Shepherd I T, Taylor J S
Department of Human Anatomy, University of Oxford, United Kingdom.
J Comp Neurol. 1995 Apr 17;354(4):501-10. doi: 10.1002/cne.903540403.
The early development of the uncrossed tectobulbar and the crossed tectospinal tracts was studied. These two projections arise from the same structure, the mesencephalon, and develop during the same time period, but follow divergent courses. We have traced the pathways followed by these projections and identified the positions at which axon guidance decisions are made. The first neurons differentiate either side of the entire rostrocaudal extent of the dorsal midline and initiate axons that extend dorsoventrally across the surface of the tectum. At the ventral edge of the tectum these axons turn abruptly and fasciculate to form a caudal descending projection to the hindbrain. These axons extend to the caudal hindbrain and do not project to the periphery along cranial nerve roots. We therefore consider this tract to be the tectobular, rather than the mesencephalic division of the trigeminal. While the tectobulbar projection is still developing, a second wave of axons is initiated, which arises from only the rostral part of the tectum. These axons grow beyond the tectobulbar turn point and continue toward the ventral midline, where they cross the floor plate, before turning caudally at the lateral edge of the main descending hindbrain tract, the ventrolateral tract. We discuss the development of these tracts with reference to possible guidance cues mediating their course.
对未交叉的顶盖延髓束和交叉的顶盖脊髓束的早期发育进行了研究。这两种投射起源于同一结构——中脑，且在同一时期发育，但走行不同。我们追踪了这些投射所遵循的路径，并确定了轴突导向决策发生的位置。第一批神经元在背侧中线整个头尾方向的两侧分化，并发出轴突，这些轴突在顶盖表面背腹向延伸。在顶盖的腹侧边缘，这些轴突突然转向并形成束，向后脑形成尾侧下行投射。这些轴突延伸至尾侧后脑，并不沿颅神经根投射到外周。因此，我们认为这条束是顶盖延髓束，而非三叉神经的中脑分支。当顶盖延髓束仍在发育时，第二批轴突开始发出，这批轴突仅起源于顶盖的头侧部分。这些轴突生长至顶盖延髓束的转折点之外，继续向腹侧中线延伸，在那里它们穿过底板，然后在主要下行后脑束——腹外侧束的外侧边缘转向尾侧。我们参照可能介导其走行的导向线索来讨论这些束的发育。
