The timecourse of neuronal connections of the rotundoectostriatal pathway in chicks (Gallus gallus) during embryogenesis: A retrograde transport study

Abstract

The avian retinotectofugal pathway has been suggested to be homologous to the mammalian retinotectofugal pathway. The projection of the nucleus rotundus upon the ectostriatum is equivalent to that of the pulvinar nucleus upon the extrastriate cortex in mammals. In this system, the optic tectum relays retinal input to the nucleus rotundus, which then ascends to the ectostriatum of the telencephalon. Given the fact that the chick retinotectofugal system becomes mature early during development, the present study attempted to investigate the timecourse of neuronal connections of the embryonic rotundoectostriatal pathway. We used multiple injections of cholera toxin B subunit (CTb) in the ectostriatum of chick embryos to retrogradely trace projections to the nucleus rotundus. We found CTb-labeled neurons in the nucleus rotundus at embryonic day 7.5-8. By embryonic day 8-8.5, increased numbers of CTb-labeled neurons were seen in the nucleus rotundus. It was noted that the time of this initial connection between the nucleus rotundus and the ectostriatum is nearly synchronous with that of the retinotectal and tectorotundal pathways, respectively (Crossland et al., 1975; Thanos & Bonhoeffer, 1987; Wu et al., 2000). These findings, combined with the present study, suggest that the retinotectofugal system becomes established, at least at a structural level, by embryonic day E8.

Early Development of the Thalamo‐Pallial Stage of the Tectofugal Visual Pathway in the Chicken (Gallus gallus)

“…In addition, our assays also showed the existence of a highly organized rotundo-entopallium projection from E8 onward. These findings are in line with previous retrograde tracing studies that have observed thalamic fibers reaching the pallium as early as E6 (Bielle et al 2011;Tosa et al 2015) and the presence of a distinct rotundo-entopallium projection by E8 (Wu, Charlton, and Karten 2000).…”

“…It was previously described that the TGC axons already project to the nucleus rotundus by E7.5–E8 (Wu, Charlton, and Karten 2000). We can now add that, according to our results, Rt–E and E–M systems of projections can also be detected as early as E7.5–8.…”

“…2011; Tosa et al. 2015) and the presence of a distinct rotundo‐entopallium projection by E8 (Wu, Charlton, and Karten 2000).…”

Asymmetrical Modes of Visual Bottom-Up and Top-Down Integration in the Thalamic Nucleus Rotundus of Pigeons

“…Although this possibility cannot be excluded, it probably does not explain all lateralized bottom-up effects. The thalamofugal system matures considerably later than the tectofugal one and is functional only at the time of hatch in chicks (Wu and Karten, 1998;Wu et al, 2000). Although the sequence of ontogenetic effects is comparable in chicks and pigeons, neural ontogenetic maturation speed is slower in the pigeon (Manns and Güntürkün, 1997).…”