to verify whether individual fibers target the ipsilateral equivalent of their canonical contralateral target area, overall pathfinding to gross pretectal and tectal areas appears intact ( Dingwell et al, 2000 ).…”

A Retino-retinal Projection Guided by Unc5c Emerged in Species with Retinal Waves

“…To assess a putative correlation between the expression of Unc5c in the retina and the existence of an R-R projection and its function in refinement, we analyzed Unc5c expression in the developing retinas of zebrafish and chickens. Expression of Unc5c was not detected in the developing zebrafish retina at the time that RGC axons grow to reach their targets (36–48 h postfertilization [hpf]; Figures 7C and 7D) [43]. Accordingly, monocular injections of DiI in zebrafish embryos did not reveal R-R projections (Figures 7E and 7E’).…”

Bilateral retinofugal pathfinding impairments limit behavioral compensation in near-congenital one-eyed Xenopus laevis

“…However, these axons target the thalamus rather than the tectum, and our results suggest that other mechanisms are relevant for axonal crossing in this species, either of a chemical or mechanical nature 50,51 . Even though we are unable to verify whether individual fibers target the ipsilateral equivalent of their canonical contralateral target area, overall pathfinding to gross pretectal and tectal areas appears intact 52 .…”

Bilateral Retinofugal Pathfinding Impairments Limit Behavioral Compensation in Near-Congenital One-Eyed Xenopus laevis

“…However, these axons target the thalamus rather than the tectum, and our results suggest that other mechanisms are relevant for axonal crossing in this species, either of a chemical or mechanical nature ( Koser et al, 2016 ; Murcia-Belmonte and Erskine, 2019 ). Even though we are unable to verify whether individual fibers target the ipsilateral equivalent of their canonical contralateral target area, overall pathfinding to gross pretectal and tectal areas appears intact ( Dingwell et al, 2000 ).…”

A Retino-retinal Projection Guided by Unc5c Emerged in Species with Retinal Waves

“…To assess a putative correlation between the expression of Unc5c in the retina and the existence of an R-R projection and its function in refinement, we analyzed Unc5c expression in the developing retinas of zebrafish and chickens. Expression of Unc5c was not detected in the developing zebrafish retina at the time that RGC axons grow to reach their targets (36–48 h postfertilization [hpf]; Figures 7C and 7D) [43]. Accordingly, monocular injections of DiI in zebrafish embryos did not reveal R-R projections (Figures 7E and 7E’).…”