Drenhaus U, von Gunten A, Rager G
Institute for Anatomy and Special Embryology, CH-1700 Fribourg, Switzerland.
Anat Rec. 1997 Sep;249(1):103-16. doi: 10.1002/(SICI)1097-0185(199709)249:1<103::AID-AR13>3.0.CO;2-T.
It has been suggested that retinal ganglion cells (RGCs) of Tupaia can be subdivided into three classes that correspond to the X, Y, and W classes in the cat. Estimates of these classes as determined by electrophysiological experiments and by histological studies of the retina are at variance. Because the RGC classes differ in axon diameter, this parameter could serve as a reliable criterion for the evaluation of RGC classes and their proportions.
An electronmicroscopic analysis of four optic nerves was carried out. The density of axons and their diameters was recorded from cross sections in the middle of the nerves. Based on a theoretical model, axons were classified according to the three known RGC classes. In addition, we investigated how axons of different size are distributed within the nerve.
On average, the total number of axons is 570,000. More than 99% of axons are myelinated. Axon diameters can be 0.2-3.6 microns, and fiber diameters can be 0.3-4.6 microns. The frequency distributions of axon and fiber diameter are positively skewed and multimodal. Our analysis revealed three distinct axon diameter groups with the following mean axon diameters and proportions: the small or S group: 0.55 micron, 70%; the medium-sized or M group: 0.88 micron, 20%; the large of L group: 1.39 microns, 10%. Density and axon diameter plots produced consistent patterns, according to which axons of different sizes were distributed in the optic nerve. Thick axons are located dorsotemporally and centrally in the nerve. The average diameter value decreases toward the periphery. The smallest values are found at the ventronasal border of the nerve.
The observed axon diameter groups probably arise from functionally distinct RGC types. There is evidence that these groups correspond to functional W, X, and Y RGC classes. Our study also provides the first evidence of the existence of a topographic order of fibers within the nerve of the tree shrew.
有人提出，树鼩的视网膜神经节细胞（RGCs）可分为三类，分别对应于猫的X、Y和W类。通过电生理实验和视网膜组织学研究确定的这些类别的估计值存在差异。由于RGC类别在轴突直径上有所不同，该参数可作为评估RGC类别及其比例的可靠标准。
对四条视神经进行了电子显微镜分析。从神经中部的横切面记录轴突的密度及其直径。基于一个理论模型，根据三种已知的RGC类别对轴突进行分类。此外，我们研究了不同大小的轴突在神经内的分布情况。
平均而言，轴突总数为570,000。超过99%的轴突有髓鞘。轴突直径可为0.2 - 3.6微米，纤维直径可为0.3 - 4.6微米。轴突和纤维直径的频率分布呈正偏态且多峰。我们的分析揭示了三个不同的轴突直径组，其平均轴突直径和比例如下：小或S组：0.55微米，70%；中等大小或M组：0.88微米，20%；大或L组：1.39微米，10%。密度和轴突直径图产生了一致的模式，据此不同大小的轴突分布在视神经中。粗轴突位于神经的背颞侧和中央。平均直径值向周边减小。在神经的腹鼻侧边界发现最小值。
观察到的轴突直径组可能源于功能不同的RGC类型。有证据表明这些组对应于功能性的W、X和Y RGC类别。我们的研究还首次提供了树鼩神经内纤维存在拓扑顺序的证据。
