Retinal adaptation to dim light vision in spectacled caimans (Caiman crocodilus fuscus): Analysis of retinal ultrastructure
Retinal adaptation to dim light vision in spectacled caimans (Caiman crocodilus fuscus): Analysis of retinal ultrastructure
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DOI：
10.1016/j.exer.2018.05.006
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发表时间：
2018-08-01
影响因子：
3.4
通讯作者：
Bringmann, Andreas
中科院分区：
文献类型：
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作者：
Karl, Anett;Agte, Silke;Bringmann, Andreas
研究表明，哺乳动物视网膜胶质细胞 (Muller) 充当活光纤，引导光线穿过视网膜组织到达感光细胞 (Agte 等人，2011；Franze 等人，2007)。然而，对于非哺乳动物物种，尚不清楚穆勒细胞是否也能改善视网膜光传输。此外，对于非哺乳动物物种来说，缺乏视网膜细胞的超微结构数据，而这些数据通常提供视网膜功能的基本信息，即物种的视力。对细胞超微结构的详细研究提供了研究的基本方法。因此，本研究的目的是在电子和光学显微镜水平上研究眼镜凯门鳄的视网膜，以描述其结构特征。对于电子显微镜，我们使用超快微波固定程序，以获得比普通固定技术更精确的超微结构信息。因此，我们对所有视网膜部分的详细超微结构研究显示出结构特征，强烈表明凯门鳄视网膜适应昏暗的光线和夜视。米勒细胞的各种结构特征推测，米勒细胞可以增加穿过神经视网膜的光路的光强度，从而增加眼镜凯门鳄暗视觉的敏感性。 Muller 细胞穿过神经视网膜的整个厚度，因此可以将光线从视网膜内表面引导至感光细胞核周和感光节段之间的 Muller 细胞微绒毛。厚厚的穆勒细胞树干/突起穿过包含光散射结构（即神经纤维和突触）的层。大的米勒细胞胞体贯穿内核层，并含有扁平、细长的米勒细胞核，这些细胞核沿光路排列，因此可以减少沿视网膜光路的光强度损失。内丛状层中的许多穆勒细胞干/突起和内核层中的大穆勒细胞体体的倾斜排列可能表明通过穆勒细胞的光引导增加了视觉敏感性。此外，凯门鳄视网膜对低光水平的适应得到了其他视网膜部分的详细超结构数据的有力支持，例如视网膜的详细超微结构数据。通过（i）基于鸟嘌呤的视网膜绒毡层的存在，（ii）视网膜的杆优势，（iii）穿透外界膜的感光细胞核的存在，（iv）感光细胞和神经元细胞的密度相对较低，通过（v）具有长而厚的外节的杆的存在来补偿，这可能会增加光子吸收的可能性。根据细胞数量分析，背侧绒毡层视网膜的中央和颞区支持在较暗的水中向下检测猎物，是最高的昼间对比度/色觉（即锥视觉）和最高的视网膜光敏感性（即杆视觉）的部位。
It has been shown that mammalian retinal glial (Muller) cells act as living optical fibers that guide the light through the retinal tissue to the photoreceptor cells (Agte et al., 2011; Franze et al., 2007). However, for nonmammalian species it is unclear whether Muller cells also improve the transretinal light transmission. Furthermore, for nonmammalian species there is a lack of ultrastructural data of the retinal cells, which, in general, delivers fundamental information of the retinal function, i.e. the vision of the species. A detailed study of the cellular ultrastructure provides a basic approach of the research. Thus, the aim of the present study was to investigate the retina of the spectacled caimans at electron and light microscopical levels to describe the structural features. For electron microscopy, we used a superfast microwave fixation procedure in order to achieve more precise ultrastructural information than common fixation techniques.As result, our detailed ultrastructural study of all retinal parts shows structural features which strongly in-dicate that the caiman retina is adapted to dim light and night vision. Various structural characteristics of Muller cells suppose that the Muller cell may increase the light intensity along the path of light through the neuroretina and, thus, increase the sensitivity of the scotopic vision of spectacled caimans. Muller cells traverse the whole thickness of the neuroretina and thus may guide the light from the inner retinal surface to the photoreceptor cell perikarya and the Muller cell microvilli between the photoreceptor segments. Thick Muller cell trunks/processes traverse the layers which contain light-scattering structures, i.e., nerve fibers and synapses. Large Muller cell somata run through the inner nuclear layer and contain flattened, elongated Muller cell nuclei which are ar-ranged along the light path and, thus, may reduce the loss of the light intensity along the retinal light path. The oblique arrangement of many Muller cell trunks/processes in the inner plexiform layer and the large Muller cell somata in the inner nuclear layer may suggest that light guidance through Muller cells increases the visual sensitivity.Furthermore, an adaptation of the caiman retina to low light levels is strongly supported by detailed ultra-structural data of other retinal parts, e.g. by (i) the presence of a guanine-based retinal tapetum, (ii) the rod dominance of the retina, (iii) the presence of photoreceptor cell nuclei, which penetrate the outer limiting membrane, (iv) the relatively low densities of photoreceptor and neuronal cells which is compensated by (v) the presence of rods with long and thick outer segments, that may increase the probability of photon absorption. According to a cell number analysis, the central and temporal areas of the dorsal tapetal retina, which supports downward prey detection in darker water, are the sites of the highest diurnal contrast/color vision, i.e. cone vision and of the highest retinal light sensitivity, i.e. rod vision.