Department of Biology, American University, Washington, DC 20016, USA.
Tissue Cell. 2012 Aug;44(4):264-79. doi: 10.1016/j.tice.2012.04.004. Epub 2012 May 17.
The organization, morphological characteristics, and synaptic structure of photoreceptors in the adult zebrafish retina were studied using light and electron microscopy. Adult photoreceptors show a typical ordered tier arrangement with rods easily distinguished from cones based on outer segment (OS) morphology. Both rods and cones contain mitochondria within the inner segments (IS), including the large, electron-dense megamitochondria previously described (Kim et al.) Four major ultrastructural differences were observed between zebrafish rods and cones: (1) the membranes of cone lamellar disks showed a wider variety of relationships to the plasma membrane than those of rods, (2) cone pedicles typically had multiple synaptic ribbons, while rod spherules had 1-2 ribbons, (3) synaptic ribbons in rod spherules were ∼2 times longer than ribbons in cone pedicles, and (4) rod spherules had a more electron-dense cytoplasm than cone pedicles. Examination of photoreceptor terminals identified four synaptic relationships at cone pedicles: (1) invaginating contacts postsynaptic to cone ribbons forming dyad, triad, and quadrad synapses, (2) presumed gap junctions connecting adjacent postsynaptic processes invaginating into cone terminals, (3) basal junctions away from synaptic ribbons, and (4) gap junctions between adjacent photoreceptor terminals. More vitread and slightly farther removed from photoreceptor terminals, extracellular microtubule-like structures were identified in association with presumed horizontal cell processes in the OPL. These findings, the first to document the ultrastructure of the distal retina in adult zebrafish, indicate that zebrafish photoreceptors have many characteristics similar to other species, further supporting the use of zebrafish as a model for the vertebrate visual system.
使用光镜和电镜研究了成年斑马鱼视网膜中光感受器的组织、形态特征和突触结构。成年光感受器表现出典型的有序分层排列，基于外节（OS）形态，很容易将杆状细胞与锥状细胞区分开来。杆状细胞和锥状细胞的内节（IS）中都含有线粒体，包括之前描述的（Kim 等人）大型电子致密的巨大线粒体。在斑马鱼杆状细胞和锥状细胞之间观察到四个主要的超微结构差异：（1）锥状细胞板层盘的膜与质膜的关系比杆状细胞的膜更为多样化；（2）锥状细胞足通常具有多个突触小带，而杆状细胞的球状体只有 1-2 个小带；（3）杆状细胞球状体中的突触小带比锥状细胞足中的小带长约 2 倍；（4）杆状细胞球状体的细胞质比锥状细胞足更电子致密。对光感受器末端的检查确定了锥状细胞足上的四种突触关系：（1）内陷接触位于锥状小带突触后形成二联体、三联体和四联体突触；（2）假定连接相邻内陷进入锥状细胞末端的突触后过程的缝隙连接；（3）远离突触小带的基底连接；（4）相邻光感受器末端之间的缝隙连接。在更靠近视盘的外丛状层（OPL）中，与假定的水平细胞过程相关的细胞外微管样结构被鉴定出来，并且稍微远离光感受器末端。这些发现首次记录了成年斑马鱼远侧视网膜的超微结构，表明斑马鱼光感受器具有许多与其他物种相似的特征，进一步支持了将斑马鱼作为脊椎动物视觉系统模型的使用。
