Reese B E
University of Oxford, Department of Human Anatomy, U.K.
Neuroscience. 1987 Sep;22(3):1015-24. doi: 10.1016/0306-4522(87)92977-0.
The distribution of axons according to diameter was examined in the optic nerve and optic tract of adult hooded rats. Observations were made on semithin sections, and measurements of axonal diameters were made on electron micrographs taken from various locations across thin sections through the optic nerve and tract. The distribution of axons by size differs markedly in the optic nerve and tract. Coarse (greater than 2 microns) and fine (less than or equal to 2 microns) axons are distributed throughout all regions of the optic nerve. In the optic tract, in contrast, coarse axons are especially dense dorsally, at the deep border of the tract, while they are absent ventrally, subjacent to the pial surface. No regions of the optic nerve contain densities of coarse axons as high as the deep nor as low as the superficial extremes of the optic tract. Nevertheless, even at the deep (dorsal) border of the optic tract, the coarse axons make up only a small minority (roughly 15%) of the total number of axons in that region. The axons 2 microns or smaller may be divisible into two overlapping, fine and intermediate, diameter classes, that are partially segregated within the optic tract, but not in the optic nerve: the distributions of axon diameters smaller than 2 microns are skewed to distinctly smaller diameters at the dorsal and ventral extremes of the optic tract, while in between, at mid-positions along the deep-to-superficial axis of the optic tract, the axon size distributions contain many more axons greater than 1 micron in diameter. These different axon diameter groups may arise from the morphologically distinct retinal ganglion cell types, and may underlie the components of the trimodal compound axon potential seen in the rat's primary optic pathway. Their partial segregation within the tract anticipates the partial segregation of their terminal arborizations within the laminae of the dorsal lateral geniculate nucleus. The rearrangement of axons into a partial segregation by size within the optic tract may indicate a chronology of axonal arrival during early development, proximity to the pial surface being an index of recency of arrival. As axonal outgrowth and neurogenesis appear to be directly related within the retinal ganglion cell population in mammals, the relative birthdates of the retinal ganglion cell types giving rise to the axon diameter classes in the rat may be inferred from the present results.
研究了成年带帽大鼠视神经和视束中轴突按直径的分布情况。在半薄切片上进行观察，并在通过视神经和视束的薄切片不同位置拍摄的电子显微照片上测量轴突直径。视神经和视束中轴突按大小的分布明显不同。粗（大于2微米）轴突和细（小于或等于2微米）轴突分布于视神经的所有区域。相比之下，在视束中，粗轴突在背侧、视束深部边界处特别密集，而在腹侧、软膜表面下方则没有。视神经中没有任何区域的粗轴突密度像视束深部那样高，也不像视束浅表极端处那样低。然而，即使在视束深部（背侧）边界处，粗轴突在该区域轴突总数中也只占一小部分（约15%）。直径为2微米或更小的轴突可分为两个重叠的、细直径和中等直径类别，它们在视束内部分离，但在视神经中没有：在视束背侧和腹侧极端处，直径小于2微米的轴突分布向明显更小的直径倾斜，而在两者之间，沿着视束深部到浅表轴的中间位置，轴突大小分布包含更多直径大于1微米的轴突。这些不同的轴突直径组可能源于形态学上不同的视网膜神经节细胞类型，并且可能是大鼠初级视觉通路中三峰复合轴突电位成分的基础。它们在视束内的部分分离预示着它们在背外侧膝状核层内终末分支的部分分离。视束内轴突按大小重新排列为部分分离可能表明早期发育过程中轴突到达的时间顺序，靠近软膜表面是到达时间新近程度的一个指标。由于哺乳动物视网膜神经节细胞群体中轴突生长和神经发生似乎直接相关，因此根据目前的结果可以推断出大鼠中产生轴突直径类别的视网膜神经节细胞类型的相对出生日期。
