Wu C C, Russell R M, Karten H J
Department of Neurosciences, University of California, San Diego, School of Medicine, La Jolla 92037-0608, USA.
J Comp Neurol. 2000 Jan 31;417(1):115-32.
The avian tectorotundal system has been suggested as a homologue of the mammalian colliculopulvinar system. In the tectorotundal system, neurons of the stratum griseum centrale (SGC) of the optic tectum send their axons bilaterally to the nucleus rotundus (Rt). In transit to the Rt, the axons of the SGC neurons collateralize in the nuclei posteroventralis thalami (PV), subpretectalis (SP), and interstitiopretectosubpretectalis (IPS) of the tectothalamic tract (TT). The current study used birthdating and pathway-tracing methods to investigate the neurogenesis and time course of neuronal connections of the tectorotundal pathway in chicks during embryogenesis. By using tritiated thymidine autoradiography, we observed that the SGC neurons of the tectum were generated by embryonic days 3.0-5.5 (E3.0-E5.5), the Rt by E3.5-E5.0, and the nuclei of TT by E3.5-4.5. To trace the tectorotundal pathway, we injected cholera toxin B subunit (CTb) into the tectum, and the CTb-like immunoreactivity was examined. By E4.5-E5.5, some CTb-like immunoreactive (CTb-LI) axons terminated in the ipsilateral SP/IPS. By E6.0-E6.5, CTb-LI axon bundles were seen ipsilaterally in the TT. Increased numbers of labeled axons were seen terminating in the SP/IPS. By E7.0-E7.5, heavily labeled axons in the TT were observed with diffuse terminals in areas ventral to the presumptive Rt and PV. By E7.5-E8.0, the tectal axons innervated the ipsilateral Rt, in which some of the collaterals crossed the midline to the contralateral diencephalon. The crossed tectorotundal projection was seen first by E8.0-E8.5. Also, during this stage, a few CTb-LI collaterals terminated in the contralateral SP/IPS. Between E10 and E13, the pattern of bilateral tectorotundal projections became more regionalized, whereas labeling continued to increase in the SP/IPS. At E16, the labeling pattern of all tectorecipient structures resembled that of the hatchling. The current study revealed the temporal order of development of the tectorotundal pathway during embryogenesis. The SGC cells first innervate ipsilaterally the SP/IPS and then the Rt/PV. The schedule of the crossed tectorotundal connections coincides with the schedule of tectal projections onto the contralateral intrinsic nuclei of the TT. We conclude that E8.0 (+/- E0.5) is a critical stage for the development of the tectofugal pathway. Moreover, the current study provides important insights into the relative ontogeny of the mammalian tectofugal pathway.
禽顶盖-圆核系统被认为是哺乳动物中脑丘-丘脑枕系统的同源物。在顶盖-圆核系统中，视顶盖中央灰质层（SGC）的神经元将其轴突双侧投射至圆核（Rt）。在投射至Rt的过程中，SGC神经元的轴突在顶盖-丘脑束（TT）的丘脑后腹核（PV）、顶盖前区（SP）和顶盖前间隙-顶盖前区（IPS）发生分支。本研究采用出生时间标记和通路追踪方法，研究鸡胚胎发育过程中顶盖-圆核通路的神经发生及神经元连接的时间进程。通过氚标记胸腺嘧啶核苷放射自显影，我们观察到顶盖的SGC神经元在胚胎第3.0 - 5.5天（E3.0 - E5.5）产生，Rt神经元在E3.5 - E5.0产生，TT的核团在E3.5 - 4.5产生。为追踪顶盖-圆核通路，我们将霍乱毒素B亚基（CTb）注入顶盖，并检测CTb样免疫反应性。到E4.5 - E5.5时，一些CTb样免疫反应性（CTb-LI）轴突终止于同侧的SP/IPS。到E6.0 - E6.5时，在同侧的TT中可见CTb-LI轴突束。终止于SP/IPS的标记轴突数量增加。到E7.0 - E7.5时，在TT中观察到大量标记轴突，其弥散性终末位于推测的Rt和PV腹侧区域。到E7.5 - E8.0时，顶盖轴突支配同侧的Rt，其中一些分支越过中线至对侧间脑。交叉的顶盖-圆核投射最早在E8.0 - E8.5时可见。此外，在此阶段，少数CTb-LI分支终止于对侧的SP/IPS。在E10至E13之间，双侧顶盖-圆核投射模式变得更加区域化，而SP/IPS中的标记持续增加。在E16时，所有顶盖接受区结构的标记模式类似于雏鸡。本研究揭示了胚胎发育过程中顶盖-圆核通路发育的时间顺序。SGC细胞首先同侧支配SP/IPS，然后支配Rt/PV。交叉的顶盖-圆核连接时间表与顶盖投射至TT对侧固有核的时间表一致。我们得出结论，E8.0（±E0.5）是顶盖传出通路发育的关键阶段。此外，本研究为哺乳动物顶盖传出通路的相对个体发生提供了重要见解。
