Wen Yi, Zou Jian, Fang Wei, Sun Ming, Stolz Donna B, Wei Xiangyun
Department of Ophthalmology, University of Pittsburgh, Pittsburgh, USA.
Present address: Institute of Genetics, Zhejiang University International School of Medicine, Hangzhou, Zhejiang, China.
Mol Neurobiol. 2025 Dec 5;63(1):253. doi: 10.1007/s12035-025-05490-4.
Light refraction by transparent eye tissues focuses object images on photoreceptor outer segments for visual perception. To enhance this light-focusing function, heterochromatin in the rods of nocturnal mammals adapts to organize into a central clump that acts as a miniature converging lens. However, whether and how the nuclear architectures of other retinal cells affect vision in vertebrates remains unknown. Here, we examine chromatin organization patterns in the zebrafish retina under transmission electron microscopy (TEM) and compare them with those of other vertebrates. In the outer vertebrate retinal cells, chromatin is segregated in various patterns into more refractive heterochromatin and less refractive euchromatin, which we now name collectively "dualchromatin." By contrast, in the inner retinal cells, chromatin adopts a uniform architecture with no clear distinction between heterochromatin and euchromatin observable under TEM; we thus name such chromatin "unichromatin." The unichromatic architecture in the "inverted" vertebrate retinas is neither conserved in the "everted" retinas of cephalopods and gastropods nor in non-visual sensory systems of vertebrates. By basic optics, we infer that heterogeneous dualchromatin may distort light more than homogeneous unichromatin. Considering that the dualchromatic nuclear architecture may facilitate gene expression better than unichromatin, we propose that the geographically differential utilization of unichromatin and dualchromatin in the vertebrate retinas is a compromising evolutionary visual adaptation to balance the conflicting demands on nuclear architectures imposed by transcriptomic fitness and optical clarity.
透明眼组织对光线的折射将物体图像聚焦在光感受器外段以实现视觉感知。为增强这种聚光功能，夜行性哺乳动物视杆细胞中的异染色质会适应性地组织成一个中央团块，其作用类似于一个微型会聚透镜。然而，其他视网膜细胞的核结构是否以及如何影响脊椎动物的视觉仍然未知。在这里，我们在透射电子显微镜（TEM）下检查斑马鱼视网膜中的染色质组织模式，并将它们与其他脊椎动物的模式进行比较。在脊椎动物的外视网膜细胞中，染色质以各种模式分离为折射性更强的异染色质和折射性较弱的常染色质，我们现在将它们统称为“双染色质”。相比之下，在内视网膜细胞中，染色质呈现出均匀的结构，在透射电子显微镜下观察不到异染色质和常染色质之间的明显区别；因此我们将这种染色质称为“单染色质”。“倒置”脊椎动物视网膜中的单染色质结构在头足类动物和腹足类动物的“外翻”视网膜中以及脊椎动物的非视觉感觉系统中均不保守。根据基本光学原理，我们推断异质性双染色质可能比同质性单染色质使光线发生更大的畸变。考虑到双染色质核结构可能比单染色质更有利于基因表达，我们提出脊椎动物视网膜中单染色质和双染色质在空间上的差异利用是一种折衷的进化视觉适应，以平衡转录组适应性和光学清晰度对核结构的相互冲突的要求。
