Singman E L, Scalia F
Department of Anatomy and Cell Biology, State University of New York, Brooklyn 11203.
J Comp Neurol. 1990 Dec 22;302(4):792-809. doi: 10.1002/cne.903020411.
The proportion of ganglion cells connected to the several central targets of the retinal projection varies in different species. In the frog, the retinotectal projection is clearly the largest branch of the optic pathway and the relative size of the tectally projecting population can be expected to be correspondingly great. However, there have been no studies aimed at quantifying the size of this population and at partitioning its contralateral and ipsilateral components. We injected the tectum with horseradish peroxidase (HRP) dried onto fine needles to count the numbers of retinal ganglion cells labeled by retrograde transport. The retinas were prepared as flat-mounts to facilitate the cell counting. The tecta were injected either unilaterally or bilaterally in mirror-symmetric loci. Specimens included completely normal frogs and frogs which had undergone unilateral optic nerve regeneration, although only normal retinas are presented in the current study. The retrograde transport interval was varied progressively (from 3 to 5 days), and single or multiple injections of HRP were placed singly or as clusters, in order to increment the cell counts toward a level of saturation. Approximately 70.9% of the neurons in the ganglion cell layer could be labeled by this method. Correcting for the presence of displaced amacrine cells, estimated to comprise approximately 16% of the neurons in the ganglion cell layer (Scalia et al., '85, Brain Res. 344:267-280), we calculate that approximately 84.4% of the retinal ganglion cells project contralaterally to the optic tectum. Flat-mounted retinas ipsilateral to unilaterally injected tecta of completely normal frogs were also examined for labeled cells. The results of injections in the rostrolateral, caudomedial, and caudolateral tectum were studied. We found that ipsilaterally labeled cells comprised no more than 2.3% of the overall population of ganglion cells in the ganglion cell layer. The ipsilaterally projecting cells were found in loci which were approximately mirror-symmetric to the regions of maximal cell labeling in the contralateral retinas from the same animals. The ipsilateral population was always displaced toward the periphery of the retina with respect to the contralateral population, regardless of whether the contralateral locus was centered in the temporal, ventronasal, or dorsonasal sector of the retina. Because the ipsilaterally projecting ganglion cells form such a minor population, and because they exist in the monocular as well as the binocular parts of the retina, it seems likely that they may not play a significant role in visual function in the frog.
连接到视网膜投射的几个中枢目标的神经节细胞比例在不同物种中有所不同。在青蛙中，视网膜-脑顶盖投射显然是视觉通路中最大的分支，因此可以预期投射到脑顶盖的神经节细胞群体的相对大小也会相应地很大。然而，目前还没有研究旨在量化这个群体的大小，并区分其对侧和同侧成分。我们将干燥在细针上的辣根过氧化物酶（HRP）注入脑顶盖，以计数通过逆行运输标记的视网膜神经节细胞的数量。将视网膜制成平铺标本以便于细胞计数。脑顶盖单侧或双侧注射，注射位点呈镜像对称。标本包括完全正常的青蛙和经历过单侧视神经再生的青蛙，不过本研究仅展示正常视网膜的情况。逆行运输的时间间隔逐渐变化（从3天到5天），HRP单次或多次注射，单次或成簇注射，以便使细胞计数增加到饱和水平。通过这种方法可以标记神经节细胞层中约70.9%的神经元。校正估计约占神经节细胞层神经元16%的移位无长突细胞的存在（Scalia等人，《脑研究》，1985年，第344卷：267 - 280页），我们计算出约84.4%的视网膜神经节细胞向对侧投射到视神经脑顶盖。还检查了完全正常青蛙单侧注射脑顶盖同侧的平铺视网膜中的标记细胞。研究了在脑顶盖的前外侧、后内侧和后外侧注射的结果。我们发现同侧标记的细胞不超过神经节细胞层神经节细胞总数的2.3%。同侧投射细胞位于与同一动物对侧视网膜中最大细胞标记区域大致镜像对称的位置。同侧群体相对于对侧群体总是向视网膜周边移位，无论对侧位点位于视网膜的颞侧、腹鼻侧还是背鼻侧扇形区域。由于同侧投射的神经节细胞形成的群体如此之小，并且它们存在于视网膜的单眼和双眼部分，所以它们似乎不太可能在青蛙的视觉功能中发挥重要作用。
