Eckmiller M S
C. und O. Vogt Institut für Hirnforschung, Medizinische Einrichtungen der Universität Düsseldorf, Federal Republic of Germany.
J Cell Biol. 1987 Nov;105(5):2267-77. doi: 10.1083/jcb.105.5.2267.
Because cone outer segments (COS) are now known to be continually renewed, I reexamined COS morphogenesis in retinas of Xenopus tadpoles (prepared by standard histologic techniques and viewed by light and electron microscopy) to clarify how COS incorporate new membrane. I observed that developing COS underwent an unexpected shape change: they were always conical, but their taper (width divided by length) continually decreased. Ultrastructural examination revealed that many of the membrane foldings within distal COS were partial or incomplete, not extending across the full COS width but ending at variable distances from the ciliary side. Because these partial folds represented infoldings of the plasma membrane of an existing lamella, and they occurred at all COS levels except the base, I have termed them distal invaginations (DI). The completion of each DI increased COS length by one lamella but caused no noticeable change in local COS width; thus the formation of many DI throughout the distal COS presumably resulted in the observed decrease in overall COS taper. Based on these findings, I suggest that DI indicate growing membrane fronts and may represent sites where newly synthesized membrane is incorporated into COS. Because DI occur in developing and adult COS of various vertebrate species, I propose that DI formation plays an important role in the generation of COS taper during development and the remodeling of COS taper in mature cones after tip shedding.
由于现已知道视锥细胞外段（COS）会不断更新，我重新研究了非洲爪蟾蝌蚪视网膜中COS的形态发生（通过标准组织学技术制备并利用光镜和电镜观察），以阐明COS如何整合新的膜。我观察到发育中的COS发生了意想不到的形状变化：它们始终呈圆锥形，但其锥度（宽度除以长度）不断减小。超微结构检查显示，远端COS内的许多膜折叠是部分的或不完整的，没有延伸至整个COS宽度，而是在距纤毛侧不同距离处终止。由于这些部分折叠代表现有薄片质膜的内陷，且它们在除基部外的所有COS水平均有出现，我将其称为远端内陷（DI）。每个DI的完成使COS长度增加一个薄片，但局部COS宽度没有明显变化；因此，整个远端COS中许多DI的形成大概导致了观察到的COS整体锥度减小。基于这些发现，我认为DI表明了正在生长的膜前沿，可能代表新合成的膜整合到COS中的位点。由于DI出现在各种脊椎动物物种的发育中和成年的COS中，我提出DI的形成在发育过程中COS锥度的产生以及成熟视锥细胞在尖端脱落后端锥度的重塑中起重要作用。
