Retinotopic order in the optic nerve and superior colliculus during development of the retinocollicular projection in the wallaby ( Macropus eugenii )

Abstract

Retinotopic order of optic axons in the optic nerve and superior colliculus of the marsupial mammal, the wallaby (Macropus eugenii), has been examined and compared during development of the retinocollicular projection to investigate the role of order in the nerve in map formation. Small groups of axons from different retinal quadrants were labelled in vivo with a carbocyanine dye from just after axons first reached the colliculus to when the projection was mature. The distribution and branching patterns of axons and their arbors on the colliculus were assessed quantitatively during this period, as was the degree of order in the nerve. Initially, axons accumulated in coarse retinotopic order in the colliculus, with little branching and no sign of arborization to form terminal zones. Axons labelled from deposits covering a mean of 2.2% of the retina reached a mean collicular coverage of around 30% at 41-47 days, at which time they began arborizing in their retinotopically correct positions. By 55 days axons from all retinal quadrants had formed terminal zones in their retinotopically correct positions. Axons did not arborise in incorrect positions as has been reported in the rat. By 61-68 days coverage had decreased to around 10%. By 90-95 days only axons supplying terminal zones were present and terminal zones were smaller. In the nerve, axons showed a coarse and consistent order throughout development. This order was retinotopic only immediately behind the eye. Temporal and nasal axons occupied corresponding halves of the nerve along its course. Axons from dorsal and ventral retina shifted from dorsal and ventral positions in the nerve, respectively, to opposite sides of the nerve just before the chiasm. This would assist in positioning them in the appropriate lateral and medial optic tracts, respectively, in the positions they occupied as they approached the colliculus. However, the position in the nerve was not related to the ability to arborize in the correct collicular position. In particular, the increase in retinotopic order in the colliculus late in development was not accompanied by an increase in order in the nerve. Since the final organization in the colliculus shows greater order than is ever seen in the nerve, additional mechanisms must be involved in the maturation of the collicular map.

Overexpression of Ten‐m3 in the retina alters ipsilateral retinocollicular projections in the wallaby (Macropus eugenii)

“…The ages examined spanned the period when retinal axons are growing into and forming connections in the SC (Mark et al, 1993a; Ding and Marotte, 1996, 1997). At P12–17, retinal axons are growing into both the ipsilateral and contralateral SC in coarse retinotopic order and by P25–33 have fully covered the contralateral SC although arborisation has not commenced.…”

“…This is consistent with the in situ hybridisation data which showed retinotopically corresponding high ventral to low dorsal gradients of Ten‐m3 mRNA expressed in retinal ganglion cells, and high medial to low lateral gradients of Ten‐m3 within the retinorecipient layers of the SC. This suggests that retinal ganglion cell afferents form connections with target cells in the SC that express Ten‐m3 at similar relative levels: the ventral retina which expressed higher levels of Ten‐m3 than dorsal retina connects with medial SC (Mark et al, 1993b; Ding and Marotte, 1997) which also expressed higher levels than lateral SC. Results from the in situ hybridisation experiments revealed that the gradients were more pronounced in younger age groups, with one of the younger age groups (P27–37) showing the greatest fold differences in qPCR data in regional expression in both the retina and SC.…”

GAP-43 Immunoreactivity in the brain of the developing and adult wallaby ( Macropus eugenii )

“…Invasion of the superior colliculus by retinocollicular axons appears to coincide with the initial development of laminar cytoarchitecture in the superior colliculus and the appearance of strong GAP-43 ir in rostrocaudally running fibers in the superficial layers, where these axons are located (Ding and Marotte 1997). A more complex laminar specific pattern of GAP-43 ir fibers appears by P19, but the deeper fibers seen at that time are probably from sources other than the retina.…”

Electrical field analysis of extracellular electrical stimulation of optic nerve with spiral cuff electrode

“…The influence can be ignored. The relationship between stimulating patterns and the position, size and shape of phosphene are also not clear [14]. The distribution of electric field of electrode is an important factor to evaluate the stimulation effect.…”